SOIL MANAGEMENT FOR SMALL FARMS

EB1895

SOIL MANAGEMENT FOR

SMALL FARMS

This publication is part of the Farming West of the Cascades series

Craig Cogger

What Is Soil?

Soil is a natural mixture of weathered rock fragments and organic matter

that has formed at the surface of the earth. It is biologically active—a home

to countless microorganisms, invertebrates, and plant roots. It varies in depth

from a few inches to five or more feet. Soil is roughly 50% pore space, a

complex network of pores of varying sizes, much like those in a sponge.

Soil provides nutrients, water, and physical support for plants, and air for

plant roots. Soil organisms are nature’s prime recyclers, turning dead cells

and tissue into nutrients, energy, carbon dioxide, and water to fuel new life.

Soil and Water

Soil Pores, Water, and Productivity

A productive soil can take in and hold water and supply water to plants.

A soil’s permeability and water holding capacity depend on its network

of pores.

• Large pores, or macropores, control the permeability and aeration of a

soil. Macropores include earthworm channels and many root channels.

They are large enough that water moves through them rapidly by gravity,

allowing rainfall and irrigation water to infiltrate into the soil and

excess water to drain through the soil.

• Micropores are fine soil pores, typically a fraction of a millimeter in

diameter. They are responsible for the water holding capacity of soil.

Micropores hold water by capillary forces, like the fine pores in a

sponge or towel. Much of the water held in micropores is available

to plants, while some is held so tightly that plant roots cannot tap it.

Soil that has a balance of macropores and micropores will provide adequate

permeability and water holding capacity for good plant growth. Soils that

contain mostly macropores will take in water readily, but they will not hold

2 — Soil Management for Small Farms

much water. As a result, they need more frequent

irrigation. Soils that contain mostly micropores will

have good water holding capacity, but they will take

longer to dry and warm in the spring. They do not

take in water readily, thus rainfall and irrigation

water may run off the soil surface.

What Affects the Porosity of Your Soil?

A number of soil properties affect the abundance of

macropores and micropores. These include texture,

structure, compaction, and organic matter. You can

evaluate these properties to understand how they

affect the porosity of your soil. The only tools you

need are your eyes, your fingers, and a shovel.

Soil texture. Texture describes how coarse or fine

a soil is; it depends on the relative amounts of sand,

silt, and clay particles in the soil. The coarsest soil

particles are sand. They are visible to the eye, and

they give soil a gritty feel. Silt particles are smaller

than sand, about the size of individual particles of

white flour. They give soil a smooth, floury feel.

Sand and silt particles look like miniature rocks.

Clay particles are the finest, similar in size to tiny

bacteria and viruses, and they typically have a flat

shape. Soils rich in clay feel very hard when dry,

but they are easily shaped and molded when moist.

Although all of these particles seem small, the rel-

ative difference in their sizes is quite large. If a typical

clay particle were the size of a penny, a sand particle

would be as large as a house.

Soil texture affects porosity. Pores between sand

particles tend to be large, while pores between silt

and clay particles tend to be small. Thus, sandy soils

contain mostly macropores, promoting permeability

but limiting water holding capacity. Clayey soils

contain mostly micropores, creating high water

holding capacity but reducing permeability.

Particle size also affects the surface area in a volume

of soil. Surface area is important because surfaces are

the most active part of the soil, holding plant nutri-

ents, binding contaminants, and providing a home for

microorganisms. Clay particles have a large surface

area relative to their volume; a small amount of clay

makes a large contribution to the surface area of a

soil.

Nearly all soils contain a mixture of particle sizes

and a pore network of varying pore sizes. A soil that

has roughly equal influence of sand, silt, and clay

particles is called a loam. Loams usually make good

agricultural and garden soils because they have a

balance between macropores and micropores. Loams

usually have good water holding capacity and moder-

ate permeability.

A sandy loam is similar to a loam, except that it

contains more sand. It feels gritty, yet has enough silt

and clay to hold together in your hand. Sandy loams

usually have low to moderate water holding capacity

and good permeability. Silt loams are richer in silt,

and feel smooth rather than gritty. They are pliable

when moist, but they are not very sticky. Silt loams

usually have high water holding capacity and low

to moderate permeability. Clays and clay loams are

very hard when dry, and sticky when wet. They can

be molded into wires and ribbons when moist. They

generally have high water holding capacity and low

permeability.

Many soils contain coarse fragments—gravel and

rocks. Coarse fragments do not contribute to the

productivity of a soil, and they can be a nuisance

when tilling. Most agricultural soils have less than

15% coarse fragments in the plow layer.

Soils with many different textures can be suitable for

farming, as long as you are aware of the soil’s limita-

tions and use appropriate management. Sandy soils

need lighter, more frequent irrigation and fertilization,

but you can till them earlier in the spring. Clay soils

hold more water, but they are harder to till, and they

dry more slowly in the spring.

Soil structure. Individual particles of sand, silt, and

clay tend to cluster and bind together in soil, forming

aggregates called peds. Aggregation is a natural

process in soil, caused largely by biological activity,

Table 1. Soil particle sizes.

Particle Diameter

Sand 0.05—2 mm

Silt 0.002—0.05 mm

Clay <0.002 mm

Soil Management for Small Farms — 3

100

1090

2080

70 30

60 40

50 50

6040

30 70

8020

10 90

100

90 80 70 60 50 40 30 20 10

Percent Clay

Percent Silt

Percent Sand

Clay

Silty

clay

Silty clay

loam

Clay loam

Sandy

clay

Sandy

clay loam

Sandy loam

Loamy

sand

Sand

Loam

Silt loam

Silt

including earthworm burrowing, root growth, and

microbial action. Soil organic matter is an important

binding agent that stabilizes and strengthens the peds,

providing structure to the soil. Dig up a piece of grass

sod and examine the soil around the roots. The

granules of soil you see hanging onto the grass roots

are examples of peds—containing sand, silt, clay,

and organic matter.

In medium- to fine-textured soils, good structure is

important because it increases the macroporosity of

the soil. The spaces between peds are macropores,

improving permeability, drainage, and recharge of

air into the soil profile. The pores within peds are

predominantly micropores, contributing to the

water holding capacity of the soil.

Compaction and loss of structure. Soil

structure is fragile and can be damaged

or destroyed by compaction, exces-

sive tilling, or tilling when the soil

is too wet. Loss of organic matter

also weakens structure. Compaction alters the struc-

ture of the soil, squeezing macropores into micro-

pores and creating horizontal aggregates that resist

root penetration and water flow. You can protect the

structure of your soil by avoiding unnecessary traffic

on the soil, and by postponing tillage until the soil has

become dry enough to till. If you can mold a piece of

soil into a wire or worm in your hand, it is too wet for

tilling. If the soil crumbles when you try to mold it, it

is dry enough to till.

Sometimes a compacted layer or “plow pan” forms

just below the depth of tillage. Occasionally, tilling

deeper helps break up a plow pan.

Organic matter. Adding organic matter is the

best way to improve the plant environment in

nearly all soils. Organic matter helps build

and stabilize soil structure in fine-textured

and compacted soils, improving soil perm-

eability and aeration, and reducing the

risk of runoff and erosion. The

Percentages of clay, silt,

and sand in the basic

soil textural classes.

biological decomposition of organic materials pro-

duces natural glues, which bind and strengthen soil

aggregates. Organic matter also helps sandy soils

hold water and nutrients. Refer to pages 20–22 for

information on amending soil with organic matter.

Effect of Porosity on Irrigation

Most areas in the Northwest require summer irriga-

tion for peak crop production. Irrigation is essential

on sandy soils. The need for irrigation varies, depend-

ing on soil water holding capacity, weather, site aspect,

and crop requirements. In most cases, the goal of

irrigation is to recharge the available water in the top

foot or so of the soil. For a sand, one inch of irriga-

tion water will recharge the water holding capacity.

Any more will leach through the root zone, carrying

nutrients with it. A silt loam or clay can hold more

than 2 inches of water, but you may need to irrigate

more slowly than for a sandy soil to avoid runoff.

Site and Landscape Factors

Landscape Position

Landscape position affects the suitability of a site

for the production of specific crops. Ridge tops and

sideslopes tend to shed water, and soils in these land-

scape positions are likely to be droughty and subject

to erosion. Soils at the bottom of slopes and in low

areas collect water, and are likely to be wet late into

the spring. Soils on level ground can also be wet

during the winter and spring, especially if they have

a fine-textured or compacted subsurface layer that

restricts the downward movement of water.

Wet Soils

If your soil stays wet in the spring, you will have to

delay working the soil and planting. Working wet soil

can damage the structure, and planting in cold, wet

soil reduces germination. Some plants don’t perform

well in wet soils. Raspberries, for example, become

infected by a root disease in wet soils, lose vigor and

may die.

Soil color gives clues to the wetness of a soil. If the

subsoil is a brown or reddish color, the soil is usually

well drained with few wetness problems. Gray and

mottled subsoils are often saturated during the wet

season.

If you have wet areas on your farmland, avoid the

temptation to till too early in the season, and avoid

crops that are sensitive to wet conditions. Mid-season

annual crops such as sweet corn, green beans, and

squash, and some perennial forages are good choices

for wet soils. Blueberries can be a suitable crop on

moderately wet soils.

Some farmland has subsurface drainage or ditches

that lower the water table and help the soil dry more

quickly. If your land has drainage you can maintain

and repair it to keep it functioning well. If your land

is wet and undrained, you may not be able to install

drainage because of wetland regulations. If you have

questions about field drainage, check with your local

Natural Resources Conservation Service (NRCS)

office. You can find it in the government pages of

the phone book listed under Federal Government,

Department of Agriculture.

Raised beds can improve drainage in marginal situa-

tions. The simplest raised beds involve hilling soil

in rows during tillage. Raspberry growers frequently

hill soil around raspberry plants because the raspberry

roots will grow into the more aerated soil in the hilled

area, reducing problems with root rot. More sophisti-

cated raised beds can be quite expensive, and usually

they are more suitable for gardens than farms. Raised

beds may be economical for farmers in some cases;

for farmers growing a small area of high value crops

under intensive management, installing raised beds

may make sense.

Runoff and Erosion

Runoff and soil erosion can be a serious problem on

sloping ground. Erosion affects soil quality and crop

productivity by reducing the depth of topsoil. Runoff

and erosion can also affect water quality when eroded

soil or dissolved contaminants run off into surface

water. If you are farming sloping ground, follow

recommended conservation practices to reduce run-

off and erosion. These practices include minimum

tillage, cover cropping, contour planting, and strip

rotations. The key to these practices is keeping

vegetative cover or crop residues on the surface as

4 — Soil Management for Small Farms

much as possible to help water soak into the soil

rather than run off. Check with your local NRCS

office for information on conservation practices

that are appropriate for your farm.

Site Aspect

Site aspect has an important effect on crop growth.

South- and southwest-facing exposures collect the

most sunlight and heat and use the most water. North-

and northeast-facing exposures are cooler and retain

more water. Low-lying areas can be prone to early

and late frosts. South and southwest exposures are

a good location for crops grown for early-season

markets and crops that need a lot of heat units to

ripen. Always consider site aspect when looking to

lease or purchase land, or when planning for crop

production.

Soil Horizons and Depth

Soil typically has several layers or horizons that were

formed by natural weathering processes. Sometimes

different layers formed during different geological

events.

The surface soil, or topsoil, is the darkest color and

contains the most organic matter. It is the most bio-

logically active layer and contains the largest proportion

of available nutrients. Topsoils in western Washington

range in depth from about 3 inches to 12 inches.

The subsoil contains less organic matter than the

topsoil, and it is lighter in color. Its texture can be

coarser, finer, or similar to the topsoil. The subsoil

provides additional water and nutrients to crops.

Deep subsoils with moderate to high water holding

capacity greatly increase the ability of deep-rooted

crops to survive drought. Well-drained subsoils have

uniform brown or reddish colors, while wet subsoils

are usually gray, flecked with bright-colored mottles.

Beneath the subsoil is relatively unweathered material

called parent material. The parent material in most

soils contains few roots and little or no structure.

Biological activity is much lower than in the topsoil

or subsoil.

Some soils have layers that restrict root growth. In

western Washington, the most common restrictive

layers are compact “hardpans” in glacial soils, or

coarse gravelly layers that hold little water. Other

restrictive layers include tight clay horizons and

shallow bedrock. Soils with shallow root zones will

have less available water and fewer nutrients than a

similar soil with a deeper root zone.

Soils of Western Washington

Most of the agricultural soils in western Washington

are in the lowlands, below 1,200 feet elevation. This

section describes the major types of lowland soils in

western Washington, how they were formed, and

their suitability for agriculture.

Alluvial Soils

The alluvial soils in the major river valleys through-

out western Washington are by far the best farmlands

in the area. These soils were formed by repeated

flooding cycles that have occurred since the most

recent glacial retreat about 15,000 years ago. The

valley soils are deep, level, and nearly free from

rocks. Most are sandy loam to silt loam in texture.

They have good to excellent water holding capacity,

good nutrient holding capacity, and low erosion

potential. They are easy to till with light equipment

and suitable for a variety of crops. Some areas are

wet late into the spring and are not suitable for early

crops or crops sensitive to wet soils. Other areas are

well drained. Most of the alluvial soils in King and

Pierce counties have been lost to development, and

development encroaches on alluvial farmland in

other counties as well.

Glacial Soils

Most of the other soils in the Puget Sound area formed

from glacial materials on low plateaus. The glacial

soils developed from three main types of glacial

material: till, outwash, and lacustrine (lakebed)

deposits.

Glacial till is material left behind by glacial ice. Soils

developed from till typically have a sandy loam to

loam texture, containing more than 15% gravel and

rocks. These soils are usually 18 to 36 inches deep

and are underlain by a “hardpan” that consists of very

dense and cemented till that was compacted by the

Soil Management for Small Farms — 5

weight of the glacial ice sheet. The dense layer

restricts root growth and water movement, and it is

too thick and too compact to break up. Glacial till

soils have a moderate water holding capacity. They

are frequently sloping and somewhat rocky, and low

areas tend to be wet. Organic matter levels are gener-

ally low. Despite these limitations they are suitable

for pastures and moderately productive for row crops.

Organic matter, conservation tillage, and careful water

and nutrient management will make these soils more

productive.

Glacial outwash was deposited by glacial melt-

water streams. Outwash soils are found throughout

the Puget Sound area and in some parts of southwest

Washington. Outwash soils are usually coarse tex-

tured—sandy or gravelly. In Whatcom County, the

outwash has a cap of silty material about a foot thick

that was deposited by wind. Some sandy outwash

soils are moderately productive and are good soils

for early crops and crops needing well-drained con-

ditions. Careful irrigation management and nutrient

management are essential to successful crop produc-

tion. Gravelly outwash soils are too droughty for

farming. Outwash soils with a silty cap hold more

water than other outwash soils, and they naturally

contain higher levels of organic matter. They vary

in drainage, and are suitable for a variety of crops.

Lacustrine soils formed in material deposited at the

bottom of ancient glacial lakes. They typically have a

silt loam texture in the surface horizon, and silt loam

to clay loam texture in the subsoil. They have a high

water holding capacity and can be productive under

good management. Limitations include wetness late

into spring, and risk of runoff and erosion on sloping

ground. They can be hard when dry.

Volcanic Soils

Areas of eastern King and Pierce counties have soils

developed from volcanic mudflow materials. These

soils are level with a black, loamy topsoil and a dense,

rocky subsoil. Most mudflow soils have restricted

drainage, and are wet during the winter and spring.

They are well suited to pastures and acceptable for

mid-season row crops. They are too wet for early

crops or crops that require good drainage.

Volcanic ash and sediments dominate some soils in

Lewis and Cowlitz counties. Suitability for farming

varies, depending on slope and texture.

Weathered Soils of Southwest Washington

Most areas south of Olympia were not covered by

glacial ice 15,000 years ago, and the soils are quite

different from soils in the Puget Sound area. They

tend to be older, more weathered, and higher in clay

content than the glacial soils. They generally have

fewer coarse fragments and a more stable structure.

They formed in sediments from old terraces, ancient

glacial material, and upland material. Most of these

soils range in texture from loam to clay, and are found

on a variety of slopes. Gently sloping soils on well-

drained landscapes are productive agricultural soils

when good conservation practices are used. Maintain-

ing organic matter and soil structure are essential in

the finer textured soils. Wetter and more sloping soils

are better suited for pasture than row crops.

Evaluating Soils

Evaluating the soil is an important part of choosing

farmland. If you are planning to buy or lease farm-

land, learn as much about the soils as you can, keep-

ing the following in mind: soil texture, structure,

compaction, depth, drainage and wetness, landscape

position, and site aspect. All these will affect site

productivity and suitability for different crops. Don’t

limit your investigation to the topsoil. Dig or probe

to a depth of three feet in a few spots to determine

the depth and properties of the underlying soil.

Soil surveys are a tool you can use to identify poten-

tial farmland or learn more about land you already

lease or own. Each county has a soil survey that

6 — Soil Management for Small Farms

contains maps showing locations of different soil types

and descriptions of each soil type. Because each soil

type can have a range of properties and because several

soil types often are mixed together on the landscape,

the soil survey map does not necessarily match what

you find on a piece of land. Use the survey as a guide

for understanding soils in an area, but walk and dig

on a piece of land to confirm what the soil is like there.

You can request a copy of a soil survey at your local

NRCS office. Some surveys are out of print, but you

can visit the NRCS office to look at one of their

copies.

Soil Organisms

Soil abounds with life. Besides plant roots, earthworms,

insects, and other creatures that we can see, soil is

home to an abundant and diverse population of micro-

organisms. A single gram of topsoil (about one quarter

of a teaspoon) may contain a billion microorganisms

(Table 2). Microorganisms are most abundant in the

rhizosphere—the thin layer of soil surrounding plant

roots.

The main function of soil organisms is to decompose

the remains of plants and other organisms, releasing

energy, nutrients, and carbon dioxide, and creating

soil organic matter. Organisms at all levels, from tiny

bacteria to insects and earthworms, take part in this

food web. Mammals such as moles and voles are

also part of the food web, feeding on insects and

earthworms.

Some soil organisms play other beneficial roles as well.

Mycorrhizae are fungi that infect plant roots and

increase the roots’ ability to take up nutrients from

the soil. Rhizobia bacteria are responsible for nitrogen

fixation. Earthworms mix large volumes of soil and

create macropore channels that improve permeability

and aeration.

Not all soil organisms are beneficial to agriculture.

Some are pathogens, causing a variety of diseases,

such as root rot of raspberries and scab on potatoes.

The activity of soil organisms depends on soil mois-

ture and temperature. Microorganisms are most active

between 70° and 100°F, while earthworms are most

active and abundant at about 50°F. Most organisms

prefer moist soil. Because organic matter is at the

base of the soil food web, soils with more organic

matter tend to have more organisms. Just about any

activity affects the population and diversity of soil

organisms—including tillage, the use of fertilizers,

manures, and pesticides, and choice of crop rotations.

The relationships between farming practices, microbial

populations, and soil quality are complex and often

poorly understood. Amending soils with organic matter,

returning crop residues to the soil, and rotating plantings

are practices that tend to increase the number and

diversity of beneficial organisms.

Nutrient Management, Fertilizers,

and Manures

Soil supplies 13 essential plant nutrients (Table 3).

Each nutrient plays one or more specific roles in the

function of the plant. Nitrogen, for example, is part

of the structure of molecules of chlorophyll, amino

acids, proteins, DNA, and many plant hormones. It

plays a vital role in nearly all aspects of the growth

and development of the plant, and plants need large

amounts of nitrogen to grow well. By contrast, molyb-

denum is involved in the function of a few enzymes,

and plants need only tiny amounts. Molybdenum is

nonetheless essential, and plants do not grow well

in a soil that is deficient in molybdenum.

Nutrients are classified as primary nutrients, secondary

nutrients, and micronutrients, based on the amounts

of them plants need (Table 3). When the soil nutrient

Soil Management for Small Farms — 7

Table 2. Approximate abundance of

microorganisms in agricultural topsoil.

Organism Number per gram

(dry weight basis)

Bacteria 100 million to 1 billion

Actinomycetes 10 million to 100 million

Fungi 100 thousand to 1 million

Algae 10 thousand to 100 thousand

Protozoa 10 thousand to 100 thousand

Nematodes 10 to 100

supply is deficient, farmers use fertilizers to provide

the additional nutrients needed for healthy plant

growth.

Nutrient Deficiencies

The most common nutrient deficiencies are for the

primary nutrients, N, P, and K. Nearly all agricultural

soils lack enough available N for ideal plant growth.

Sulfur deficiencies are common in western Washing-

ton, and calcium and magnesium may be deficient in

acid soils. Except for boron and zinc, growers in this

region seldom encounter micronutrient deficiencies.

Boron deficiencies occur in western Washington,

particularly in root crops, brassica crops, and cane-

berries, such as raspberry. Zinc deficiency is most

often associated with high pH soils, especially on

tree fruit. Plants with nutrient deficiencies sometimes

have characteristic symptoms; they also grow more

slowly, yield less, and are less healthy than plants

with adequate levels of nutrients.

Excess Nutrients

Excess nutrients can be a problem for plants and the

environment. Excesses usually result from applying

too much of a nutrient, or applying it at the wrong

time. Too much boron is toxic to plants. Too much

nitrogen can lead to excessive production of foliage

(increasing the risk of disease and wind damage),

delayed flowering and fruiting, and delayed dormancy.

Extra available nitrogen left in the soil at the end

of the growing season can leach into groundwater,

degrading the quality of drinking water. Excess

levels of nitrogen most often occur on soils where

large amounts of manure have been used. The key

8 — Soil Management for Small Farms

Table 3. Essential plant nutrients. Plants obtain these elements from soil,

fertilizers, crop residues, and other amendments. Plants also require

carbon, hydrogen, and oxygen, which they derive from water and air.

Name Chemical Plant-Available Solubility

Symbol Ions in Soil Water

Primary Nutrients

Nitrogen N NH

, NO

,NO

high

Phosphorus P HPO

, H

very low

Potassium K K

low

Secondary Nutrients

Sulfur S SO

high

Calcium Ca Ca

low

Magnesium Mg Mg

low

Micronutrients

Zinc Zn Zn

very low

Iron Fe Fe

, Fe

+++

very low

Copper Cu Cu

, Cu

very low

Manganese Mn Mn

, Mn

++++

very low

Boron B H

medium

Molybdenum Mo MoO

low

Chlorine Cl Cl

high

to applying fertilizers and manures is to meet plant

nutrient needs without creating excesses that can

harm plants or the environment.

How Nutrients Become Available to Plants

Plants can only take up nutrients that are dissolved in

soil water (in solution). Most nutrients in soil are not

in solution; they exist in the soil minerals and organic

matter in insoluble forms.

Soil nutrients become available to plants only after

they dissolve into soil solution. This occurs by weather-

ing of mineral matter and biological decomposition

of organic matter. Weathering of mineral matter is a

very slow process, releasing small amounts of nutri-

ents each year. The rate of nutrient release from soil

organic matter is somewhat faster, depending on the

amount of biological activity in the soil. Nutrient

release is fastest when soils are warm and moist, but

it is nearly zero when soils are cold or dry. About 1

to 4% of the nutrients in soil organic matter are

released in soluble form each year.

Soluble, available nutrients are in ionic form. An ion

has either positive or negative charges. Positively-

charged ions are cations, and negatively charged ions

are anions. Clay particles and soil organic matter have

negative charges on their surfaces, and they can attract

cations (such as potassium, calcium, and magnesium).

The clay and organic matter surfaces hold nutrient

cations in a ready reserve form that can be released

rapidly into soil solution to replace nutrients taken up by

plant roots. This reserve supply of nutrients contributes

to the fertility of a soil. The capacity of a soil to hold

cations is called its cation exchange capacity, or CEC.

Nitrogen and Phosphorus

Nitrogen and phosphorus present the greatest nutrient

management challenges on most farms. Plants need

large amounts of both nutrients, but excess levels of

either nutrient increase the risk of water quality prob-

lems. Understanding the availability and cycling of

these nutrients can help growers become better

nutrient managers.

The nitrogen cycle. Most nitrogen in soil is in the

organic matter in forms such as humus and proteins.

This organic nitrogen is not available to plants. As

the soil warms in the spring, soil microbes begin

Soil Management for Small Farms — 9

Most nutrients are in in-

soluble form, as part of the

structure of the soil mineral

matter and organic matter.

Weathering of mineral matter and

biological decomposition of organic

matter slowly release nutrients into

soluble, available forms.

- -

Plant roots take up

available nutrients

from the soil solution.

Clay and organic

matter surfaces act

as a ready reserve

supply of cationic

(+ charged) nutrients.

These nutrients are

released rapidly to

the soil solution by

cation exchange.

Table 4. Common forms of nitrogen in soil.

Organic N

Main form in soil. Found in organic matter in

forms such as proteins, lignin, amino acids,

and humus. Not available to plants. Mineral-

ized to ammonium by soil microorganisms.

Ammonium N (NH

)

Soluble form. Available to plants. Converted

to nitrate by soil microorganisms.

Nitrate N (NO

)

Soluble form. Available to plants. Can be lost

by leaching. Converted to gases in wet soils.

Atmospheric N (N

)

Comprises about 80% of soil atmosphere.

Source of N for N-fixing plants. Not used by

other plants.

Unavailable and available forms of plant

nutrients.

to decompose organic matter, releasing some of the

nitrogen as ammonium (NH

). Ammonium is a soluble

ion that is available to plants and soil microbes. When

the soil is warm, a group of microbes called nitrifiers

convert the ammonium to nitrate (NO

). Nitrate is

also soluble and available to plants. The ammonium

and nitrate ions released from soil organic matter

are the same as ammonium or nitrate contained in

processed fertilizers.

Because nitrate has a negative charge, it is not held

to the surfaces of clay or organic matter, and it can

be lost readily by leaching. Nitrate remaining in the

soil at the end of the growing season will leach during

the fall and winter, and it may leach to groundwater

where it becomes a contaminant. In soils that become

saturated during the wet season, soil microbes convert

nitrate to nitrogen gases, which diffuse back into the

atmosphere.

Nitrogen cycle: (a) Legumes, soil organic matter, crop residues and organic additions (manures, composts, etc.) are

sources of organic N. (b) Organic N is mineralized into ammonium (NH

) by soil microbes. (c) Commercial fertilizer

supplies N as ammonium or nitrate (NO

). (d) Microbes nitrify ammonium to nitrite and then nitrate. (e) Plants,

microorganisms, leaching below the root zone, and release of gaseous N to the atmosphere remove N from the

root zone soil solution. (f) Crop harvest removes N stored in plants. (g) Nitrogen present in both crop residues and

soil microorganisms becomes a part of the soil organic N content.

Organic Additions

(manure, compost, etc.)

Soil Organic

Nitrogen

Crop Residues

N-fixation (legumes)

Mineralization (microbes)

(ammonium)

Nitrification (microbes)

(nitrite)

(nitrate)

Leaching

Harvest removal

Gaseous

nitrogen loss

Commercial Fertilizer

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Plants and soil

microorganisms

Ammonium and nitrate taken up by plants become

organic forms in the plant tissue. When plant residues

are returned to the soil, they decompose, slowly

releasing nitrogen into available forms and complet-

ing the cycle.

The nitrogen cycle is a leaky one, with losses to leach-

ing and to the atmosphere. Harvesting crops removes

more nitrogen. To maintain an adequate nitrogen

supply, nitrogen must be added back into the system

through fixation or fertilization. Nitrogen fixation is

a natural process involving certain plants and Rhizo-

bia bacteria. The Rhizobia form nodules in the plant

roots; through these nodules they are able to supply

atmospheric nitrogen (N

) from the soil air to the host

plant. Legumes such as peas, beans, alfalfa, clover,

and scotch broom are common nitrogen-fixers. Alder

trees also fix nitrogen. When tissue from N-fixing

plants decomposes, the fixed nitrogen becomes

10 — Soil Management for Small Farms

available to other plants. Farmers use legumes as cover

crops or in crop rotations to supply nitrogen to future

crops. About one half of the nitrogen in a legume cover

crop will become available to the following crop.

Buying feed to raise animals brings nitrogen onto

the farm in the feed. The animals cycle a portion of

the feed nitrogen to the soil through their manure.

Manure is a good source of nitrogen for crops and

pastures, but adding excessive amounts of manure

leads to over-fertilization, increasing the risk of harm

to crops and water quality. For more information on

using manure, see pages 13–16 and Fertilizing with

Manure, PNW0533, part of the Farming West of the

Cascades series available from Washington State

University Cooperative Extension.

Phosphorus. Available forms of phosphorus are

released from the mineral and organic fractions of

the soil through the weathering and decomposition

processes described on page 9. Unlike nitrogen, the

available forms of phosphorus have limited solubil-

ity, and they revert to insoluble forms in the soil.

In the spring, when soils are still cool, organic matter

decomposition is slow, and little phosphorus is avail-

able for plants. It is especially difficult for seedlings

or transplants to obtain phosphorus early in the season

because their limited root system compounds the effect

of low availability. The plants often have a purplish

tinge associated with phosphorus deficiency. Many

crops respond to phosphorus-rich starter fertilizer

placed near the seed or transplants to help overcome

early deficiencies. Most plants outgrow the deficien-

cies as the season continues, because phosphorus

availability increases in warmer soils, and root systems

grow larger and become more able to tap available

phosphorus.

Phosphorus levels can be quite high in soils with a

history of manure application, although you may

still see some signs of early season phosphorus

deficiency in these soils. The risk of water quality

problems from excess phosphorus is higher in soils

with high phosphorus levels. Phosphorus can be a

problem in surface water, where it can lead to the

excessive growth of aquatic plants. In the Northwest,

lakes are usually the most sensitive to phosphorus.

Phosphorus can enter surface water in runoff, in

eroded sediments, or through shallow groundwater.

The environmental risk depends on the capacity of the

soil to hold phosphorus in unavailable forms, the amount

of phosphorus added to the soil, the amount of runoff

and erosion, and the sensitivity of surface water to

phosphorus.

Understanding Fertilizers

Fertilizers supplement the native nutrient supply of the

soil. They are essential to good plant growth when soil

nutrient supply is inadequate. You can use processed

fertilizers, organic fertilizers, or a combination of the two.

Comparing processed and organic fertilizers.

Processed fertilizers are manufactured or refined from

natural ingredients to make them more concentrated

and more available to plants. Typically they are pro-

cessed into soluble, ionic forms that will be immedi-

ately available to plants.

Organic fertilizers are natural materials that have under-

gone little or no processing. They include both biologi-

cal (plant and animal) and mineral materials (Table 5).

Organic fertilizers release nutrients through natural

processes in the soil, including chemical weathering

of mineral materials, and biological decomposition of

organic matter. The released nutrients are available to

plants in a water-soluble form. These soluble forms

of nutrients are the same as those supplied by pro-

cessed fertilizers.

Compared with processed fertilizers, organic fertiliz-

ers usually contain smaller amounts of nutrients, and

they release nutrients more slowly. You need to apply

larger amounts of organic fertilizers, but their effects

last longer. Organic fertilizers contain a variety of

nutrients, but the amounts are not always balanced

according to plant needs.

Using organic fertilizers recycles materials that other-

wise would be discarded as waste. Production of pro-

cessed fertilizers, on the other hand, can create waste

and use substantial amounts of energy.

Slow release of nutrients. Organic fertilizers are

slow-release fertilizers because their nutrients become

available to plants during the course of the growing

season through the nutrient cycling process described

above. The rate of release of nutrients from organic

materials depends on the activity of soil microorgan-

isms, just as it does for soil organic matter. Tempera-

Soil Management for Small Farms — 11

Table 5. Comparing organic and processed fertilizers.

Organic fertilizers Processed fertilizers

Source Natural materials; little or no processing Manufactured or extracted from

natural materials, often undergoing

extensive processing

Examples Manure, cottonseed meal, rock Ammonium sulfate, processed urea,

phosphate, fish by-products, ground potassium chloride

limestone

Nutrient Usually slow-release; nutrients are Nutrients usually are immediately

Availability released by biological and chemical available to plants

processes in soil

Nutrient Usually low concentration Usually high concentration

Concentration

ture and moisture conditions that favor plant growth also

favor the release of nutrients from organic fertilizers.

Some organic fertilizers contain immediately avail-

able nutrients as well as slow-release nutrients. These

materials can supply nutrients to plants both early in

the season and later. Fresh manure and fish emulsions

are examples of organic fertilizers containing avail-

able nutrients as well as slow-release ones. As manure

ages, the most readily available fraction is lost into

the air or leached into the soil, leaving slow-release

material in the aged manure.

Some material in organic fertilizers decays so slowly

that the nutrients do not become available the first

season after application. Repeated application of organic

fertilizers builds up a pool of material that releases

nutrients very slowly. In the long run, this will decrease

the amount of fertilizer needed each year.

Fertilizer Labels

The labels on fertilizer containers tell the amount of

each of the three primary nutrients in the fertilizer,

expressed as a percent of total fertilizer weight: Nitrogen

(N) is always listed first, phosphorus (P) second, and

potassium (K) third. Historically, fertilizer labels have

not listed the amount of phosphorus as P, but as units

of P

This convention is still used today for

fertilizer labels and recommendations, even though

there is no practical reason for doing so, except that

people are accustomed to it. Similarly, fertilizer labels

list potassium as K

O. For example, a bag of fertilizer

labeled 5-10-10 contains 5% nitrogen expressed as

N, 10% phosphorus expressed as P

, and 10% potas-

sium expressed as K

O. This information is the called

the fertilizer analysis.

For processed fertilizers the analysis guarantees the

amount of available nutrients in the fertilizer. For

organic fertilizers the analysis is for the total amount

of nutrients rather than available nutrients. The amount

of available nutrients will be less than the total, because

nutrients in most organic fertilizers are initially unavail-

able to plants and are released slowly.

Examples of Processed Fertilizers

Nitrogen. The raw material for processed nitrogen

fertilizer is nitrogen gas from the atmosphere. The

manufacturing process is the chemical equivalent

of biological nitrogen fixation, and it requires a sub-

stantial amount of fossil fuel energy. Examples of

processed nitrogen fertilizers are listed in Table 6.

Phosphorus and potassium. Processed phosphorus

fertilizers (Table 7) come from phosphate rock. The

rock is treated with acid, releasing the phosphorus

into plant-available forms.

12 — Soil Management for Small Farms

If you need to convert from P to P

, the conversion is 1lb P = 2.3 lb

. For potassium the conversion is 1lb K = 1.2 lb K

Soil Management for Small Farms — 13

The most common raw material for potassium fertiliz-

ers is sylvinite (Table 7), a mixture of sodium chloride

and potassium chloride salts. The potassium in sylvinite

is already in soluble form, but the sylvinite is treated

to remove the sodium salts, making it suitable to use

as a fertilizer. Some other potassium fertilizers are

potassium sulfate salts, which supply sulfur as well

as potassium.

Mixed fertilizers. Mixed fertilizers contain all three

primary nutrients blended in varying ratios. Many

farmers find these are more convenient to use than

fertilizers providing individual nutrients, although

they tend to be more expensive. Use soil test results

and recommendations from Cooperative Extension

publications to determine which ratios best meet

your needs.

Common Organic Fertilizers

Animal manure. Manure is a good source of plant

nutrients and organic matter, and it is readily avail-

able for many growers. Properly managed manure

applications recycle nutrients to crops, improve soil

quality, and protect water quality. Animal manures

vary widely in nutrient content and nutrient availabil-

ity, depending on the type of animal that produced

Table 6. Examples of processed nitrogen fertilizer materials.

Material Analysis Comments

Urea 46-0-0 Rapidly converted to ammonium in soil.

Ammonium sulfate 21-0-0 Also contains 24% available sulfur. Used

with acid-loving plants such as blueberries.

Diammonium phosphate 18-46-0 Used in mixed N-P-K fertilizers as a source of

nitrogen and phosphorus.

Ammonium nitrate 34-0-0 Contains N in nitrate and ammonium forms.

Table 7. Examples of processed phosphorus and potassium fertilizers.

Material Typical Analysis Comments

Triple superphosphate 0-46-0 Concentrated phosphorus fertilizer.

Monoammonium phosphate 11-52-0 Used in mixed fertilizers as a source of

nitrogen and phosphorus. Also used as a

starter fertilizer.

Diammonium phosphate 18-46-0 Used in mixed fertilizers as a source of

nitrogen and phosphorus.

Potassium chloride 0-0-60 Concentrated source of potassium.

Potassium magnesium sulfate 0-0-22 Also contains 11% magnesium and 18%

sulfur.

Potassium sulfate 0-0-50 Also contains 18% sulfur.

the manure and the age and handling of the manure.

Farmers must be able to understand and reduce that

variability to make best agronomic and environmental

use of manure.

This section is a brief introduction to using manure

as a fertilizer. For details on manure use, including

manure testing, determining application rates, and

spreader calibration, see Fertilizing with Manure,

PNW0533.

• Sources of manure. You can obtain manure in bulk

from manure processors, organic fertilizer dealers,

or directly from livestock producers. Manure from

processors is often more uniform and will have a

guaranteed nutrient analysis. Manure processors

often compost manure to destroy pathogens and

weed seeds. Manure from livestock producers is

usually less expensive, or even free, but it may

be more variable in quality and may not have an

analysis. If the manure is not composted or well

aged, it may contain weed seeds and pathogens.

Be sure to read the sidebar on manure safety

before using fresh manures.

• Nutrient content. Not knowing the nutrient content

of manure can lead to large errors in application

rate. We strongly advise that you test the manure

you plan to use. If you buy manure from a commer-

cial source they should be able to provide you with

nutrient test values, and you would not need to do

further testing.

In the absence of test values, use the published

values in Table 8 as a starting point. Remember

that these are average values and they may not

accurately represent your situation.

• Applying manure. Manure application rates are

usually based on N because N is usually the nutrient

needed in the largest quantity for crop growth.

Manure is not like commercial fertilizer in that it

does not come with a guaranteed N availability.

Nitrogen availability from manure varies greatly,

depending on the type of animal, type and amount

of bedding, and age and storage of manure. There

is no simple test to determine N availability for an

individual manure sample. Use Table 8 as a guide-

line for estimating N availability.

Horse manure or other manures with lots of woody

bedding may remove available nitrogen from the

soil (N immobilization), rather than supply nitro-

gen for crop growth. Woody material contains so

little N that microorganisms must use soil N to

supply their metabolic needs as they break the

material down. Expect nitrogen immobilization

from manures containing less than 1% N.

Experiment with manure applications and observe

the performance of your crops to fine-tune your

application rate. It’s better to be conservative with

your application rate and add more nutrients if the

crops appear deficient.

14 — Soil Management for Small Farms

Table 8. Typical nutrient content, solids content, bulk density, and estimate of nitrogen

availability for animal manure at the time of application.

Bulk Nitrogen

Type N P

O Solids density availability

lb / ton as-is % lb/cu yard %

Broiler with litter 73 63 46 70 900 40–70

Laying hen 37 56 32 40 1400 40–70

Sheep 18 9 24 28 1400 25–50

Beef 12 6 12 23 1400 20–40

Dry stack dairy 9 4 13 35 1400 20–40

Separated dairy solids 5 2 2 19 1100 0–20

Horse 9 6 11 37 1400 0–20

Soil Management for Small Farms — 15

The best of manure application estimates will not

be useful if you don’t know how much you’re

applying once you get into the field. You will need

a spreader with capacity matched to the size of

your farm, and you will need to calibrate it so that

you have confidence in your application rates. See

Fertilizing with Manure, PNW0533, for details on

estimating nutrient availability, application rates,

and spreader calibration.

• Timing manure applications. The best time to

apply manure to row crops is in the spring before

Fresh manure sometimes contains disease-causing

pathogens that can contaminate produce. Salmo-

nella bacteria are among the most serious pathogens

found in animal manure. Pathogenic strains of E. coli

bacteria can be present in cattle manure. Manure

from swine and carnivores can contain helminths,

which are parasitic worms.

These pathogens are not taken up into plant tissue,

but they can adhere to soil on plant roots, or on the

leaves or fruit of low-growing crops. Cooking

destroys pathogens, but raw food carries a risk of

pathogen exposure. Although washing and peeling

raw produce removes most pathogens, some may

remain. The risk from pathogens is greatest for root

crops (e.g., carrots and radishes) or leaf crops (e.g.,

lettuce or spinach), where the edible part touches

planting. You also can apply manure in the fall, but

some of the nutrients will be lost during the winter

if you apply manure to bare ground. Environmental

risks of leaching and runoff also increase. If you

do apply manure in the fall, apply it early, and plant

a cover crop to help capture nutrients and prevent

runoff. You can apply manure to pastures from late

February through mid October in most parts of

western Washington, as long as the applications

are at moderate rates.

Biosolids. Biosolids are a by-product of wastewater

treatment. They are processed wastewater solids that

meet federal and state criteria for application to land.

A common form of biosolids is a spongy, black sub-

stance called “cake.” Biosolids cake is about 20 to 25%

dry matter and 75 to 80% water. It typically contains

about 3 to 6% nitrogen and 2 to 3% phosphorus on

a dry weight basis, plus small amounts of potassium

and trace elements. Some of the nitrogen in biosolids

is immediately available to plants. The rest is released

slowly. Most of the biosolids produced in Washington

are used to fertilize agricultural and forest crops. Typical

agricultural application rates range from 2 to 5 dry

tons per acre depending on the nitrogen content of the

biosolids and the nitrogen requirement of the crop.

the soil. The risk is negligible for crops such as sweet

corn, which does not come in contact with the soil, or

for any crop that is cooked thoroughly.

Avoid using fresh manure where you grow high-risk

crops. Composting manure at high temperatures will

kill pathogens, but you need careful quality control to

make sure that all of the manure reaches conditions

for pathogen kill. Refer to the On-Farm Composting

Handbook for details on composting procedures.

Commercial manure composts are composted under

conditions to destroy pathogens. Bacterial pathogens

die naturally in the environment during a period of

weeks or months, and well-aged manure should not

contain them. Helminths in swine manure can persist

in soil for years, however. High temperature com-

posting will kill helminths.

Using Manure Safely

Some commercial and municipal composters use bio-

solids as an ingredient in making compost. Biosolids

composts behave like other composts, slowly releas-

ing nutrients. They are a good source of organic matter

and will provide small amounts of nutrients to plants.

There are two classes of biosolids based on pathogen

removal. Class A biosolids include biosolids composts

and heat-treated biosolids. They are virtually free of

pathogens, and are safe to use on any crop.

Class B biosolids are processed to reduce, but not

eliminate pathogens. Pathogens remaining in Class B

biosolids are similar to those in fresh manure. After

land application any remaining pathogens in class B

biosolids are killed by exposure to sunlight, drying,

soil microorganisms and other environmental factors.

To allow time for the pathogens to die off, federal

regulations require waiting periods between the

application of class B biosolids and the harvest of

crops. Waiting periods are longest for crops where

the edible part touches the soil (more than 1 year for

aboveground crops and more than 3 years for root

crops). Because of the length of the waiting periods,

it is usually impractical to use Class B biosolids on

vegetable crops. The most commonly used crops for

Class B biosolids application are grain crops and

pastures, which have much shorter waiting periods.

Biosolids contain small amounts of trace elements.

Some trace elements are micronutrients (such as zinc,

copper, and molybdenum) which can benefit crops.

Other trace elements (such as lead and cadmium) have

no known beneficial effects. Large amounts of trace

elements can be toxic to crops, animals, or humans.

Levels of trace elements in biosolids produced in

Washington are low compared with federal standards.

When growers apply Washington biosolids at rates to

meet crop nitrogen requirements, the trace element

accumulations in the crop are insignificant.

You can usually obtain biosolids free or at low cost

from wastewater treatment plants. In many cases the

treatment plant or their contractor will apply the bio-

solids for you. Even though the risks associated with

biosolids use are no greater than risks with manure,

the Department of Ecology requires that wastewater

treatment operators obtain permits for biosolids applica-

tion. As a farmer you do not have to apply for a permit,

but the permit may affect your farm management. For

16 — Soil Management for Small Farms

example, the permit will specify application rates and

timing, and application buffers to ditches, streams,

and property boundaries.

If you consider using biosolids, ask the following

questions:

• Are the biosolids Class A or Class B?

• What is the content of nutrients and trace ele-

ments in the biosolids?

• Are they a slurry (more odor) or a solid?

• What site management and waiting period

requirements are there?

• When will the biosolids be available, and does

the timing fit in with your farm management?

• Who will apply the biosolids and what equipment

will they use?

• Will buyers accept crops treated with biosolids?

(Some food processors and consumers will not buy

vegetable crops fertilized with biosolids, but buyer

acceptance is less likely an issue for grain crops.)

Although biosolids behave like organic fertilizers,

they are not certified as such, and certified organic

farmers should not use biosolids. Their best use is

for grain crops and pastures on farms that do not

seek organic certification. For more information

on biosolids, read the extension bulletin PNW0508,

Fertilizing with Biosolids.

Commercial organic fertilizers. Many organic by-

products and some unprocessed minerals are sold as

commercial organic fertilizers. Table 9 shows approxi-

mate nutrient contents of some of these materials.

Numbers represent total nutrient contents; because

most are slow-release fertilizers, not all of the nutri-

ents will be available the same year they are applied.

The table shows that each fertilizer contains one main

nutrient. The other nutrients are present in smaller

amounts. Several companies produce balanced organic

fertilizers, a combination of materials blended into a

single product that provides all of the primary nutri-

ents.

Choosing organic fertilizers. Choosing organic

fertilizers involves tradeoffs in cost and convenience.

Farmyard manure is usually inexpensive or free, but it

is less convenient than packaged, commercial materi-

als. If you or your neighbors have livestock, it makes

both environmental and economic sense to recycle

the manure produced by the livestock.

Soil Management for Small Farms — 17

Commercial organic fertilizers can be expensive, but

you may choose them where convenience or quick

availability of nutrients is important or for small areas

of land under intensive production. The cost per pound

of nutrients in organic fertilizers varies widely, depend-

ing on the type of material, the concentration of nutri-

ents, and the size of the package. Compare costs and

nutrient availability when shopping for organic

fertilizers.

Estimating How Much Fertilizer to Use

The goal of applying fertilizer is to supply enough

nutrients to meet plant needs, without accumulation

of excess nutrients that could harm water quality.

Farmers should have a regular soil testing program

to assess nutrient status and to plan fertilizer

applications.

Soil tests. A soil test will give you (1) information on

the levels of nutrients in your soil, and (2) a recommen-

dation for how much fertilizer to add each year based

on your soil test results and the crops you are grow-

ing. You don’t need to test each field every year. You

can rotate your tests around the farm, testing each

field at least once every 2 to 3 years.

A basic soil test typically includes the following

nutrients: phosphorus, potassium, calcium, magne-

sium, and boron. The test also includes soil pH and

a recommendation for lime if needed. Many soil test

labs don’t test routinely for nitrogen because there is

no simple, reliable test for predicting nitrogen avail-

ability in soils. The lab will give a nitrogen recom-

mendation, however, based on the crops you are

growing and information you provide about the soil

(such as whether there is a history of manure applica-

tions that would increase soil available nitrogen).

Some specialized nitrogen tests are done, such as the

pre-sidedress nitrate test for corn (see Oregon State

University bulletin EM 8650), but samples for these

tests are collected at different times from the basic test.

The best way to take a soil sample is to collect multiple

cores (at least 15) from a field, air-dry them, and mix

the cores together well. Use a cylindrical soil-sam-

pling probe to get uniform samples. Send about a pint

of the dried, mixed sample to the lab. The samples

you collect should be from the top foot (0 to 12-inch

depth) of your soil. Avoid atypical areas such as the

site of an old manure pile, burn pile, or building, or

areas that are unusually wet or eroded.

Farmers generally collect different samples for each

field, crop, and soil type. If you are growing a large

variety of crops on a small acreage, it will not be

economical to do a soil test for each crop, and you

will want to group crops for soil tests. For more infor-

mation on sample collection, see Oregon State Univer-

sity Extension Bulletin EC 628, How To Take a Soil

Sample…And Why, available on the OSU publications

web site at http://eesc.orst.edu, or University of Idaho

Bulletin 704, Soil Sampling, available on the UIdaho

web site at http://info.ag.uidaho.edu/. For information

on interpreting soil tests see OSU bulletin EC 1478,

Soil Test Interpretation Guide, available on the OSU

web site listed above.

Washington State University and Oregon State Univer-

sity no longer test soils, but private labs in both states

do. Cooperative Extension county offices have lists

of testing labs. If you have not worked with a lab

before, call them to make sure they are set up to test

and make recommendations for agricultural soils.

Table 9. Total nitrogen, phosphate and

potash content typical of some organic

fertilizers.

Material % Nitrogen % P

% K

Cottonseed Meal 6–7 2 1

Blood Meal

12–15 1 1

Alfalfa 2 0.5 2

Bat Guano

10 3 1

Fish Meal

10 4 0

Fish Emulsions

3–5 1 1

Bone Meal 1–4 12–24 0

Rock Phosphate

0 25–30 0

(only 2–3% available)

Greensand 0 0 3–7

Kelp Meal 1 0.1 2–5

These materials contain a substantial amount of quickly

available nitrogen that plants can use early in the season.

Very low availability. Useful only in acid soils.

Ask the lab:

• Do you routinely test soils for plant nutrients and

pH?

• Do you use WSU or OSU test methods and

fertilizer guides?

• Do you give recommendations for fertilizer

applications?

• Are there forms to complete? What information

do you need?

• How should the sample be packaged and sent?

• How much does a test cost?

• How quickly will you send results?

Extension publications. Extension publications are a

good source of information on crop nutrient needs.

Use them together with soil test results for planning

fertilizer applications. Check other bulletins in the

Farming West of the Cascades series, or use the WSU

publications web site (caheinfo.wsu.edu) or the OSU

site (eesc.orst.edu) to find appropriate bulletins.

Fertilizer calculations. Fertilizer recommendations

are usually given in pounds of nutrient (such as nitro-

gen) per acre. You will need to convert the fertilizer

recommendations from pounds of nutrient to actual

pounds of fertilizer.

Example

You plan to make a mid-season application of 100 lb/

acre of nitrogen to your corn crop. You are growing 5

acres of corn, using urea (46-0-0).

1. Divide the amount of nitrogen recommended for

1 acre (100 lb) by the fraction of nitrogen in the

fertilizer (46% or .46).

100 lb N/acre / .46 = 218 lb urea/acre

2. Calculate the total amount of fertilizer needed by

multiplying the area of your field by the fertilizer

rate calculated in step 1:

218 lb/acre x 5 acres = 1,090 lb urea

If you are growing crops intensively on a small area

of land, the acre-based calculation for each crop may

not be convenient. To convert the calculations to units

per 1,000 square feet, divide the recommendations per

acre by 44.

Example for a Small Area

You are growing carrots and plan to make a nitrogen

application of 80 lb N/acre. Your carrot bed covers

2,000 square feet and you are using urea (46-0-0).

1. Divide the amount of nitrogen recommended for

1 acre (80 lb) by the fraction of nitrogen in the

fertilizer (46% or .46).

80 lb N/acre / .46 = 174 lb urea/acre

2. Divide the urea rate by 44 to calculate the amount

of urea needed per 1,000 square feet.

174 lb urea/acre / 44 = 4.0 lb urea/1,000 square feet

3. Calculate the total amount of fertilizer needed by

multiplying the area of your field (in 1,000 square

foot units) by the fertilizer rate calculated in step 2.

(4.0 lb urea/1,000 square feet) x 2 = 8.0 lb urea

Tips for Estimating Organic Fertilizer Rates

Estimating how much organic fertilizer to use can be

a challenge because you must estimate the availability

of the nutrients in the organic fertilizer.

• Organic fertilizers having large proportions of

available nutrients (such as bat guano and fish

emulsions) can be substituted in direct proportion

for processed fertilizers.

• For other commercial organic fertilizers, apply

according to their nutrient availability. Composts,

rock phosphate, and plant residues generally have

lower nutrient availability than more concentrated

animal products (bloodmeal, bone meal, and

chicken manure). If you use packaged organic

fertilizers, the recommendations on the package

often are a good guideline for application rates.

Check the recommendations against other prod-

ucts to make sure they seem reasonable.

• The nutrient concentration and availability in

farmyard manures varies widely depending on

the type of manure and its handling. For guide-

lines for determining appropriate application

rates for different types of manures see Fertilizing

with Manure, PNW0533.

18 — Soil Management for Small Farms

• Whatever fertilizer you use, observe your crops

carefully. It is sometimes hard to estimate how

much organic fertilizer to use. Lush plant growth

and delayed fruiting and flowering are signs of

high amounts of available nitrogen, indicating

overfertilization. You can experiment with different

fertilizer rates in different rows and see if you

notice differences. Plan your experiment care-

fully, so you are confident that any results come

from the fertilizer rates, rather than differences

in soil, watering, sunlight, or other management.

• Use soil testing to track changes in nutrient

availability and modify application rates.

Timing Fertilizer Applications

In most cases, the best time to apply fertilizers is close

to the time the plant needs the nutrients. Proper timing

of applications reduces the potential for loss of nutrients

before they are taken up by the plants. Loss of nutri-

ents is not only inefficient, but the lost nutrients may

become contaminants in groundwater or surface water.

Plants need the largest amount of nutrients when they

are growing most rapidly—early to midsummer for

corn and squash, earlier for spring plantings of lettuce

and other greens. Plants also need available nutrients

(especially phosphorus) shortly after seeding and

transplanting. For a long-season crop such as corn,

farmers often add a small amount of fertilizer as a

starter at the time of seeding and a larger amount in

early summer, just before the period of rapid growth.

If the entire application was made in the spring, some

of the nutrients (especially nitrogen) could be lost by

leaching before the plant was ready to use them.

When using organic fertilizers, a single application

is usually adequate because nutrient release occurs

throughout the growing season. If you apply organic

fertilizers to a crop that matures early, the crop will

not take up nutrients that are released from the fertilizer

during the late summer and fall. Nitrogen released

after crop maturity is likely to leach into groundwater

during the winter. Planting cover crops between the

rows or immediately after crop harvest can capture

some of the nutrients released late in the season by

organic fertilizers.

For perennial plants, timing depends on the growth

habit of the plant. Blueberries, for example, benefit

most from fertilizer applied early in the season at

budbreak, while June-bearing strawberries are fertil-

ized after harvest. For crop-specific information on

timing fertilizer applications, refer to the appropriate

extension bulletins.

Calibrating Fertilizer Spreaders

Depending on the size and type of your operation,

the type of spreader you use will vary. If you have

a small, intensively managed crop, you may apply

fertilizer by hand. For a larger area, a hand-operated

whirlybird applicator may work, or you may use a

broadcast or band applicator towed behind a tractor.

Whatever equipment you use, be sure to calibrate it

to apply the appropriate amount of fertilizer.

For tractor-operated applicators you control the rate

by adjusting the fertilizer settings.

1. Select a setting that is likely to be close to your

desired rate, and place a known weight of fertil-

izer in the spreader.

2. Measure a known distance (50 or 100 feet is

adequate) and drive the spreader over that distance.

3. At the end of the run, weigh the fertilizer remain-

ing in the spreader. The difference between the

initial and final weights is the amount spread.

4. Calculate the area spread by multiplying the

distance traveled by the width spread. Convert

into acres by dividing by 43,560.

Soil Management for Small Farms — 19

5. Divide the weight of fertilizer applied by the

application area in acres. This is the application

rate in lb/acre.

6. Compare with your target rate.

7. Adjust the settings as needed and repeat the

process until you are within 10% of the desired

fertilizer rate.

8. Record the settings for future use.

Example

1. You need to apply urea at a rate of 218 lb/acre.

Your spreader width is 5 feet, and the length of

your test run is 100 feet. You apply to the test

area and use 2.1 lb of urea.

2. The area spread is 5 ft x 100 ft = 500 square feet

3. The area spread (in acres) is 500 / 43560 = 0.0115

acre

4. The application rate in lb/acre is 2.1 / 0.0115 =

183 lb/acre

5. The difference from the target rate is 218 - 183 =

35 lb /acre

6. This is more than 10% below the target so you

will need to open the settings and test again.

You can adapt the above method to work with hand-

operated whirlybird spreaders. It may be more conve-

nient to calculate on the basis of 1,000 square foot

units rather than acre units. You can adjust rates with

whirlybird spreaders by changing the settings or chang-

ing your walking speed.

You can also calibrate a spreader by placing it on

blocks so that the wheels can spin freely. Place fertilizer

in the spreader, and place a tarp beneath the spreader.

Turn the wheels, counting the number of turns. Deter-

mine the distance traveled by multiplying the circum-

ference of the wheel by the number of turns. Weigh

the fertilizer on the tarp, and continue with the above

calculation. For calibrating manure spreaders see

Fertilizing with Manure, PNW0533.

If you change fertilizer formulation or application

rate, you will need to recalibrate the spreader for the

new conditions.

Adding Organic Matter

Organic matter builds and stabilizes soil structure,

improving the porosity, infiltration, and drainage of

the soil, and reducing erosion. It holds water and

nutrients for plants and soil organisms. Organic matter

also is a long-term, slow-release storehouse of nitro-

gen, phosphorus, and sulfur.

The value of organic amendments varies, depending

on their nitrogen content, or more specifically on their

carbon to nitrogen (C:N) ratio. Organic materials that

have a low C:N ratio, such as undiluted manure or

bloodmeal, are rich in nitrogen. They are a good source

of nutrients, but growers must use them sparingly to

avoid overfertilization.

Materials with an intermediate C:N ratio (including

many composts, leaf mulches, and cover crop residues)

have lower nutrient availability. They are the best

materials to replenish soil organic matter. Because

they are relatively low in available nutrients, they

can be added to the soil in large amounts.

Materials with a high C:N ratio (such as straw, bark,

and sawdust) contain so little nitrogen that they will

reduce levels of available nitrogen when they are

mixed into the soil. Soil microorganisms use available

nitrogen from the soil when they decompose these

materials, leaving little nitrogen for the plants. This

process is called immobilization and it results in

nitrogen deficiency. If you amend your soil with

materials with a high C:N ratio, you will need to add

extra nitrogen fertilizer to compensate for immobili-

zation. The best use for these materials is as mulches

around perennial crops or in walkways. They will not

immobilize nitrogen until you mix them into the soil.

Green Manure

Green manures are cover crops grown specifically to

be tilled into the soil. Planting green manure is a way

to grow your own organic matter. The value of cover

crops goes beyond their contribution of organic

matter. Cover crops also can do the following:

20 — Soil Management for Small Farms

• Capture and recycle nutrients that otherwise

would be lost by leaching during the winter.

• Protect the soil surface from rainfall impact

during the winter.

• Reduce runoff and erosion.

• Help suppress weeds.

• Supply nitrogen (if legumes are grown).

No one cover crop provides all of these benefits

(Table 10). Deciding which cover crop or crop

combination to grow depends on which benefits

are most important to you, and which cover crops

fit best into your farm plan.

Farmers usually plant cover crops in the fall and till

them as green manure in the spring, before planting.

The earlier cover crops are planted, the more benefits

they will provide. Research in western Washington

showed that cereal rye planted in September captured

three times the amount of nitrogen as an October

planting. Legumes such as vetch and crimson clover

Table 10. Examples of cover crops grown

in Washington.

Cereal Rye

Very hardy, grows quickly, matures rapidly in

spring. Helps suppress winter weeds, protects

soil surface from raindrop impact and erosion.

Winter Wheat

Leafy, covers soil well, matures slowly. Helps

suppress winter weeds, protects soil surface

from raindrop impact and erosion.

Hairy Vetch

Legume, fixes nitrogen, starts slowly, grows

quickly in spring, good companion crop for

cereal rye.

Crimson Clover

Legume, fixes nitrogen, slower growth than

vetch.

Buckwheat

Fast growing, frost-sensitive, ready to till in 30

days. Helps suppress weeds. Produces biomass

quickly. Use as a summer cover crop.

need an early start to cover the soil before cold weather

arrives.

You will not get much benefit from cover crops planted

after October. Plant cover crops in areas you harvest

early, and consider applying compost mulches or using

minimum tillage in areas you harvest later. You can

also start cover crops between rows of late crops

where space allows.

Till cover crops into the soil before they flower. After

flowering, the plants become woody and decline in

quality. Also, tilling the crop into the soil becomes

more difficult if the plants grow too large.

The organic matter benefits from cover crops last only

about one year. Where possible, make cover crops an

annual part of your crop rotation. The WSU Cooperative

Extension Bulletin EB1824, Cover Crops for Gardens

in Western Washington and Oregon, gives details on

choosing and managing cover crops in areas west of

the Cascades. It is useful for small-acreage farmers as

well as gardeners. For more details on specific cover

crops, refer to the Oregon State University Extension

Service cover crops series. These publications are

available on the Web (see page 18 for WSU and OSU

publications web addresses).

Composts

Composts provide an excellent source of organic

matter. They also supply a modest amount of nutrients

released slowly over the long term. You need to apply

Vetch

Soil Management for Small Farms — 21

large amounts of compost (100 to 200 yards per acre)

to see substantial benefits, so the cost of purchased

composts can be prohibitive for all but small areas

of intensively managed crops.

Commercial composts. Yard debris is the major

raw material in most commercial composts sold in

Washington and Oregon. Commercial composts may

also contain animal manure, biosolids, food waste,

or wood waste. Commercial composts are made on a

large scale, with aeration and/or frequent turning to

meet time and temperature requirements to kill weed

seeds, plant pathogens, and human pathogens. They

are high quality materials, but they are usually too

expensive for general agricultural use.

On-farm composting. An alternative to commercial

composts is making compost on your farm using crop

residues, manure, or other appropriate materials gen-

erated on the farm. In some counties you can import

material to compost and use on-farm without a permit,

while other counties require a permit. If you sell or

distribute your compost off-farm you will need a permit.

Contact your local health department if you have

questions about composting permits.

To produce high-quality compost you need to gener-

ate high temperatures within the pile and turn the pile

frequently to make sure all material is exposed to high

temperatures. High-temperature composting demands

time and attention, and it’s not for every farmer. Com-

posting will still occur at lower temperatures, but you

will not get a complete kill of weed seeds or patho-

gens. You can still produce suitable compost at lower

temperatures, as long as you avoid raw materials that

are full of weed seeds or pathogens.

For detailed information about on-farm composting,

including raw materials, methods, and equipment,

refer to the On-Farm Composting Handbook, avail-

able from the Northeast Regional Agricultural Engi-

neering Service, Cornell University Cooperative

Extension, Ithaca, New York, 14853-5701.

Partially Composted Yard Debris

Some commercial composters will provide partially

composted yard debris to farmers during periods of

peak flow. These periods usually occur during the

spring when homeowners ship large volumes of yard

debris to composters. If composters can divert some

of the flow to farms, they can avoid overloading their

composting facilities. The timing is usually good for

farmers because it occurs when they are preparing

land for spring crops.

Research in western Washington has shown that

partially composted yard debris is a good source of

nutrients and organic matter. Application rates of 20

to 30 dry tons per acre per year can supply all the

nutrients needed for a corn crop and increase soil

organic matter levels. Because the material is not fully

composted, there is some risk of weeds, but no weed

problems have been observed in the experiments and

on-farm demonstrations done by researchers at WSU

Puyallup.

Yard debris is inexpensive for farmers, but prices

could rise depending on supply and demand. Process-

ing of yard debris differs, depending on the facility.

Some yard debris is composted actively for a few

days before delivery to the farm, while others do not

have active composting. Contact local composters

to find out if they have a yard debris program for

agriculture. Find out about their procedures, costs,

and timing to see if it fits into your farm plan.

Soil pH and Liming

Soil pH measures the acidity or alkalinity of a soil.

A pH of 7 is neutral—where acidity and alkalinity

are balanced. Acidity increases by a factor of 10 with

each 1 unit drop in pH below 7. For example, a pH

of 5.5 is ten times as acidic as a pH of 6.5. Alkalinity

increases by a factor of 10 with each 1-unit change in

pH above 7. Native soil pH depends on the minerals

present in the soil and the amount of rainfall. Soils

tend to be near neutral in the low rainfall areas of

western Washington (around Sequim, Port Townsend,

and Coupeville) and acid in the moderate to high

rainfall areas. Farming practices also affect soil pH;

many nitrogen fertilizers tend to reduce soil pH,

while liming increases soil pH.

Soil pH influences plant growth in three ways:

• by affecting availability of plant nutrients

• by affecting availability of toxic metals

• by affecting the activity of soil microorganisms,

which in turn affects nutrient cycling and disease

risk.

22 — Soil Management for Small Farms

The availability of phosphorus decreases in acid soils,

while the availability of iron increases. In alkaline

soils, the availability of iron and zinc can be quite

low. In acid soils aluminum availability increases.

Aluminum is one of the most common elements in

soil, but it is not a plant nutrient, and it is toxic to

plants in high concentrations. Very little aluminum

is in solution in soils above pH 6 and it causes no

problems to plants. As pH declines and aluminum

availability increases, aluminum toxicity becomes a

problem.

Soil pH affects microbes in a similar way. The most

numerous and diverse microbial populations exist in

the middle of the pH range; fewer organisms are adapted

to strongly acid or strongly alkaline soils. Nutrient

cycling is slower in acid and alkaline soils because

of reduced microbial populations. Soil pH also affects

pathogenic microbes, and growers can adjust pH to

manage some plant diseases.

Many crops perform best between pH 6 and 7.5, but

some (such as blueberries) are adapted to more strongly

acid soils. Before amending the soil to adjust pH, you

must know the preferred pH ranges of your crops.

Increasing soil pH with lime. Lime is ground lime-

stone, a rock containing calcium carbonate. It is a

certified organic amendment, suitable for use in organic

agriculture. Lime raises the pH of acid soils and sup-

plies the essential nutrient, calcium. Dolomitic lime

contains magnesium as well as calcium, and it can

correct magnesium deficiencies in soil as well as raise

soil pH. Dolomitic lime is more expensive and slower

acting than agricultural lime, so use it only when you

need to.

A basic soil test gives you a lime recommendation in

tons of agricultural lime (calcium carbonate) per acre.

Lime is a slow-release material. A fall application will

benefit a spring crop. Do not lime areas where you

grow acid-loving plants.

Gypsum (calcium sulfate) is not a substitute for lime.

It provides calcium and sulfur to soils, but it has little

effect on soil pH. Gypsum has been promoted as a

soil amendment to improve soil structure, but it does

not work in our environment. Gypsum improves

structure only when the problem results from excess

sodium in the soil, a rare condition west of the

Cascades. Use organic amendments to improve soil

structure, as described earlier.

Decreasing soil pH. Elemental sulfur lowers the pH

of a soil. If your soil pH is too high for acid-loving

crops that you are growing, ask your soil test lab for

an acidification recommendation. Ammonium sulfate

fertilizer also lowers pH, but the effect is not as fast

as for sulfur. Urea reduces pH slowly, as do some

organic fertilizers.

About the Author

Craig Cogger is an Extension Soil Scientist, Depart-

ment of Crop and Soil Sciences, Washington State

University, Puyallup, WA. He specializes in research

and extension programs in the use of animal manures,

composts, and other organic byproducts for crop

production.

Soil Management for Small Farms — 23

The series Farming West of the Cascades is a project of the WSU Food and Farm Connections Team. The Food and Farm

Connections Team is a group of Cooperative Extension faculty and staff seeking to promote and enhance sustainable,

community-based food and fiber systems through research, education, and partnerships. The team is supported by the WSU

Center for Sustaining Agriculture and Natural Resources (CSANR). For more information about the team or CSANR,

visit our website at <http://foodfarm.wsu.edu>, or call (253) 445-4514.

Funding for this project was provided by WSU Cooperative Extension and the King County Agriculture Commission.

A list of WSU publications is available online <http://caheinfo.wsu.edu> or order through the Bulletin office 1-800-723-1763.

Issued by Washington State University Cooperative Extension, Oregon State University Extension Service, University of

Idaho Cooperative Extension System, and the U. S. Department of Agriculture in furtherance of the Acts of May 8 and June

30, 1914. Cooperative Extension programs and policies comply with federal and state laws and regulations on nondis-

crimination regarding race, sex, religion, age, color, creed, national or ethnic origin; physical, mental, or sensory disability;

marital status, sexual orientation, and status as a Vietnam-era or disabled veteran. Evidence of noncompliance may be

reported through your local Cooperative Extension office. Trade names have been used to simplify information; no

endorsement is intended. Published July 2000. Subject code 371. C. EB1895

K ING C OUNTY

Department of Natural Resources

King County Agriculture Commission

COOPERATIVE EXTENSION

24 — Soil Management for Small Farms