soils and fertilizer - professional and continuing … · pile and leaving it until it decomposes....

Soils and Fertilizers ❂2TOPICS IN THIS CHAPTER❂

By Craig Cogger, Extension soil scientist,Washington State University.

■ Soil and water■ Soil organisms■ Soil nutrients■ Understanding fertilizers■ How much fertilizer to use■ When to fertilize■ Adding organic matter■ Soil pH■ Soil salinity

Soil is a mixture of weathered rock fragments andorganic matter at the earth’s surface. It is biologi-cally active—a home to countless microorgan-

isms, invertebrates, and plant roots. It varies in depthfrom a few inches to 5 feet or more. Soil is roughly50 percent pore space. This space forms a complexnetwork of pores of varying sizes, much like those in asponge.

Soil provides nutrients, water, and physical supportfor plants as well as air for plant roots. Soil organismsare nature’s primary recyclers, turning dead cells andtissue into nutrients, energy, carbon dioxide, and waterto fuel new life.

Soil and waterSoil pores, water, and productivity

A productive soil is permeable to water and is ableto supply water to plants. A soil’s permeability andwater-holding capacity depend on its network of pores:• Large pores (macropores) control a soil’s perme-

ability and aeration. Macropores include earth-worm and root channels. Because they are large,water moves through them rapidly by gravity.Thus, rainfall and irrigation infiltrate into the soil,and excess water drains through it.

• Micropores are fine soil pores, typically a fraction ofa millimeter in diameter. They are responsible for asoil’s water-holding capacity. Like the fine pores ina sponge or towel, micropores hold water againstthe force of gravity. Much of the water held inmicropores is available to plants, while some isheld so tightly that plant roots cannot use it.

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36 • Soils and Fertilizers—Chapter 2

Soils and fertilizers terminologyAggregation—The process by which

individual particles of sand, silt, and claycluster and bind together to form peds.

Anion—A negatively charged ion. Plantnutrient examples include nitrate (NO3

-),phosphate (H2PO4

-), and sulfate (SO42-).

Aspect—Direction of exposure to sunlight.Biosolids—A by-product of wastewater

treatment sometimes used as a fertilizer.Capillary force—The action by which water

molecules bind to the surfaces of soilparticles and to each other, thus holdingwater in fine pores against the force ofgravity.

Cation—A positively charged ion. Plantnutrient examples include calcium (Ca++)and potassium (K+).

Cation exchange capacity (CEC)—A soil’scapacity to hold cations as a storehouseof reserve nutrients.

Clay—The smallest type of soil particle(less than 0.002 mm in diameter).

C:N ratio—The ratio of carbon to nitrogenin organic materials. Materials with a highC:N ratio are good bulking agents incompost piles, while those with a low C:Nratio are good energy sources.

Cold composting—A slow compostingprocess that involves simply building apile and leaving it until it decomposes.This process may take months or longer.Cold composting does not kill weed seedsor pathogens.

Compaction—Pressure that squeezes soilinto layers that resist root penetrationand water movement. Often the result offoot or machine traffic.

Compost—The product created by thebreakdown of organic waste underconditions manipulated by humans.

Cover crop—A crop that is dug into the soilto return organic matter and nitrogen tothe soil. Also called green manure.

Decomposition—The breakdown of organicmaterials by microorganisms.

Fertilizer—A natural or synthetic productadded to the soil to supply plantnutrients.

Fertilizer analysis—The amount ofnitrogen, phosphorus (as P2O5), andpotassium (as K2O) in a fertilizerexpressed as a percent of total fertilizerweight. Nitrogen (N) always is listed first,phosphorus (P) second, and potassium(K) third.

Green manure—Same as cover crop.Hot composting—A fast composting

process that produces finished compostin 6 to 8 weeks. High temperatures aremaintained by mixing balanced volumesof energy materials and bulking agents,keeping the pile moist, and turning itfrequently to keep it aerated.

Humus—The end product of decomposedanimal or vegetable matter.

Immobilization—The process by which soilmicroorganisms use available nitrogen asthey break down materials with a highC:N ratio, thus reducing the amount ofnitrogen available to plants.

Infiltration—The movement of water intosoil.

Ion—An atom or molecule with eitherpositive or negative charges.

Leaching—Movement of water and solublenutrients down through the soil profile.

Loam—A soil with roughly equal influencefrom sand, silt, and clay particles.

Macropore—A large soil pore. Macroporesinclude earthworm and root channelsand control a soil’s permeability andaeration.

Micronutrient—A nutrient used by plantsin small amounts (iron, zinc, molyb-denum, manganese, boron, copper, andchlorine). Also called a trace element.

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Chapter 2—Soils and Fertilizers • 37

Soils and fertilizers terminology (continued)Micropore—A fine soil pore, typically a

fraction of a millimeter in diameter.Micropores are responsible for a soil’sability to hold water.

Mycorrhizae—Beneficial fungi that infectplant roots and increase their ability totake up nutrients from the soil.

Nitrifier—A microbe that convertsammonium to nitrate.

Nitrogen cycle—The sequence ofbiochemical changes undergone bynitrogen as it moves from livingorganisms, to decomposing organicmatter, to inorganic forms, and back toliving organisms.

Nitrogen fixation—The conversion ofatmospheric nitrogen into plant-available forms by Rhizobia bacteria.

Organic fertilizer—A natural fertilizermaterial that has undergone little or noprocessing. Can include plant, animal,and/or mineral materials.

Organic matter—Any material originatingfrom a living organism (peat moss,compost, ground bark, manure, etc.).

Pathogen—A disease-causing organism.Pathogenic soil organisms includebacteria, viruses, fungi, and nematodes.

Ped—A cluster of individual soil particles.Permeability—The rate at which water

moves through a soil.pH—A measure of acidity or alkalinity.

Values from 0 to 7 indicate acidity, avalue of 7 is neutral, and values from7 to 14 indicate alkalinity. Most soilshave a pH between 4.5 and 9.

Phosphate—The form of phosphoruslisted in most fertilizer analyses (P205).

Potash—The form of potassium listed inmost fertilizer analyses (K20).

Primary nutrient—A nutrient required byplants in a relatively large amount(nitrogen, phosphorus, and potassium).

Processed fertilizer—A fertilizer that ismanufactured or is refined from naturalingredients to be more concentrated andmore available to plants.

Quick-release fertilizer—A fertilizer thatcontains nutrients in plant-available formssuch as ammonium and nitrate.

Rhizobia bacteria—Bacteria that live inassociation with roots of legumes andconvert atmospheric nitrogen to plant-available forms, a process known asnitrogen fixation.

Rhizosphere—The thin layer of soilimmediately surrounding plant roots.

Sand—The coarsest type of soil particle(0.05 to 2 mm in diameter).

Secondary nutrient—A nutrient needed byplants in a moderate amount (sulfur,calcium, and magnesium).

Silt—A type of soil particle that isintermediate in size between sand and clay(0.002 to 0.05 mm in diameter).

Slow-release fertilizer—A fertilizer materialthat must be converted into a plant-available form by soil microorganisms.

Soil—A natural, biologically active mixture ofweathered rock fragments and organicmatter at the earth’s surface.

Soil salinity—A measure of the total solublesalts in a soil.

Soil solution—The solution of water anddissolved minerals found in soil pores.

Soil structure—The arrangement ofaggregates (peds) in a soil.

Soil texture—How coarse or fine a soil is.Texture is determined by the proportionsof sand, silt, and clay in the soil.

Soluble salt—A compound often remainingin soil from irrigation water, fertilizer,compost, or manure applications.

Water-holding capacity—The ability of asoil’s micropores to hold water for plantuse.


Soil that has a balance of macroporesand micropores provides adequate perme-ability and water-holding capacity forgood plant growth. Soils that containmostly macropores drain readily, but aredroughty and need more frequent irriga-tion. Soils that contain mostly microporeshave good water-holding capacity, buttake longer to dry out and warm up in thespring. Runoff of rainfall and irrigationwater also is more likely on these soils.

What affects soil porosity?Several soil properties affect porosity,

including texture, structure, compaction,and organic matter. You can evaluate yourgarden soil with respect to these proper-ties to understand how they affect itsporosity. The only tools you need are youreyes and fingers and a shovel.

Soil textureTexture describes how coarse or fine a

soil is. The coarsest soil particles aresand. They are visible to the eye and givesoil a gritty feel. Silt particles are smallerthan sand—about the size of individualparticles of white flour. They give soil asmooth, floury feel. On close inspection,sand and silt particles look like miniaturerocks (Figure 1).

Clay particles are the smallest—aboutthe size of bacteria and viruses—and canbe seen only with a microscope. Theytypically have a flat shape, similar to asheet of mica. Soils rich in clay feel very

hard when dry, but are easily shaped andmolded when moist.

Although all of these particles seemsmall, the relative difference in their sizesis quite large. If a typical clay particlewere the size of a penny, a sand particlewould be as large as a house.

Soil texture directly affects porosity.Pores between sand particles tend to belarge, while those between silt and clayparticles tend to be small. Thus, sandysoils contain mostly macropores andusually have rapid permeability butlimited water-holding capacity. Micro-pores predominate in soils containingmostly silt and clay, creating high water-holding capacity, but reducing permeabil-ity.

Particle size also affects the surfacearea in a volume of soil. Surface area isimportant because surfaces are the mostactive part of the soil. They hold plantnutrients, bind contaminants, and providea home for microorganisms. Clay particleshave a large surface area relative to theirvolume, and a small amount of clay makesa large contribution to a soil’s surfacearea.

Nearly all soils contain a mixture ofparticle sizes and have a pore networkcontaining a mixture of pore sizes(Figure 2). A soil with roughly equalinfluence from sand, silt, and clay par-ticles is called a loam. Loams usuallymake good agricultural and garden soilsbecause they have a balance of macro-pores and micropores. Thus, they usuallyhave good water-holding capacity andmoderate permeability.

A sandy loam is similar to a loam,except that it contains more sand. It feelsgritty, yet has enough silt and clay to holdtogether in your hand. Sandy loamsusually have low to moderate water-holding capacity and good permeability.

Silt loams are richer in silt and feelsmooth rather than gritty. They are pli-able when moist, but not very sticky. Silt

Sand(0.05–2 mm diameter)

Silt(0.002–0.05 mm diameter)

Clay(<0.002 mm diameter)

Figure 1.—Shapes of soil particles.


loams usually have high water-holdingcapacity and low to moderate permeabil-ity.

Clays and clay loams are very hardwhen dry, sticky when wet, and can bemolded into wires and ribbons whenmoist. They generally have high water-holding capacity and low permeability.

Almost any texture of soil can be suit-able for gardening, as long as you areaware of the soil’s limitations and adjustyour management to compensate. Claysoils hold a lot of water, but are hard todig and dry slowly in the spring. Sandysoils need more frequent watering andlighter, more frequent fertilization, but

you can plant them earlier in the spring.All soils can benefit from additions oforganic matter, as described below under“Adding organic matter.”

Many soils contain coarse fragments,i.e., gravel and rocks. Coarse fragments donot contribute to a soil’s productivity andcan be a nuisance when you are digging.Don’t feel compelled to remove them allfrom your garden, however. Coarse frag-ments aren’t harmful, and your time isbetter spent doing other gardening tasks.The only time rocks are a problem is whenyou have nothing but rocks on your land.Then, water- and nutrient-holding capaci-ties are so low that it is difficult to growhealthy plants.

Figure 2.—Percentages of clay, silt, and sand in the basic soil textural classes.

10

20

30

40

50

60

70

80

90

100

100

90

80

70

60

50

40

30

20

10

100 90 80 70 60 50 40 30 20 10

Perc

ent c

lay

Percent sand

Percent silt

Clay

Siltyclay

Silty clayloam

Silt loamLoam

Clay loam

Sandy clay loam

Sandy loam

Loamy sandSand

Silt

Sandyclay


Soil structureIndividual particles of sand, silt, and

clay tend to cluster and bind together,forming aggregates called peds, whichprovide structure to a soil. Dig up a pieceof grass sod and examine the soil aroundthe roots. The granules of soil clinging tothe roots are examples of peds. Theycontain sand, silt, clay, and organic mat-ter.

Aggregation is a natural process causedlargely by biological activity such asearthworm burrowing, root growth, andmicrobial action. Soil organic matter is animportant binding agent that stabilizesand strengthens peds.

The spaces between peds are a soil’smacropores, which improve permeability,drainage, and recharge of air into the soilprofile. The pores within peds are pre-dominantly micropores, contributing tothe soil’s water-holding capacity. A well-structured soil is like a sponge, allowingwater to enter and soak into the micro-pores and letting excess water draindownward through the macropores. Goodstructure is especially important inmedium- to fine-textured soils, because itincreases the soil’s macroporosity, thusimproving permeability and drainage.

Compaction and loss of structureSoil structure is fragile and can be

damaged or destroyed by compaction,excessive tillage, or tillage when the soil istoo wet. Loss of organic matter alsoweakens structure.

Compaction squeezes macropores intomicropores and creates horizontal aggre-gates that resist root penetration andwater movement (Figure 3). Compactionoften occurs during site preparation orhouse construction, creating a difficultenvironment for establishing plants.Protect your soil from compaction byavoiding unnecessary foot or machinetraffic.

Tilling when soil is too wet also dam-ages soil structure. If you can mold apiece of soil into a wire or worm in your

hand, it is too wet to till. If the soilcrumbles when you try to mold it, it is dryenough to till.

Structural damage caused by humanactivity usually is most severe within thetop foot of soil and can be overcome byproper soil management. In some soils,there is deeper compaction resulting frompressure from ancient glaciers. Glaciallycompacted subsoils (a type of hardpan)are common in the Puget Sound area,where the compacted layer often begins18 to 36 inches below the soil surface.Where the land surface has been cut,leveled, or shaped for development, thecompacted layer may be much closer tothe surface. This layer looks like concreteand is so dense and thick that it is nearlyimpossible to work with. If your gardenhas a glacially compacted layer close to

Figure 3.—Compacted soil resists root penetra-tion and water movement.

Compaction layer

Tiretrack


the soil surface, consider using raisedbeds to increase soil depth.

Organic matterAdding organic matter is the best way

to improve the environment for plants innearly all soils. Organic matter helps buildand stabilize soil structure in fine-texturedand compacted soils, thus improvingpermeability and aeration and reducingthe risk of runoff and erosion. Whenorganic matter decomposes, it formshumus, which acts as a natural glue tobind and strengthen soil aggregates.Organic matter also helps sandy soils holdwater and nutrients. See “Adding organicmatter” later in this chapter for informa-tion on amending soil with organic matter.

Slope, aspect, depth, and waterSlope, aspect (direction of exposure),

and soil depth affect water availability anduse in a soil. Choose plants that are bestsuited to conditions on your property.

Ridgetops and side slopes tend to shedwater, while soils at the bottoms of slopesand in low areas collect water (Figure 4).Often, soils that collect water have highwinter water tables, which can affect thehealth of some plants. Soils on ridgetopsare more likely to be droughty.

Site aspect also is important. South- andsouthwest-facing exposures collect themost heat and use the most water.

Soil depth also affects water availabilityby determining the rooting zone. Soildepth is limited by compacted, cemented,or gravelly layers, or by bedrock. A shal-low soil has less available water simplybecause the soil volume available to rootsis smaller. Dig below the topsoil in yourgarden. The deeper you can dig beforehitting a restrictive layer, the greater thesoil volume for holding water.

Water management in your gardenSoils and irrigation

Most gardens in the Northwest requiresummer irrigation. The need for irrigationvaries, depending on soil water-holdingcapacity, weather, site aspect, the plantsgrown, and their growth stage.

In most cases, the goal of irrigation is torecharge the available water in the topfoot or so of soil. For sandy soil, 1 inch ofirrigation water is all you need. Any morewill leach (move downward) through theroot zone, carrying nutrients with it. A siltloam or clay soil can hold more than2 inches of water, but you may need toirrigate more slowly to prevent runoff.

Wet soilsIf your soil stays wet in the spring, you

will have to delay tilling and planting.Working wet soil can damage its structure,and seeds are less likely to germinate incold, wet soil.

Some plants don’t grow well in wet soil.Raspberries, for example, often becomeinfected by root diseases in wet soil andlose vigor and productivity.

A soil’s color gives clues to its tendencyto stay wet. If a subsoil is brown or red-dish, the soil probably is well drained andhas few wetness problems. Gray subsoils,especially those with brightly coloredmottles, often are wet. If your soil is grayand mottled directly beneath the topsoil,it probably is saturated during the wetseason.

Ridgetop

Slope

Bottomland

Figure 4.—Ridgetops and slopes tend to shedwater, while soils at the bottoms of slopes and inlow areas collect water.


Sometimes, simple actions can reducesoil wetness problems. For example:• Divert runoff from roof drains away

from your garden.• Avoid plants that perform poorly in

wet conditions.• Use raised beds for perennials that

require well-drained soil and for early-season vegetables.

• Investigate whether a drain on a slopewill remove excess water in yoursituation. Installing drainage can beexpensive, however. When consideringdrainage, make sure there is a place todrain the water. Check with localregulatory agencies to see whetherthere are restrictions on the project.

Soil organismsSoil abounds with life. Besides the plant

roots, earthworms, insects, and othercreatures you can see, soil is home to anabundant and diverse population ofmicroorganisms. A single gram of topsoil

(about 1⁄4 teaspoon) can contain as manyas a billion microorganisms (Table 1).Microorganisms are most abundant in therhizosphere—the thin layer of soil sur-rounding plant roots.

The main function of soil organisms isto break down the remains of plants andother organisms. This process releasesenergy, nutrients, and carbon dioxide, andcreates soil organic matter.

Organisms ranging from tiny bacteria toinsects and earthworms take part in acomplex soil food web (Figure 5). Mam-mals such as moles and voles also are

Table 1.—Approximate abundance of micro-organisms in agricultural topsoil.

Number per gramOrganism (dry weight basis)Bacteria 100 million to 1 billionActinomycetes 10 million to 100 millionFungi 100,000 to 1 millionAlgae 10,000 to 100,000Protozoa 10,000 to 100,000Nematodes 10 to 100

MesopredatorsMacropredators

HerbivoresFungivoresMicrobial feedersDetrivores

Micropredators

Dead plants Bacteria Fungi Plant roots

Figure 5.—The soil food web.


part of the food web, feeding on insectsand earthworms.

Some soil organisms play other benefi-cial roles. Mycorrhizae are fungi that infectplant roots and increase their ability totake up nutrients from the soil. Rhizobiabacteria are responsible for convertingatmospheric nitrogen to plant-availableforms, a process known as nitrogen fixa-tion. Earthworms mix large volumes of soiland create macropore channels thatimprove permeability and aeration(Figure 6).

Not all soil organisms are beneficial.Some are pathogens, which cause diseasessuch as root rot of raspberries and scabon potatoes. Moles can damage crops andlawns, and slugs are a serious pest inmany Northwest gardens.

The activity of soil organisms dependson soil moisture and temperature as wellas on the soil’s organic matter content.Microorganisms are most active between70° and 100°F, while earthworms are mostactive and abundant at about 50°F. Mostorganisms prefer moist soil. Becauseorganic matter is at the base of the soilfood web, soils with more organic mattertend to have more organisms.

The relationships between gardeningpractices, microbial populations, and soilquality are complex and often poorlyunderstood. Almost all gardening activi-ties—including tillage; the use of fertiliz-ers, manures, and pesticides; and thechoice of crop rotations—affect thepopulation and diversity of soil organ-isms. For example, amending soils withorganic matter, returning crop residues tothe soil, and rotating plantings tend toincrease the number and diversity ofbeneficial organisms.

Soil nutrientsSoil supplies 13 essential plant nutri-

ents. Each nutrient plays one or morespecific roles in plants. Nitrogen, forexample, is a component of chlorophyll,

amino acids,proteins, DNA, andmany plant hor-mones. It plays avital role in nearlyall aspects of plantgrowth and devel-opment, andplants need a largeamount of nitro-gen to grow well.In contrast, plantsneed a only tinyamount of molyb-denum, which isinvolved in thefunctioning of only a few plant enzymes.Molybdenum nonetheless is essential, andplant growth is disrupted if it is deficient.Plants also require carbon, hydrogen, andoxygen, which they derive from water andair.

A soil nutrient is classified as a primarynutrient, secondary nutrient, or micronutri-ent, based on the amount needed byplants (Table 2). If a soil’s nutrient supplyis deficient, fertilizers can provide theadditional nutrients needed for healthyplant growth.

Figure 6.—Earthworm channelscreate macropores, which improve asoil’s permeability and aeration.

✿See Chapter 1,Botany.

Table 2.—Essential plant nutrients.Name Chemical symbolPrimary nutrientsNitrogen NPhosphorus PPotassium KSecondary nutrientsSulfur SCalcium CaMagnesium MgMicronutrientsZinc ZnIron FeCopper CuManganese MnBoron BMolybdenum MoChlorine Cl


Nutrient deficienciesThe most common nutrient deficiencies

are for the primary nutrients—N, P, andK—which are in largest demand by plants.Nearly all soils lack enough available N forideal plant growth.

Secondary nutrients also are deficientin some soils in the Pacific Northwest.Sulfur deficiencies are common west ofthe Cascades. Calcium and magnesiummay be deficient in acid soils, also typi-cally west of the Cascades.

Except for boron and zinc, micronutri-ents rarely are deficient in the Northwest.Boron deficiencies occur most often westof the Cascades, particularly in rootcrops, brassica crops (e.g., broccoli), andcaneberries (e.g., raspberries). Zincdeficiency usually is associated with highpH soils and most often affects tree fruits.

Each nutrient deficiency causes charac-teristic symptoms. In addition, affectedplants grow more slowly, yield less, andare less healthy than plants with adequatelevels of nutrients.

Excess nutrientsExcess nutrients can be a problem for

plants and the environment. Excessesusually result because too much of anutrient is applied or because a nutrient isapplied at the wrong time.

Too much boron is toxic to plants. Toomuch nitrogen can lead to excessivefoliage production, increasing the risk ofdisease; wind damage; and delayed flow-ering, fruiting, and dormancy. Availablenitrogen left in the soil at the end of thegrowing season can leach into ground-water and threaten drinking water quality.

The key to applying fertilizers is to meetplant needs without creating excessesthat can harm plants or the environment.

Nutrient availability to plantsPlants can take up only nutrients that

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

they are tied up in soil mineral andorganic matter in insoluble forms. Thesenutrients become available to plants onlyafter they are converted to soluble formsand dissolve into the soil solution.

This process occurs through weather-ing of mineral matter and biologicaldecomposition of organic matter. Weath-ering of mineral matter is a very slowprocess that releases small amounts ofnutrients each year. The rate of nutrientrelease from soil organic matter is some-what faster and depends on the amount ofbiological activity in the soil.

Nutrient release from soil organicmatter is fastest in warm, moist soil andnearly nonexistent in cold or dry soil.Thus, the seasonal pattern of nutrientrelease is similar to the pattern of nutrientuptake by plants. About 1 to 4 percent ofthe nutrients in soil organic matter arereleased in soluble form each year.

Soluble, available nutrients are in ionicform. An ion has either positive or nega-tive charges. Positively charged ions arecations, and negatively charged ions areanions. Potassium, calcium, and magne-sium are examples of cations. Chlorine isan example of an anion.

Clay particles and soil organic matterhave negative charges on their surfacesand can attract cations. They hold nutri-ent cations in a ready reserve form thatcan be released rapidly into soil solutionto replace nutrients taken up by plantroots. This reserve supply of nutrientscontributes to a soil’s fertility. A soil’scapacity to hold cations is called its cationexchange capacity or CEC.

The nitrogen cycleManaging nitrogen is a key part of

growing a productive and environmentallyfriendly garden. Nitrogen is the nutrientneeded in the largest amount by plants,but excess nitrogen can harm plants anddegrade water quality. Understanding howthe nitrogen cycle affects nitrogen avail-ability can help you become a betternutrient manager (Figure 7).

✿See Chapter 1,Botany.

✿See Chapter 6,Water Quality.


Organic additions(manure, compost, etc.)

Soilorganicnitrogen

Crop residuesN-fixation(legumes)

NH4+

(ammonium)

NO2-

(nitrite)

NO3-

(nitrate)

Leaching

Commercialfertilizer

Plants and soilmicroorganisms

Harvest removal

Gaseousnitrogen loss

Nitrification(microbes)

Figure 7.—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(NH4

+) by soil microbes. (c) Commercial fertilizer supplies N as ammonium or nitrate (NO3-).

(d) Microbes nitrify ammonium to nitrite and then nitrate. (e) Plants, microorganisms, leachingbelow the root zone, and release of gaseous N to the atmosphere remove N from the root zone soilsolution. (f) Crop harvest removes N stored in plants. (g) Nitrogen present in both crop residuesand soil microorganisms becomes a part of the soil organic N content. (Note: An alternate sche-matic of the nitrogen cycle is illustrated in Chapter 6, Water Quality.)

Mineralization(microbes)

(a) (a)

(a)

(b)

(c)

(c)

(g)

(e)

(e)

(e)(e)

(f)

(d)


Nitrogen is found in four different formsin the soil (Table 3). Only two of them—ammonium and nitrate—can be useddirectly by plants.

Most nitrogen in soil is tied up inorganic matter in forms such as humusand proteins. This organic nitrogen is notavailable to plants. As soil warms in thespring, soil microbes begin breaking downorganic matter, releasing some of thenitrogen as ammonium (NH4

+). Ammoniumis a soluble ion that is available to plantsand soil microbes. When the soil is warm,a group of microbes called nitrifiers con-vert the ammonium to nitrate (NO3

-).Nitrate also is soluble and available toplants. The ammonium and nitrate ionsreleased from soil organic matter are thesame as the ammonium and nitrate con-tained in processed fertilizers.

Because nitrate has a negative charge, itis not held to the surface of clay ororganic matter, so it can be lost readily byleaching. Nitrate remaining in the soil atthe end of the growing season will leachduring the fall and winter and may reachgroundwater, where it becomes a contami-nant. In soils that are saturated during thewet season, soil microbes convert nitrate

to nitrogen gases,which diffuse backinto the atmosphere.

Ammonium andnitrate taken up byplants are convertedback to organicforms in planttissue. Whenplant residuesare returned tothe soil, they decompose, slowly releasingnitrogen back into available forms.

The nitrogen cycle is a leaky one, withlosses to leaching and to the atmosphere.Harvesting crops also removes nitrogen.To maintain an adequate nitrogen supply,nitrogen must be added back into thesystem through fixation or fertilization.

Nitrogen fixation is a natural processinvolving certain plants and Rhizobiabacteria. The Rhizobia form nodules inthe plant roots, and through these nod-ules they are able to take atmosphericnitrogen (N2 gas) from the soil air andconvert it to available N within the plant.Legumes such as peas, beans, alfalfa,clover, and Scotch broom are commonnitrogen fixers. Alder trees also fix nitro-gen. Growing legumes as a cover crop willsupply nitrogen to future crops.

Understanding fertilizersFertilizers supplement a soil’s native

nutrient supply. They are essential togood plant growth when the soil nutrientsupply is inadequate. Rapidly growingplants such as annual vegetable cropsgenerally need more nutrients than slowlygrowing plants such as established peren-nials.

You can use processed fertilizers,organic fertilizers, or a combination of thetwo to supply soil nutrients.

Table 3.—Common forms of nitrogen in soil.Form of nitrogen CharacteristicsOrganic N Primary form of N in soil. Found in

proteins, lignin, amino acids,humus, etc. Not available toplants. Mineralized to ammoniumby soil microorganisms.

Ammonium N (NH4+) Inorganic, soluble form. Available

to plants. Converted to nitrate bysoil microorganisms.

Nitrate (NO3-) Inorganic, soluble form. Available

to plants. Can be lost by leaching.Converted to gases in wet soils.

Atmospheric N (N2) Makes up about 80 percent of thesoil atmosphere. Source of N forN-fixing plants. Not available toother plants.

✿See Chapter 6,Water Quality.


Comparing organicand processed fertilizers

Processed fertilizers are manufacturedor are refined from natural ingredients tomake them more concentrated and moreavailable to plants. Typically, they areprocessed into soluble, ionic forms thatare immediately available to plants.

Organic fertilizers are natural materialsthat have undergone little or no process-ing. They include both biological (plantand animal) and mineral materials(Table 4). Once in the soil, organic fertiliz-ers release nutrients through naturalprocesses, including chemical weatheringof mineral materials and biological break-down of organic matter. The releasednutrients are available to plants in water-soluble forms. These soluble forms ofnutrients are the same as those suppliedby processed fertilizers.

When compared with processed fertiliz-ers, organic fertilizers usually have alower concentration of nutrients andrelease nutrients more slowly. Thus, largeramounts of organic fertilizers are needed,but their effects last longer.

Using organic fertilizers recycles materi-als that otherwise would be discarded aswastes. Production of processed fertiliz-ers, on the other hand, can create wastesand use substantial amounts of energy.

Choosing organic fertilizers involvestrade-offs in cost or convenience.

Farmyard manure usually is inexpensiveor free, but can be inconvenient to apply.Packaged organic blends, on the otherhand, are convenient but often expensive.

Nutrient releaseNutrients in most processed fertilizers

are available immediately. Processedfertilizers can furnish nutrients to plantsin the spring before the soil is warm.However, nitrogen in these fertilizers isvulnerable to leaching loss from heavyrainfall or irrigation. Once nitrogen movesbelow the root zone, plants no longer canuse it, and it may leach into groundwater.

Organic fertilizers are slow-releasefertilizers because their nutrients becomeavailable to plants over the course of thegrowing season. The rate of nutrientrelease from organic materials depends onthe activity of soil microorganisms, just asit does for nutrient release from soilorganic matter. Temperature and moistureconditions that favor plant growth alsofavor the release of nutrients from organicmatter.

Some organic fertilizers contain immedi-ately available nutrients as well as slow-release nutrients. These fertilizers cansupply nutrients to plants both early inthe season and later. Fresh manure,biosolids, and fish emulsion are examplesof organic fertilizers containing availablenutrients. As manure ages, the mostreadily available nutrients are lost into the

Table 4.—Comparing organic and processed fertilizers.Organic fertilizers Processed fertilizers

Source Natural materials; little or no Manufactured or extracted from naturalprocessing materials; often undergo extensive

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

phosphate, fish by-products, potassium chlorideground limestone

Nutrient Usually slow-release; nutrients are Nutrients usually are immediatelyavailability released by biological and chem- available to plants

ical processes in soilNutrient Usually low Usually highcontent


air or leached into the soil, leaving slow-release material in the aged manure.

Some material in organic fertilizersbreaks down so slowly that it is notavailable the first season after application.Repeated application of organic fertilizersbuilds up a pool of material that releasesnutrients very slowly. In the long run, thisnutrient supply decreases the need forsupplemental fertilizer.

Fertilizer labelsThe labels on fertilizer packages tell the

amount of each of the three primarynutrients in the fertilizer, expressed as apercent of total fertilizer weight. Nitrogen(N) always is listed first, phosphorus (P)second, and potassium (K) third.

Historically, the amount of phosphorusin fertilizer has been expressed not as P,but as units of P2O5 (phosphate). Similarly,fertilizer potassium is expressed as K2O(potash). This practice still is used forfertilizer labels and recommendations,even though there is no practical reasonfor the system except that people areaccustomed to it. (If you need to convertfrom P to P2O5, the conversion is1 lb P = 2.3 lb P2O5. For potassium, theconversion is 1 lb K = 1.2 lb K2O.)

Thus, a bag of fertilizer labeled 5-10-10contains 5 percent nitrogen, 10 percentphosphorus expressed as P2O5, and10 percent potassium expressed as K2O.This information is called a fertilizeranalysis.

The analysis for processed fertilizersguarantees the amount of available

nutrients in the fertilizer. The analysis fororganic fertilizers represents the totalamount of nutrients rather than availablenutrients. Because nutrients in mostorganic fertilizers are released slowly, theamount of immediately available nutrientsis less than the total.

Common processed fertilizersNitrogen

The raw material for processed nitrogenfertilizer is nitrogen gas from the atmo-sphere. The manufacturing process is thechemical equivalent of biological nitrogenfixation and requires a substantial amountof fossil fuel energy. Examples of pro-cessed nitrogen fertilizers available forhome garden use include those listed inTable 5.

Phosphorus and potassiumProcessed phosphorus fertilizers come

from phosphate rock. The rock is treatedwith acid to release phosphorus intoplant-available forms.

The most common raw material forpotassium fertilizers is sylvinite, a mixtureof sodium chloride and potassium chlo-ride salts. The potassium in sylvinitealready is in soluble form, but thesylvinite is treated to remove the sodiumsalts to make it suitable for use as afertilizer. Some other potassium fertilizersare potassium sulfate salts, which supplysulfur as well as potassium.

Table 6 lists examples of processedphosphorus and potassium fertilizers.

Table 5.—Examples of processed nitrogen fertilizer materials.Material Analysis CommentsUrea 46-0-0 Rapidly converted to ammonium in soil.Ammonium sulfate 21-0-0 Also contains 24 percent available sulfur. Used with

acid-loving plants.Diammonium phosphate 18-46-0 Used in mixed fertilizers as a source of nitrogen and

phosphorus.Sulfur-coated urea (SCU) 35-0-0 Sulfur coating slows release of available N, making

this a slow-release fertilizer.


Mixed fertilizersMixed fertilizers contain all three pri-

mary nutrients. The ratios can vary.Fertilizers for annual gardens typicallyhave N:P2O5:K2O ratios in the range of1:1:1 or 1:2:2. Examples include 8-8-8 and10-20-20 blends. Fertilizer blends forstarting plants usually have a higherproportion of phosphorus. Lawn fertiliz-ers are higher in nitrogen; an example is a12-4-8 blend.

Common organic fertilizersAnimal manure

Farmyard manure can be an inexpen-sive source of nutrients. If you or yourneighbors have livestock, it makes envi-ronmental and economic sense to recyclethe manure as fertilizer. Packaged manureproducts cost more than manure off thefarm, but they usually are more uniformand convenient to handle.

Animal manures vary widely in nutrientcontent and nutrient availability, depend-ing on the type of animal that producedthe manure and the age and handling ofthe manure. For example:• Fresh manure has higher levels of

nutrients than aged manure.• Manure diluted with large amounts of

bedding has fewer nutrients thanundiluted manure.

• Exposure to rain leaches nutrients.• Composting under cover retains more

nutrients, but reduces nutrient avail-ability.

Table 7 compares averagenutrient contents of typicalmanure products.

It doesn’t take much freshmanure to fertilize a garden. One5-gallon bucket of fresh cowmanure is enough for about50 square feet of garden. Thesame amount of fresh poultrymanure covers 100 to 150 squarefeet. Larger amounts can harmcrops and leach nitrogen intogroundwater.

Fresh manure can carrydisease-causing pathogens.Be sure to read the sidebaron manure safety beforeusing fresh manure.

It takes larger amounts ofaged, diluted, or leachedmanure to provide the same

Table 6.—Examples of processed phosphorus and potassium fertilizer materials.Material Analysis CommentsTriple superphosphate 0-46-0 —Monoammonium phosphate 11-52-0 Used in mixed fertilizers as a source of nitrogen and

phosphorus.Diammonium phosphate 18-46-0 Used in mixed fertilizers as a source of nitrogen and

phosphorus.Potassium chloride 0-0-60 —Potassium magnesium sulfate 0-0-22 Also contains 11 percent magnesium and 18 percent sulfur.Potassium sulfate 0-0-50 Also contains 18 percent sulfur.

Table 7.—Average nitrogen, phosphate, and potash content ofcommon manures available to gardeners.Source Nitrogen P2O5 K2O

(%) (%) (%)Composted chicken manure* 3.0 2.0 2.0Separated dairy solids** 1.5 1.0 0.6Dry stack cattle manure** 1.3 2.0 2.0Horse manure with bedding 0.5 0.2 0.6*Chicken manure compost data are from labels on commercialproducts sold in the Northwest.**Data on dry stack manure and separated dairy solids comefrom Washington and Oregon.Remember that manure nutrient content can vary depending on thesource and the age and handling of the manure.


amount of nutrients as fresh manure.Increase the amount of manure appliedbased on how much it is aged, diluted,and/or leached. Composted manure solidsfrom dairy farms also have low nutrientavailability, so you can apply them athigher rates than fresh manure. Use thesemanures as much for the organic matterthey supply as for nutrients. You can addas much as 1 to 2 inches of a manure withvery low nutrient availability.

To fine-tune your application rate,experiment with the amount you applyand observe your crops’ performance. It’sbetter to be conservative and add morenutrients later if crops seem deficientthan to risk over-fertilizing.

To use manure, simply spread it overthe soil and turn it in if you wish.

Timing manure applicationsThe best time to apply manure is in the

spring before planting. You also can applymanure in the fall, but some of the nutri-ents may be lost during the winter, espe-cially west of the Cascades. Environmen-tal risks of leaching and runoff alsoincrease in winter. If you do apply manurein the fall, apply it early and plant a covercrop to help capture nutrients and pre-vent runoff. See “Green manure” later inthis chapter for more information.

BiosolidsBiosolids are a by-product of waste-

water treatment. Most of the biosolidsproduced in Washington and Oregon areused to fertilize agricultural and forest

Using manure safelyFresh manure sometimes contains

disease-causing pathogens that cancontaminate garden produce. Salmo-nella bacteria are among the mostserious pathogens found in animalmanure. Pathogenic strains of E. colibacteria also can be present in cattlemanure. Manure from swine, dogs,cats, and other carnivores can con-tain helminths, which are parasiticworms.

These pathogens are not taken upinto plant tissue, but they can adhereto soil on plant roots or to the leavesor fruit of low-growing crops. Therisk is greatest for root crops(e.g., carrots and radishes) or leafcrops (e.g., lettuce), where the ediblepart touches the soil. The risk isnegligible for crops such as sweetcorn, which do not contact the soil,or for any crop that is thoroughlycooked. Avoid using fresh manurewhere you grow high-risk crops.

Cooking destroys pathogens, butraw food carries a risk. Washing andpeeling raw produce removes mostpathogens, but some may remain.

Composting manure at high tem-peratures kills pathogens, but it isvery hard to maintain rigorouscomposting conditions in a backyardpile. Commercial manure compostsare composted under controlledconditions to destroy pathogens.

Bacterial pathogens die naturallyover a period of weeks or months, sowell-aged manure should not containthem. Helminths in dog, cat, or pigmanure can persist foryears, however, so donot add these manuresto your garden orcompost pile. ✿See Chapter 5,

Composting.


crops. Biosolids also are available togardeners from some wastewater treat-ment plants.

A common form of biosolids is aspongy, black substance called “cake.”Biosolids cake is about 20 to 25 percentdry matter and 75 to 80 percent water. Ittypically contains about 3 to 6 percentnitrogen and 2 to 3 percent phosphoruson a dry weight basis, as well as smallamounts of potassium and trace elements.Some of the nitrogen in biosolids is imme-diately available to plants. The rest isreleased slowly. A 5-gallon bucket ofbiosolids cake is enough to fertilize50 square feet of garden. Apply biosolidsas you would apply manure.

Biosolids also are an ingredient in somecommercial composts. Like other com-posts, biosolids compost releases nutri-ents very slowly. It is a good source oforganic matter and provides smallamounts of nutrients to plants.

Use only biosolids that are free ofdisease-causing pathogens (Class Abiosolids). Examples include biosolidscompost and some heat-treated biosolids.Class B biosolids have not been treated tothe same extent and may contain patho-gens, including some that are long-lived inthe soil. Check with the wastewatertreatment plant to find out whether theirbiosolids meet Class A requirements andare available for home use.

Biosolids contain small amounts oftrace elements. Some trace elements aremicronutrients, which can be beneficial tocrops. However, large amounts can betoxic to crops, animals, or humans. Whenyou apply biosolids at proper rates toprovide nutrients, the risk of applyingharmful amounts of trace elements isnegligible.

Because biosolids come from the waste-water treatment process, they containsynthetic materials that were present inthe wastewater or added during treat-ment. Biosolids are not certified asorganic fertilizers.

Biosolids do have two important char-acteristics common to organic fertilizers:• Their nutrients are released slowly

from the organic form by naturalprocesses in the soil.

• They are a product of the wastestream that can benefit crop growth.

Commercial organic fertilizersMany organic by-products and some

unprocessed minerals are sold as organicfertilizers. Table 8 shows approximatenutrient contents of some of these materi-als. The numbers represent total nutrientcontent; because most are slow-releasefertilizers, not all of the nutrients areavailable the year they are applied.

Table 8 shows that most organic fertil-izer materials contain one main nutrient.The other nutrients are present in smalleramounts. Thus, although organic fertiliz-ers contain a variety of nutrients, theymay not be present in the proportionsneeded by plants. Several companiesproduce balanced organic fertilizers byblending these materials into a singleproduct that provides all of the primarynutrients in balanced proportions.

Commercial organic fertilizers tend tobe more expensive per pound of nutrients

Table 8.—Total nitrogen, phosphate, and potash content ofsome organic fertilizers.Material Nitrogen P2O5 K2O

(%) (%) (%)Cottonseed meal 6–7 2 1Blood meal* 12–15 1 1Alfalfa 2 0.5 2Bat guano* 10 3 1Fish meal* 10 4 0Fish emulsion* 3–5 1 1Bone meal 1–4 12–24 0Rock phosphate** 0 25–30 0Greensand 0 0 3–7Kelp meal 1 0.1 2–5*Contains a substantial amount of quickly available nitrogen thatplants can use early in the season.**Very low P availability (only 2–3 percent). Useful only in acidsoils.


than either processed fertilizers ormanures. Sometimes the difference inprice is substantial. Nevertheless, manygardeners use these products because ofconvenience or quick availability ofnutrients. They are most economical forsmall gardens where little fertilizer isneeded.

The cost-per-pound of nutrients inorganic fertilizers varies widely, depend-ing on the type of material, the concentra-tion of nutrients, and the package size.Compare costs and nutrient availabilitywhen shopping for organic fertilizers.

How much fertilizer to useThe goal of applying fertilizer is to

supply enough nutrients to meet plantneeds without accumulating excessnutrients in the soil that could leach intogroundwater or run off into surface water.Soil tests and Extension publications aretwo standard methods for estimatingfertilizer needs.

Soil testsA soil test gives information on the

levels of nutrients in your soil and recom-mends how much fertilizer to add eachyear based on the test results and thecrops you grow. You don’t need to testyour soil every year; every 3 to 5 years isoften enough.

A garden soil test typically includes thefollowing nutrients: phosphorus, potas-sium, calcium, magnesium, and boron.The test also includes soil pH and recom-mends lime if needed to raise pH. In aridareas, a test for soluble salts can beworthwhile.

Soil test labs don’t routinely testfor nitrogen, because there is no

simple way to predictnitrogen availability.The lab will give ageneral nitrogenrecommendation,

however, based on the plants you aregrowing and on information you provideabout the soil (such as whether there is ahistory of manure applications, whichwould increase soil-available nitrogen).

To take a soil sample, first collectsubsamples from at least 10 differentspots in your garden. Avoid any unusualareas, such as the site of an old trashdump, burn pile, or rabbit hutch. Samplethe top foot of soil (0- to 12-inch depth).Air-dry the samples and mix themtogether well. Send about a pint of themixed sample to the lab.

Because management and fertilizerrecommendations vary for different crops,such as vegetables, lawns, and berries,send separate samples for each area.

Washington State University andOregon State University do not test soils,but private labs in both states do. WSUCooperative Extension and OSU ExtensionService county offices have lists of testinglabs. Before choosing a lab, call to makesure they test and make recommendationsfor garden soils. Ask the lab:• Do you routinely test garden soils for

plant nutrients and pH?• Do you use WSU or OSU test methods

and fertilizer guides?• Do you give recommendations for

garden fertilizer applications?• Are there forms to fill out? What

information do you need?• How much does a test cost?• How quickly will you send results?

Extension publicationsIf the cost of a soil test is large com-

pared with your normal gardening costs,you can estimate fertilizer needs usingExtension publications. These publica-tions usually give recommendations forprocessed fertilizers, but some giveguidelines for organic fertilizers as well.OSU and WSU have published fertilizerrecommendations for a variety of crops.


See “For more information” at the end ofthis chapter. Also see other chapters inthis handbook for information on specificcrops.

Typical Extension recommendations arefor 2 lb of nitrogen per 1,000 square feet ofgarden, usually applied in a mixed fertil-izer with a 1:1:1 ratio. Gardens with ahistory of fertilizer application may needless phosphorus and potassium thanthese rates supply. Fast-growing cropssuch as sweet corn need more nitrogen.

Calculating fertilizer amountsFertilizer recommendations usually are

given in pounds of nutrient (such asnitrogen) per unit area (typically 100 or1,000 square feet for gardens). You willneed to convert the recommendation frompounds of nutrient to pounds of fertilizer.

Example: You are following a fertilizerrecommendation in an Extension publica-tion that calls for adding 2 lb of N per1,000 square feet of garden, using a fertil-izer with a 1:1:1 ratio of nitrogen, phos-phorus, and potassium. Follow thesesteps to find out how much fertilizer touse:1. Choose a fertilizer with an appropriate

analysis. For example, you can choosean 8-8-8 fertilizer, but not a 21-4-4.

2. Calculate how much 8-8-8 is needed for1,000 square feet. Divide the amount ofnitrogen recommended for 1,000square feet (2 lb) by the fraction ofnitrogen in the fertilizer (8% or 0.08):

2 lb ÷ 0.08 = 25 lb per 1,000 square feet

3. Calculate the area of your garden. If it isa rectangle, the area is length timeswidth. For example, a garden 25 feetlong by 20 feet wide has an area of:

25 ft x 20 ft = 500 square feet

If your garden is an odd shape, divideit into rectangles, calculate the area ofeach rectangle, and then add themtogether.

4. Calculate the amount offertilizer needed for yourgarden. Divide the area ofyour garden (500 squarefeet) by the area inthe fertilizer recom-mendation (1,000square feet). Thenmultiply by the fertilizer amountcalculated in step 2 above:

(500 ÷ 1,000) x 25 = 12.5 lb of 8-8-8 fertilizer

This is the amount of fertilizer neededfor your garden.

Estimating organic fertilizer ratesEstimating how much organic fertilizer

to use can be a challenge because youmust estimate the availability of nutrientsin the fertilizer. Here are some tips:• Organic fertilizers with large propor-

tions of available nutrients (such asbat guano and fish emulsion) can besubstituted for processed fertilizerson a one-to-one basis. Use the samequantity as called for in the processedfertilizer recommendation.

• Apply other packaged fertilizersaccording to their nutrient availability.Composts, rock phosphate, and plantresidues generally have lower nutrientavailability than more concentratedanimal products (e.g., blood meal,bone meal, and chicken manure). Therecommended application rates onpackaged organic fertilizers are a goodguideline. Check these recommenda-tions against other products to makesure they seem reasonable.

• Nutrient concentration and availabilityin farmyard manures vary widely,depending on the type of manure andits age and handling. Application ratesrange from 5 gallons per 100 to150 square feet for high-nitrogenchicken manure to 1 to 2 inches deepfor steer or horse manure compostedwith bedding. Estimate applicationrates based on the type of manure.


• Observe your crops carefully. Lushplant growth and delayed floweringand fruiting are signs of high amountsof available nitrogen and may indicateover-fertilization.

• Experiment with different fertilizerrates in different parts of a row, andsee whether you notice differences incrop performance. Plan your experi-ment carefully so you are confidentthat differences are the result ofdifferent fertilizer rates, rather thandifferences in soil, water, sunlight, ormanagement practices.

• Soil testing is valuable in understand-ing your soil’s nutrient status. Manyestablished gardens have high levelsof soil fertility, and crops grow wellwith little fertilizer.

When to fertilizeIn most cases, the best time to apply

fertilizer is close to the time when plantsneed the nutrients. This timing reducesthe potential for nutrients to be lostbefore they are taken up by plants. Nutri-ent loss not only is inefficient, but maycontaminate groundwater or surfacewater.

Plants need the largest amount ofnutrients when they are growing mostrapidly. Rapid growth occurs in midsum-mer for corn and squash, earlier forspring plantings of lettuce and othergreens. Plants also need available nutri-ents (especially phosphorus) shortly afterseeding or transplanting.

For a long-season crop such as corn,many gardeners apply a small amount offertilizer as a starter at the time of seedingand then add a larger amount in earlysummer, just before the period of rapidgrowth. When using organic fertilizers, asingle application usually is adequate,because nutrient release usually is fastestjust before plant demand is greatest.

For perennial plants, timing depends onthe plant’s growth cycle. Blueberries, forexample, benefit most from fertilizerapplied early in the season at bud break,while June-bearing strawberries arefertilized after harvest. Refer to otherchapters in this handbook and to Exten-sion publications for information ontiming fertilizer applications for specificcrops.

Adding organic matterOrganic matter builds and stabilizes soil

structure, thus reducing erosion andimproving soil porosity, infiltration, anddrainage. It holds water and nutrients forplants and soil organisms. It also is a long-term, slow-release storehouse of nitrogen,phosphorus, and sulfur, which continu-ously become available as soil micro-organisms break down organic matter.

The value of organic materials varies,depending on their nitrogen content(more specifically, their carbon to nitro-gen, or C:N, ratio). Organic materials witha low C:N ratio, such as undiluted manureor blood meal, are rich in nitrogen. Theyare a good source of nutrients, but mustbe used sparingly to avoid over-fertiliza-tion.

Materials with an intermediate C:N ratio(including many composts, leaf mulches,and cover crop residues) have lowernutrient availability. They are the bestmaterials to replenish soil organic matter.Because they are relatively low in avail-able nutrients, you can add them to thesoil in large amounts.

Materials with a high C:N ratio (such asstraw, bark, and sawdust) contain so littlenitrogen that they reduce levels of avail-able nitrogen when mixed into the soil.Soil microorganisms use available nitro-gen when they break down these materi-als, leaving little nitrogen for plants. Thisprocess is called immobilization and✿See

Chapters 7–13.


results in nitrogen deficiency. If you usematerials with a high C:N ratio in yourgarden, add extra nitrogen fertilizer tocompensate for immobilization. The bestuse for these materials is for mulchesaround perennial crops or in walkways.They do not cause nitrogen immobiliza-tion until you mix them into the soil.

CompostCompost is an excellent source of

organic matter for garden soils. Compost-ing also closes the recycling loop byturning waste materials into a soil amend-ment. You can make compost at home orbuy commercially prepared compost.

Making compostThe key to composting is

to supply a balance of air,water, energy materials(materials with a low C:Nratio, such as grass clip-pings, green garden trim-mings, or fresh manure), and bulkingagents (materials with a high C:N ratio,such as corn stalks, straw, and woodymaterials). You don’t need additives tostimulate your compost pile. You justneed to provide conditions favorable fornatural composting organisms.

Home composting can be done in hot orcold piles, in worm bins, or in soiltrenches:• Hot (fast) composting produces high-

quality, finished compost in 6 to8 weeks. To maintain a hot compostpile, mix together balanced volumes ofenergy materials and bulking agents,keep the pile moist, and turn it fre-quently to keep it aerated.

• Cold (slow) composting requires lesswork than hot composting. Build thepile and leave it until it decomposes.This process may take months orlonger. Cold composting does not kill

weed seeds or pathogens. Rats andother pests can be attracted to ediblewastes in cold compost piles.

• You can compost fruit and vegetablescraps in a worm bin. This methodworks well for urban gardeners whohave little space.

• You can bury fruit and vegetablescraps and allow them to decomposein the soil.

Commercial compostYard debris is the major raw material in

most commercial compost sold in Wash-ington and Oregon. Commercial compostalso may contain animal manure,biosolids, food waste, or wood waste.Commercial compost is made on a largescale, with frequent aeration and/orturning to create conditions that kill weedseeds, plant pathogens, and humanpathogens.

Using compostAdding 1 to 2 inches of compost each

year helps build a productive garden soil.You can till or dig compost directly into

your garden or use it as a mulch beforeturning it into the soil. One cubic yard ofcompost covers about 300 square feet1 inch deep.

In the first year after application, par-tially decomposed woody compost mayimmobilize some soil nitrogen, resulting innitrogen deficiency for plants. If plantsshow signs of nitrogen deficiency(e.g., poor growth or yellow leaves),add extra nitrogen fertilizer(either organic or inorganic).In following years, mostcompost contrib-utes smallamounts ofavailablenitrogen to thesoil.

✿See Chapter 5,Composting.


Green manure (cover crops)Green manures are cover

crops grown specifically to betilled or dug into the soil. Plantinggreen manure is a way to growyour own organic matter. The valueof cover crops goes beyond theircontribution of organic matter,however. They also can do thefollowing:• Capture and recycle nutri-

ents that otherwise wouldbe lost by leaching duringthe winter.

• Protect the soil surface from rainfallimpact.

• Reduce runoff and erosion.• Suppress weeds.• Supply nitrogen (legumes only).

No one cover crop provides all of thesebenefits. Deciding which cover crop orcombination to grow depends on whichbenefits are most important to you and onwhich cover crops best fit into yourgarden plan (Table 9). With the exceptionof buckwheat, all of the cover crops listedin Table 9 are suitable for fall planting andspring tillage.

Gardeners usually plant covercrops in the fall and till them into

the soil before planting in thespring. The earlier cover crops

are planted, the more bene-fits they provide.Research in westernWashington showed thatcereal rye planted inSeptember captured

three times as much nitrogen asan October planting. Legumessuch as vetch and crimson cloverneed an early start to achieve

enough growth to cover the soil beforecold weather arrives.

If your garden contains crops intoNovember or December, it will not bepossible to plant early cover crops overthe entire area. In this case, plant covercrops in areas you harvest early, and usemulch in those you harvest later. Forexample, plant a cover crop in a sweetcorn bed immediately following harvest inSeptember, and mulch a bed of fall greensafter you remove the crop in November.You also can start cover crops betweenrows of late crops if space allows.

Till or dig cover crops into the soilbefore they flower. After flowering, plants

Table 9.—Examples of cover crops grown in Washington and Oregon.Cover crop CharacteristicsAnnual ryegrass Hardy, tolerates wet soils in winter, difficult to till once established.Austrian winter pea Legume, fixes nitrogen, does not compete well with winter weeds.Buckwheat Fast-growing, frost-sensitive, summer cover, ready to till in 30 days.Cereal rye Very hardy, grows quickly, matures rapidly in spring.Common vetch Legume, fixes nitrogen, easier to till than hairy vetch.Crimson clover Legume, fixes nitrogen, grows more slowly than vetch.Fava bean Legume, fixes nitrogen, grows quickly in fall.Hairy vetch Legume, fixes nitrogen, starts slowly, grows quickly in spring, good

companion crop for cereal rye, can be difficult to till.Spring barley Not as hardy as rye, tolerates drought, leafy growth in spring.Spring oats Not hardy, tolerates wet soils.Winter triticale Produces more vegetation than cereal rye, may decompose more easily.Winter wheat Leafy, covers soil well, matures slowly.


become woody and decline in quality.Also, digging plants into the soil becomesquite difficult if they grow too large. If youcannot till a cover crop before it blooms,cut it off and compost it for later use. Youstill will get the short-term benefit oforganic matter from the crowns and rootswhen you till your garden.

The organic matter benefits of covercrops last only about 1 year. Make covercrops an annual part of your gardenrotation. If they do not fit into your gardenplan, you can use winter mulches as asubstitute.

Soil pHSoil pH measures the acidity or alkalin-

ity of a soil. At a pH of 7 (neutral), acidityand alkalinity are balanced. Acidityincreases by a factor of 10 with each1-unit drop in pH below 7. For example, apH of 5.5 is 10 times as acidic as a pH of6.5. Alkalinity increases by a factor of 10with each 1-unit change in pH above 7.

Native soil pH depends on the mineralspresent in the soil and on rainfall. Soils inarid areas tend to be alkaline, and those inrainy areas tend to be acid. Gardening andfarming also affect soil pH; for example,many nitrogen fertilizers tend to reducepH, while liming increases pH.

Soil pH influences plant growth in threeways:• It affects availability of plant nutrients

(Figure 8).• It affects availability of toxic metals.• It affects the activity of soil micro-

organisms, which in turn affectsnutrient cycling and disease risk.

The availability of phosphorusdecreases in acid soils, while the availabil-ity of iron increases. In alkaline soils, theavailability of iron and zinc can be quitelow.

Aluminum availability increases in acidsoils. Aluminum is one of the most com-mon elements in soil, but it is not a plantnutrient and is toxic to plants in high

concentrations. Very little aluminum is insolution in soils above pH 6, and it causesno problems for plants. As pH declinesand aluminum availability increases,aluminum toxicity can become a problem.

Microbes also are affected by soil pH.The most numerous and diverse microbialpopulations exist in the middle of the pHrange. Fewer organisms are adapted tostrongly acid or strongly alkaline soils.Nutrient cycling is slower in acid andalkaline soils because of reduced micro-bial populations.

Many garden crops perform best in soilwith pH of 5.5 to 7.5, but some (such asblueberries and rhododendrons) areadapted to more strongly acid soils.Before amending soil to adjust pH, it isimportant to know the preferred pHranges of your plants.

Increasing soil pHThe most common way to increase soil

pH is to add lime. Lime is ground lime-stone, a rock containing calcium carbon-ate. It is an organic (natural) amendment,suitable for use by organic gardeners.

4.0 5.0 6.0 7.0 8.0 9.0 10.0Acid pH Neutral pH Alkaline pH

Nitrogen

Calcium

Magnesium

Molybdenum

Boron

Fe, Cu, Zn, Mn, Co

Potassium and sulfur

Phosphorus

Figure 8.—Effect of soil pH on the availability of plant nutrients.


Lime raises the pH of acid soils andsupplies calcium, an essential nutrient.Dolomitic lime contains magnesium aswell as calcium. It is a good choice forgardeners in western Washington andOregon, where soils often are deficient inmagnesium.

The best way to determine whetheryour soil needs lime is to have it tested. Inthe absence of a soil test, gardeners westof the Cascades can add about 50 lb oflime per 1,000 square feet per year. Do notlime areas where you grow acid-lovingplants, because they are adapted to acidsoils.

Lime is a slow-release material. Apply itin the fall to benefit spring crops.

Wood ashes are a readily availablesource of potassium, calcium, and magne-sium. Like lime, they also raise soil pH.High rates of wood ashes may causeshort-term salt injury, so apply less than15 to 25 pounds per 1,000 square feet. Wedo not recommend using wood ashes inalkaline soils.

Gypsum (calcium sulfate) is not asubstitute for lime. It supplies calciumand sulfur, but has little effect on soil pH.Gypsum has been promoted as a soilamendment to improve soil structure. Inthe vast majority of cases, it does notwork. Gypsum improves structure onlywhen poor structure results from excesssodium in the soil, a rare condition in theNorthwest. Use organic amendments toimprove soil structure, as describedearlier under “Adding organic matter.”

Some composts can increase soil pH.

Decreasing soil pHYou may need to decrease soil pH if you

wish to grow acid-loving plants. Elementalsulfur lowers soil pH. Soil testing is thebest way to determine whether sulfur isneeded and, if so, how much. In theabsence of a soil test, add about 50 lb ofsulfur per 1,000 square feet.

Ammonium sulfate fertilizer also lowerspH, but it takes longer than sulfur to havean effect. Urea also reduces pH slowly, asdo some organic fertilizers.

Soil salinitySoil salinity can be a problem in irri-

gated soils in arid areas of eastern Wash-ington and Oregon. Salts from irrigationwater, fertilizer, compost, and manureapplications can accumulate to the pointwhere they harm plant growth. In areaswith more rainfall, salts are leached fromthe soil each winter and do not accumu-late in the root zone.

A salinity test measures the totalsoluble salts in a soil. Table 10 shows howto interpret a salinity test.

You can leach salts from soil by apply-ing irrigation water in excess of the water-holding capacity of the soil. The excesswater must drain downward through thesoil to carry away excess salts. Whenleaching, apply water slowly enough thatit drains freely through the subsoil. Threeinches of excess water removes about halfof the soluble salts in a soil. Five inches ofwater removes about 90 percent.

Table 10.—Soil salinity measured in conductivity units (millimhos/cm) and potential effects on plants.Conductivity (mmho/cm) Interpretation4 or above Severe accumulation of salts. May restrict growth of many

vegetables and ornamental plants. Reduce salt by leaching.2 to 4 Moderate accumulation of salts. Will not restrict plant growth,

but may require more frequent irrigation to prevent wilting.Less than 2 Low salt accumulation. Will not affect plants.


For more informationBoth WSU and OSU also publish fertil-

izer guides for a wide variety of specificcrops.

WSU Cooperative Extension publicationsCover Crops for Gardens in Western Wash-

ington and Oregon (EB1824)Fertilizing Landscape Trees and Shrubs

(EB1034)Growing Small Fruits in the Home Garden

(EB1640)Home Lawns (EB0482)How Much Fertilizer: Conversion Guide for

Gardeners (EB1123)Role of Lime in Turfgrass Management

(EB1096)Soil Treatment Procedures for the Home

Gardener (EB1158)

OSU Extension publicationsLa Construcción de Camas Elevadas

(EC 1537-S)Cover Crops for Home Gardens (FS 304)Los Cultivos de Cobertura: Una Manera

Fácil de Mejorar el Suelo (EC 1538-S)Fertilizing Shade and Ornamental Trees

(FS 103)Fertilizing Your Garden: Vegetables, Fruits,

and Ornamentals (EC 1503)Gardening and Water Quality Protection:

Understanding Nitrogen Fertilizers(EC 1492)

Gardening and Water Quality Protection:Using Nitrogen Fertilizers Wisely(EC 1493)

Gardening with Composts, Mulches, andRow Covers (EC 1247)

A List of Analytical Labs Serving Oregon(EM 8677)

Raised Bed Gardening (FS 270)Soil Sampling for Home Gardens and Small

Acreages (EC 628)Willamette Valley Soil Quality Card Guide

(EM 8710)

Other publicationsBohn, H.L., B.L. McNeal, and G.A.

O’Connor. Soil Chemistry, 2nd edition(John Wiley & Sons, New York, 1985).360 pp.

Brady, N.C. The Nature and Properties ofSoils (Macmillan Publishing Co., Inc.,New York, 1998). 896 pp.

Craul, P.J. Urban Soil in Landscape Design(John Wiley & Sons, New York, 1987).396 pp.

Donahue, R.L., R.W. Miller, and J.C.Shickluna. Soils: An Introduction toSoils and Plant Growth (Prentice-Hall,Inc., Englewood Cliffs, NJ, 1990).768 pp.

Tisdale, S.L., and W.L. Nelson. Soil Fertilityand Fertilizers (Macmillan PublishingCo., Inc., New York, 1985). 754 pp.

soils and fertilizer - professional and continuing … · pile and leaving it until it decomposes....

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