This publication will help you understand theimportance of soil organic matter levels to goodplant performance. It also contains suggestionsfor suitable soil amendments. Any soil, nomatter how compacted, can be improved by theaddition of organic matter. The result will be abetter environment for almost any kind of plant.

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What gardening problems arecaused by poor soil quality?

Many problems with home vegetable gar-dens, fruit trees, shrubs, and flower gardens arecaused not by pests, diseases, or a lack ofnutrients, but by poor soil physical conditions.Symptoms of poor soil quality include thefollowing.• The soil is dried and cracked in summer.• Digging holes in the soil is difficult, whether

it is wet or dry.• Rhododendrons, hydrangeas, and other

shrubs wilt in hot weather, even with addedwater.

• Leaves on shrubs turn yellow and havebrown, dead sections on them, particularlyon the south side of the plant.

EC 1561 • May 2003$2.50

Neil Bell, Extension consumer horticulturist, Marion andPolk counties; Dan M. Sullivan, Extension soil scientist;Linda J. Brewer, faculty research assistant, ExtensionSmall Farms Program; and John Hart, Extension soilscientist; all of Oregon State University.

• Tomatoes and peppers get blossom-end rot,even if fertilized with calcium.

• Water tends to pool on the soil surface and todrain slowly, or it runs off the surface.

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What makes a productive soil?A productive soil provides physical support,

water, air, and nutrients to plants and soil-dwelling organisms (see “What is soil?”page 2). Like humans, roots and soil organismsbreathe and require sufficient air and water tolive. As a result, a good soil is not “solid”;rather, between 40 and 60 percent of the soilvolume is pores. The pores may be filled withwater or air, making both available to plants(see illustration on page 3).

The largest pores control aeration and move-ment of water through the soil and are largelythe result of earthworm burrowing or rootgrowth. The smaller pores store water. In agood soil, individual soil particles areaggregated into larger units, and the porearrangement remains stable over time.

Improving Garden Soilswith Organic Matter

N. Bell, D.M. Sullivan, L.J. Brewer, and J. Hart

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What is soil?Soil includes mineral and organic components,

water, and air. All of these are essential to plantgrowth. Soil formation is the result of physical,chemical, and biological processes. The processof soil formation begins when wind, rain, andfluctuating temperatures break rock down intosmaller and smaller fragments. The rock frag-ments that compose most soils, in order ofdecreasing particle size, are sand, silt, and clay.

Soil texture refers to the coarseness of the soil,which depends on the combinations of these threetypes of particles. Soils high in sand tend to befast-draining and subject to drought, while soilshigh in clay can store a lot of water but are“heavy” and not as permeable to air and water.Loam soils feel as if they have a balance of sand,silt, and clay.

The soil is also host to a large number of livingorganisms. Without soil organisms, very little soilformation would take place. Some of these organ-isms, such as bacteria, fungi, and certain insectsand mites, are microscopic. Others, such as antsand earthworms, are larger.

Soil formation results in layers of soil, muchlike the layers of a cake. The topsoil is wheremost of the organic matter is and where mostbiological activity occurs. The subsoil layers tendto be lighter in color and finer textured thantopsoil. Subsoil tends to be low in organic matterand is usually less suitable for plant growth.

Soil layers.Note thatmost of theorganicmatter isfound in thetop layers ofsoil.

Clay

Silt

Sand

Soil particle sizes. The largersand particles allow water todrain quickly through soil.Clay particles tend to packmore closely together,causing water to drain moreslowly.

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Soil structure and compactionSoil structure is another name for the units

of soil that you see when you dig into a soil.You might remember throwing dirt clods atyour siblings! The most desirable structurefor a topsoil is a granular structure (small“pebbles” or “crumbs” of soil). Soils with astable granular structure are easy to dig,accept water readily, and make a goodseedbed. This kind of ideal soil condition isoften referred to as “good tilth.”

Good soil structure is analogous to asponge. Like a sponge, pore spaces in a goodsoil are stable. In contrast, soil with poorstructure is like a bowl of popcorn, where thepore spaces are easily crushed.

A productive soil is a dynamic communitymade up of many species of fungi, bacteria,insects, and mites. This community dependson organic matter as a food and fuel source.Together with earthworms and plants, theseorganisms provide the “glue” that holds soiltogether and gives it structure.

Soil structure is delicate and is damagedby actions that compact the soil. Compactionby machinery or foot traffic is a commonproblem. Compacted soils hinder penetrationof air and water and growth of roots. Com-pacted soils are a poor environment not onlyfor plants, but also for earthworms and othersoil organisms. Tilling the soil, particularlywhen it is wet, damages soil structure andincreases the rate of soil organic matter lossthrough decomposition.

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Benefits of adding organicmatter to soil

One of the most important reasons for addingorganic matter is to improve the ability of thesoil to accept and store water (see “Soil struc-ture and compaction” at right). Amending yoursoil may mean that you can reduce the amountof water a newly planted garden requires. Thiseffect can be enhanced by the use of an organicmulch on the soil surface, which will reduceevaporation as compared to bare soil.

Adding organic matter also increases theactivity and number of soil organisms. Overtime, a well-amended soil will supply more ofthe nutrients your plants require, which willreduce fertilizer requirements.

Although you might not expect it, addingorganic matter to soil also helps to protectwater quality and the environment. Soilsamended with organic matter are a bettersponge for water. More water goes into the soil,and less water runs off the surface. Becausesurface runoff is reduced, pesticides and fertil-izers are retained in the soil instead of washinginto nearby rivers and lakes.

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Checking soil organic matter inyour garden

Here are some simple ways to assess organicmatter content of your soil.• Use your eyes. Soils with adequate organic

matter content are dark in color, both becausethey have more humus, which is dark, andbecause they hold more water.

• Look for puddling and standing water. Soilsrich in organic matter content and with goodtilth allow water to percolate below thesurface.

• Use your fingers. Look for aggregated soils.If you rub the soil between your fingers, thesoil will seem to contain “crumbs” made upof mineral and organic particles. The crumbs

Soil with goodstructure has stablepore spaces that allowwater penetration,root growth,earthworm movement,and air storage.

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are examples of aggregation and are theresult of sticky substances released by soilbacteria after feeding on organic matter.Aggregation generates soil structure.

• Use your nose. Soils with adequate organicmatter content have the rich smell of earth.Soils that have poor air circulation, a resultof reduced organic matter content, may smellsour.

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Fresh versus compostedmaterials

Both fresh and composted organic materialsare useful for amending soils. Fresh organicmaterial is rapidly decomposed by microorgan-isms in a compost pile or in the soil. Themicroorganisms use the organic material as afood source and release carbon dioxide to theatmosphere. As decomposition proceeds, theremaining organic compounds are more resis-tant to decomposition. (They have less “food”value to microorganisms.)

Decomposition is usually very rapid for thefirst 30 days after application of fresh leaves,fruits, or other vegetative material to soil(Figure 1). When decomposition of freshorganic materials takes place in soil, the stickyexudates produced by soil organisms help gluesoil particles together, improving soil structure.The volume of material will be reduced rapidlyas decomposition takes place.

Soil microorganisms require nitrogen fortheir growth, so the process of degrading freshorganic matter in the soil sometimes causes anitrogen deficiency for plants. If you use freshplant material, allow it to decompose in the soilfor several weeks before planting into it. Alsokeep in mind that very woody materials, suchas sawdust or sawdust-bedded manures, maycause nitrogen deficiency in soils for a longtime, even after composting.

When organic materials are compostedbefore use, the rapid decomposition phase takesplace in the compost pile instead of in the soil.Organic matter supplied by compost lastslonger in soil than fresh organic matter becausemuch of the decomposition has alreadyoccurred. However, composted organic matteris a poorer food source for soil organismscompared to fresh organic matter, so less of thesticky exudates that build soil structure areproduced in the soil after compost application.On the other hand, composted materials havefewer weed seeds and are less likely to carryplant disease organisms. Composted manuresare preferred over fresh manures when con-tamination of food crops with human pathogenssuch as E. coli is a concern.

In a garden situation, fresh or compostedmaterials may be used. If you are establishing araised-bed garden, compost is preferredbecause it will lose volume less rapidly andbecause it has less potential to compete withplants for nitrogen.

Days after organic matter application

0 20 40 60 80

0

20

40

60

80

100

Composted

Fresh

% o

f a

pp

lied

org

an

ic m

att

er

rem

ain

ing

in

so

il

Figure 1.—Fresh organic materials decompose morerapidly than composted organic materials. About half offresh organic matter is lost from the soil as carbondioxide gas during the first 60 days after soilincorporation. Composted materials increase soilorganic matter for a longer time. About half of thecomposted organic matter is lost in the first 1 to 2 yearsafter application.

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Common organic amendmentsBecause organic amendments are bulky,

heavy, and expensive to transport, look forsuitable amendments close to home. You canmake your own compost (see “For more infor-mation,” page 15) or use fresh organic materi-als from your yard. Local private or municipalcomposting operations offer a variety of com-post products and usually provide delivery.Also consider the types of farming, ranching, orother agricultural operations in your area, andwhat types of residuals they might produce thatwould be available at little or no cost. Youmight be able to arrange for bulk deliveries ofthese materials with a landscape supply ortrucking company. If you need only half a load,consider splitting a load with a neighbor.

Locally available amendments may include:• Yard trimmings compost• Leaves from deciduous trees• Crop residues• Manures and manure composts• Separated dairy manure solids

The following sections describe the charac-teristics of these common amendments and givesome suggestions for getting the best valuefrom each amendment.

Yard trimmings compostSometimes sold as “garden compost,” yard

trimmings compost is the most widely availablematerial suitable for high-rate incorporationinto soil. Private composting companies usuallyproduce it. Grass clippings, leaves, brush, treeand shrub prunings, or other plant materials arecomposted for 3 to 9 months in large piles, thenscreened to remove large sticks (greater than0.75 inch). Piles typically reach temperaturesabove 130°F, killing most weed seeds. Woodymaterials dominate most yard trimmings com-posts. They usually have a carbon:nitrogen(C:N) ratio of less than 20:1, a pH of 6 to 7, andrelatively low levels of ammonium-N and

soluble salts (3 to 6 mmhos/cm). (See “Inter-preting laboratory analyses for fresh organicmaterial or compost,” page 7.)

Yard trimmings compost usually increasesnitrogen fertilization requirements for the first2 months after application. Later on, it has littleor no effect on nitrogen requirements.

Composted yard trimmings decomposeslowly in soil. About half of the organic matteradded usually remains in the soil after twogrowing seasons. Apply yard trimmings com-post at a rate of 1 to 2 inches.

You can make yard trimmings compost in abackyard compost pile (see “For more informa-tion”). To make compost from woody trim-mings, you usually need to grind these materi-als prior to composting.

Be selective in the materials you include in ahome compost pile. It is quite common forweed seeds, vegetable seeds, and plant diseaseorganisms to survive the home compostingprocess.

It usually takes about 12 months to makehigh-quality yard trimmings compost in abackyard pile with minimal maintenance.Because yard trimmings compost from a back-yard pile is not screened, it usually is coarserthan purchased compost and is best used as amulch in perennial shrub beds (see “Trees andshrubs,” page 11).

Leaves from deciduous treesLeaves are perhaps the best and most readily

available organic matter source for vegetablegardens or other areas that get some annualtillage. Some cities will deliver leaves collectedfrom streets to your property at little or nocharge.

Leaf mulch, or leaf mold (partially decom-posed leaves), has a near-neutral pH (6 to 7.5).The C:N ratio typically is about 50:1 in freshleaves, decreasing to below 20:1 when fullycomposted. Most kinds of leaves are a goodsource of potassium (K); a 2-inch applicationsupplies about 0.3 to 3 lb potash (K20) per1,000 square feet.

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Because leaves decompose rapidly, they arenot as useful as yard trimmings compost forone-time applications to landscape beds toincrease organic matter.

Mulching an annual vegetable or flowergarden with 1 to 2 inches of leaves in the falladds organic matter, protects soil from raindropimpact, and smothers winter annual weeds. Inthe spring, the remaining leaf debris may bedug or rototilled into the soil. If you plant a fallcover crop, reduce leaf application or omit italtogether so that you don’t smother the covercrop.

To compost leaves, pile them in the fall, thenturn the pile several times in March and April.Leaf mulch from a home compost pile is excel-lent for summer mulching around rhododen-drons, blueberries, and other shrubs that aresensitive to summer drought, or in vegetableand flower gardens. Apply 1 to 2 inches aftersoil has warmed (June).

Partially composted leaves also can be usedto improve soil in annual planting beds.

Crop residuesFresh or composted crop residues may be

available from nearby farms, tree-trimmingcompanies, or your own kitchen. Uncompostedcrop residues may contain weed seeds, whileproperly composted residues are weed-free.

Woody materials such as hazelnut shells orground tree prunings can be used as a mulcharound trees or shrubs. Crop residues fromannual crops (fruit, leaves, straw) decomposemore rapidly in soil than do woody materials.Fruit and vegetable residues contain mostlywater and readily degradable organic matter.They can be incorporated into a backyardcompost pile or buried immediately in soil. As ageneral rule, the juicier and leafier the cropresidue, the less valuable it is for long-term soilorganic matter enhancement.

Peppermint hay, consisting of leaves andstems that have been heated to removepeppermint oil, is one of the most commonly

available residues from Willamette Valleyfarms. Freshly cooked peppermint hay, offeredfor sale in August, has roughly the same levelsof nitrogen and potassium as manure and freshgrass clippings. It also is high in soluble salts.Over half of the organic matter in fresh haydecomposes in the fall after application.

Composted peppermint hay, which is offeredfor sale in the spring, is more suitable forlandscape use than freshly cooked hay and hasgreater long-term value as a soil amendment.After composting, peppermint hay is dominatedby fibrous stems that make a good mulch or soilamendment. Peppermint can be composted withother low-nutrient materials (e.g., straw orwoody materials) to make an excellent soilamendment.

Manures and manure compostsMany manures and manure composts have

high soluble nitrogen, ammonia, or salt content,or high pH (above 8). Thus, their suitability foruse in landscapes is limited.

Composting transforms soluble nitrogen inmanure to slow-release organic forms,decreases ammonia to levels that do not injureplants, and sometimes reduces pH (to 7 to 7.5).

However, composting concentrates salts.Chicken manure and feedlot (steer) manurecomposts typically contain very high salt levels.Mushroom compost, a mixture of manure pluscottonseed or soybean meal and other inorganicamendments, is also high in salts.

In general, it is best to avoid manure andmanure composts for high-rate applications toplanting beds. Use manures in small amounts toreplace nitrogen–phosphorus–potassium fertil-izers. Ask for a compost analysis and check forsoluble salts and ammonium-nitrogen whenevaluating composted manure products. See“Interpreting laboratory analyses for freshorganic material or compost,” page 7, for moreinformation.

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Interpreting laboratoryanalyses for fresh organicmaterial or compost

Laboratory analyses describe and quantifyproduct quality. Many commercial compostsuppliers regularly test their product andprovide product information if you ask for it.

Moisture content (% “as-is” amendmentweight). Tells how much water and organicmatter are present. A material that has40 percent moisture has 60 percent dry matter.Best: 40 to 60 percent moisture content. Lessorganic matter is present at high moisturecontent (greater than 60 percent), and thematerial usually is very dense and heavy.Low-moisture materials (less than 40 percent)sometimes are dusty.

Percent organic matter (% dry weight).Percentage of a dry amendment that is organicmatter. Best: 40 to 60 percent. Low values(less than 30 percent) usually indicate thatorganic matter has been mixed with sand orsoil. High values (greater than 60 percent)indicate fresh, uncomposted material.

pH. Indicates acidity or alkalinity of thesoil. Lower values indicate greater acidity.Best: 6 to 7. Values below 5 or greater than 8may injure plants. Some plants (blueberries,rhododendrons) prefer an acidic pH, near 5.

C:N (ratio of carbon to nitrogen). Best:Stable soil organic matter has a C:N ratio of12:1 to 15:1. Ratios less than 10:1 are typicalof uncomposted manure, which will decom-pose rapidly in soil and release plant-availablenitrogen. Ratios greater than 25:1 are typicalof uncomposted woody plant materials orcrop residues such as wheat straw. Incorpora-tion of high C:N materials (greater than 25:1)usually reduces the supply of plant-availablenitrogen in the soil for several months.

Ammonium-nitrogen (NH4-N; dryweight basis). Ammonium-nitrogen is avail-able for immediate use by plants. Best: lessthan 500 ppm.

Ammonium-N concentrations above1,000 ppm (0.1 percent) typically are present inmanures that have been incompletelycomposted. Materials with high ammoniumconcentrations are not ideal for high-rateincorporation into soil because they supply toomuch water-soluble N (see “plant-availablenitrogen”). High ammonium-N levels can causeplant injury when the organic material is addedto planting holes or is not thoroughly mixedwith soil. Ammonium-N usually is converted tonitrate-N within a few weeks after application.You usually can avoid problems with ammoniatoxicity to roots by allowing a month betweenorganic matter application and planting.

Electrical conductivity (EC). A measure ofthe soluble salt content of the material. Saltcontent is measured by the electrical conductiv-ity (mmhos/cm) of the material. Best: 0 to4 mmhos/cm. Poor: above 8 mmhos/cm. Highsalt content means high conductivity, whichmay injure plants. Avoid using materials withhigh EC in planting holes because salts maydamage roots.

Nitrate-nitrogen (NO3-N; dry weightbasis). Nitrate-nitrogen is immediately avail-able to plants. Best: 200 to 500 ppm. Materialswith high nitrate concentrations supply toomuch water-soluble N (plant-available N) whenapplied at typical rates (1 to 2 inches of com-post). Materials with very low nitrate concen-trations (less than 50 ppm) and high C:N ratio(above 25:1) likely are incompletely compostedand will increase N fertilizer requirements forseveral months after application.

Plant-available, water-soluble, or inor-ganic nitrogen. This is the sum of ammonium-N plus nitrate-N (dry weight basis). One inch ofa compost with 1,000 ppm N (ammonium +nitrate-N) supplies about 1 lb of water-solublenitrogen per 1,000 square feet.

Excess water-soluble N can leach throughsoil and contaminate groundwater. Limit appli-cation rates of materials having ammonium +nitrate-N concentrations above 1,000 ppm.

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Separated dairy manure solidsOne manure by-product that is acceptable for

high-rate application to most planting beds iscalled separated dairy manure solids. Separatedsolids are available from dairies west of theCascades in fresh and composted forms. Sepa-rated dairy manure solids (composted oruncomposted) are much lower in salt, ammo-nia, and soluble nitrogen than raw dairymanure. They have a pH of 7 to 8 and solublesalt levels similar to yard trimmings compost(3 to 6 mmhos/cm). The C:N ratio of freshsolids is approximately 30:1, declining to 15:1after composting.

Fresh separated solids usually are less expen-sive, but lower in quality than compostedseparated solids. Fresh solids may contain weedseeds. Fresh dairy solids increase nitrogenfertilizer requirements for 4 to 8 weeks follow-ing application, then act as a slow-releasesource of plant-available N. About half of theorganic content of fresh separated dairy solidsis lost via decomposition during the firstmonths following soil incorporation.

Composted separated solids are a bettervalue for long-term soil organic matterenhancement because they decompose moreslowly in soil. Composted solids are essentiallyweed-free. Composted solids provide slow-release nitrogen for plant growth startingseveral weeks after application.

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Organic amendment qualityWhen applying high rates (1 to 2 inches) of

an organic amendment, look for organicamendments that will promote stable soilconditions and balanced plant nutrient levels.Organic amendments like these are:• Well-mixed and easy to spread—Quality

organic amendments have a consistenttexture and moisture content, are free of largesticks, and can be handled easily with ashovel or fork.

• Do not injure plants when applied at highrates or change usual fertilization practices—Quality organic amendments have a pleasing,earthy smell. They do not smell like ammo-nia (excessive nitrogen will burn seedlingsand tender root growth) or rotten eggs(anaerobic decomposition results in organicacids, which may prove toxic to someplants). Excessively woody materialswill rob plants of soil nitrogen as theydecompose.

• Decompose slowly—Quality organic amend-ments decompose slowly when applied tosoil because considerable decomposition hasalready occurred during storage orcomposting. Rapidly decomposing materialsmay tie up soil nitrogen temporarily, maycreate organic acids in the soil as oxygenlevels are reduced, and will undergo a reduc-tion in volume as decomposition proceeds.

Amendment quality also includes particlesize, nutrient and organic matter content, pH,and carbon-to-nitrogen ratio. Quality amend-ments have low concentrations of contami-nants, including salts, weed seed, pesticides, orother foreign substances.

Many commercial compost suppliers regu-larly test their product and provide productquality information if you ask for it. “Interpret-ing laboratory analyses for fresh organic mate-rial or compost” (page 7) discusses some of thecriteria for assessing the quality of organicmaterials and will help you interpret productinformation.

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Estimating amendment volumeneeded for a project

You can easily estimate how much amend-ment you need to cover a known area to adesired depth. Choose the depth of application(in inches), and measure the area to be amended(in square feet). Table 1 estimates the volumeof amendment (in cubic yards) you will need.

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For smaller gardens, a yard of material is toomuch, so a 5-gallon bucket makes a handymeasuring device. For example, suppose yourgarden is 10 feet by 10 feet (100 square feet),and you want to incorporate a 1-inch layer ofcompost. One inch is 1⁄12 of a foot, so you wouldneed 1⁄12 times 100 square feet (100 x 1⁄12), or8 cubic feet of compost. One and one-half5-gallon buckets equals approximately 1 cubicfoot, so you would need 12 buckets of compost(8 x 1.5). Keep in mind that there are 27 cubicfeet in 1 cubic yard of compost.

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Incorporating organic matterinto soil

There are several ways to mix organic matterinto garden soil. The most common methodsinvolve digging or rototilling (Figure 2).Rototillers are effective, but hand-operatedmachines usually are capable of working onlythe top 4 to 6 inches of soil. Tractor-mountedrototillers may enable you to mix up to 8 inchesdeep. For incorporating amendments over arelatively large area, rototillers are probably thebest option.

Excessive rototilling, however, has verydetrimental effects on soil structure, particu-larly if done when the soil is wet. Rototillingcan compact soil just below the tillage depth,

reduce the volume of pore spaces in soil for airand water, and kill earthworms.

Digging amendments into the soil is labori-ous, but will enable you to incorporate asdeeply as you choose to dig, up to 12 inches. Italso avoids many of the problems associatedwith rototillers. An ordinary spade works well

Table 1. Estimating the volume of organic amendment needed.

Depth ofamendment

Area of garden (square feet)*

desired 200 500 1,000 2,000

(inches) Organic material to add (cubic yards)

1 0.6 1.5 3.1 6.22 1.2 3.1 6.2 12.33 1.9 4.6 9.3 18.54 2.5 6.2 12.3 24.7

*To estimate square footage of a garden, multiply the length by the width(in feet).

Figure 2.—Rototilling is one way to incorporateorganic matter into soil. Hand digging can be just aseffective, however, and is less likely to damage soilstructure.

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in soil that is not too wet. To minimizethe impact of digging on soil structure,consider using a spading fork.

Cover crops provide a relativelyeasy way to add organic matter to

soil. The mat of roots formed bythe cover crop often is more

valuable in building soil struc-ture than the above-ground

biomass. Both winter andsummer cover crops can be

used. (See “For moreinformation,” page 15,

for details on covercrops suited to your

garden.) You can digin the cover cropbefore planting

your garden. Or, youcan remove the above-

ground plant material and compost it.Another option is to apply a layer of organic

amendment on the soil surface and simply plantinto it. This method has the advantage of notrequiring any disturbance of the soil structure.Also, the amendment will serve as a mulch andhelp preserve soil moisture and suppress weeds.It will take longer for the amendment toimprove the organic matter content of the soilby this method, however.

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Using organic matterfor landscape installationand maintenanceLawn establishment

Amending soil with compost prior to estab-lishing turf can help get a new lawn off to agood start by providing a better environmentfor root growth. Healthy lawns with deep rootsystems require less water during summer andare more resistant to weed invasion.

After establishment, a vigorous lawn pro-duces its own organic matter. Grass roots are

one of nature’s best soil structure builders,partly because they provide food for soil organ-isms. Living roots provide a rich food sourcefor organisms. Grass roots live for a year orless; new roots grow each year to replace thosethat die. The dead roots are continuouslysloughed from the plant and add to soil organicmatter. One of the keys to getting this naturalorganic matter factory working in yourlawn is to provide good soil quality for turfestablishment.

Use only composted organic materials forsoil amendment prior to grass establishment.Use compost that has been screened (particlesless than 0.5 inch). Sticks and other coarseorganic materials make it difficult to establish afirm seedbed prior to seeding or sodding.Screened yard trimmings compost is widelyavailable and usually is suitable. Do not usefresh organic materials, because they are toodifficult to mix evenly with soil and can causelow spots in the lawn as they decomposerapidly.

Apply 0.5 to 1 inch of compost and incorpo-rate it into 6 inches of soil with a rototiller.Higher compost application rates can cause anuneven, bumpy lawn as the result of intenseearthworm activity and loss of volume ascompost decomposes. Use less compost if thematerial is high in plant-available nitrogen(ammonium-N + nitrate-N; see “Interpretinglaboratory analyses for fresh organic material orcompost,” page 7). Use less compost if you donot till the soil to a 6-inch depth.

Lawns require adequate pH and nutrientlevels for rapid establishment. By testing yoursoil and compost, you can provide the rightamount of phosphorus, potassium, and lime forthe new lawn (see “For more information,”page 14). It’s best to incorporate these fertilizermaterials into the soil at the same time youincorporate compost.

Nitrogen fertilizer can be added before orafter you mix compost with the soil. Compostanalyses may be helpful in adjusting nitrogenfertilizer application rates.

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Vegetable and flower gardensVegetable gardens tend to contain many

annual or biennial plants. Because some, or all,of the soil can be disturbed annually, these soilscan be amended on a routine basis.

Some vegetable crops, such as rhubarb, areperennials, and many herbs are small shrubs.The deeper roots of these plants and of annualroot crops such as carrots do better in soilsamended to a depth of 10 to 12 inches.

Ornamental gardens that contain mostlyherbaceous perennial or woody plants must beamended prior to planting. Many perennialplants are dug from time to time for division,providing an opportunity to add further amend-ments every few years.

Trees and shrubsShrubs and trees are more permanent, so soil

must be well prepared at the time of planting.It’s best to amend soil to a depth of 12 inches inthese gardens. Mulching in these situations willincrease the effectiveness of the soil amend-ment, and over time some of the mulch willbecome incorporated into the soil.

Tree and shrub root systems are not a mirrorimage of the top of the tree; instead, they

resemble pancakes. Roots are found mostly inthe top 2 feet of soil, and can spread to two tothree times the diameter of the canopy, or more.Thus, it generally is better to amend the largestpossible area around these kinds of plants.

Amending only a planting hole and notamending surrounding soil is not the best wayto use organic amendments in tree and shrubplantings. If soil is compacted and has poorstructure, and only the planting hole is heavilyamended with organic matter, the planting holefunctions like a large pot. The tree roots growrapidly through the permeable, amended soil,but have difficulty penetrating the hard sur-rounding soil. This limits growth and increasesthe risk of tree blow-downs. Another problem isthat water readily enters the amended plantinghole, but drains more slowly into the surround-ing compacted soil, which may create problemswith waterlogging and cause root damage.

When planting trees and shrubs, dig theplanting hole no deeper than the root ball of theplant, but several times wider than the diameterof the root ball (Figure 3). At a minimum,loosen the top 12 inches of soil around theplanting hole to increase permeability andaeration. If the soil quality is particularly poor,you might consider adding a maximum of25 percent organic amendment (by volume) tothe loosened soil. Always use a soil amendmentthat is low in soluble salts, such as yard trim-mings compost or composted leaves. Avoidmaterials that typically have a high soluble saltcontent, such as animal manures or mushroomcompost.

Small shrubs have smaller root systems andare not a risk to blow over, so they can beplanted in raised beds amended with organicmatter. Raised beds provide better drainagethan flat plantings, and it often is easier to mixorganic matter with loose soil in a newlyformed raised bed. If you add soil or a com-post-soil mix to construct a raised bed, loosenthe existing soil with a shovel or fork beforeadding the new soil to form the raised bed.

Figure 3.—When planting a tree or shrub, dig the holeonly as deep as the root ball, but several times wider. It’sbest not to amend the planting hole with organic matter.

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Construct raised beds several inches abovethe existing grade to provide sufficient rootingvolume. The width of the raised bed shouldreflect the size of the plant at maturity. Thelarger the plant, the wider the root system islikely to be. Thus, the bed probably should beat least twice the diameter of the shrub’sexpected width at maturity.

Mulching the soil surface with 2 to4 inches of coarse compost, wood chips, bark,or similar materials is a way to add organicmatter to existing shrub beds and under trees.Mulch can suppress weeds, conserve soilmoisture, and moderate soil temperatures, all ofwhich improve plant growth even if soils arenot amended before planting. If you amended ashrub bed prior to planting, mulching the bedafterward will help maximize the benefit of thesoil amendment. Mulching to conserve soilmoisture is particularly useful for raised beds,since water tends to evaporate more readilyfrom beds than from flat soil.

Over time, earthworms and other soil organ-isms incorporate some of the mulch into thesoil, providing a natural soil amendment pro-cess similar to what happens under a forest.

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Questions and answersTopsoil was removed or buried during theconstruction of my home. Is adding moretopsoil better than amending withorganic matter?

After topsoil is removed or buried, you areleft with subsoil, a poor medium for plantgrowth. Subsoil typically is low in organicmatter and impervious to water, air, and plantroots. This problem is often addressed byspreading topsoil over the compacted subsoillayer. Before you use this approach, considerseveral potential problems:• The quality of the added topsoil may not be

significantly better than the existing soil.Unlike compost, there are no standards for

topsoil quality, so before accepting deliveryof topsoil, inspect it and try to assess organicmatter content and other quality issues suchas the presence of weeds or weed seed.

• Even if the topsoil quality is good, the layerof soil will be thin and may not allow forextensive root growth, as roots will tend notto grow into the compacted soil below. Oneresult can be reduced growth, but moreimportantly shallow root systems increasethe likelihood of blow-down in high winds,especially in the case of trees.

• The subsoil layer beneath may impededrainage, causing problems with growth androot diseases.

In most cases, the best alternative is toamend the existing soil with organic matter,then bring in topsoil for areas of the landscapewhere added soil will have the most benefit(e.g., a raised-bed garden).

Will gypsum improve soil structure?Gypsum is sometimes recommended for

improving soil structure for soils west of theCascades. Gypsum is calcium sulfate, and it isclaimed to break up and loosen clay soils. Infact, soil response to gypsum is variable. Thereis no consensus among soil scientists thatgypsum will improve soil structure in heavyclay soils.

Is there anything organic matter won’t dofor soil quality?

A single addition of organic matter to soilwill not necessarily have long-term effects onsoil quality. Over time, the organic mattercontent of the soil will decline because ofdecomposition. To maintain the organic mattercontent of the soil, further organic matterincorporation, mulching, or establishment of aperennial grass sod will be required.

Addition of organic matter will not eliminatethe need for watering flower beds and vegetablegardens during dry summer weather. Youprobably will find that gardens amended withorganic matter require less frequent irrigation,

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but, in most areas, summer water still isrequired for best growth and appearance ofmost plants.

Although in many cases incorporation oforganic matter into soils improves drainage,there are some drainage problems that organicmatter cannot improve. In low-lying areaswhere water tends to pond on the surface, thesoil will become saturated if there is no lower-lying area to which the water can drain. Insaturated soils, additional organic matter mayundergo anaerobic decomposition, and plant-toxic organic acids may result.

Will adding organic matter to soileliminate or reduce soil-borne plantdisease?

Amending soils with organic matter gener-ally improves plant health by encouraging rootsystem development, improving drainage, andallowing gas exchange within the soil. Organicmatter addition also usually increases thequantity and diversity of soil microorganismsand increases the level of competition amongthem, which generally is beneficial to the soilecosystem.

On the other hand, fresh organic matter maycontribute to plant disease by providing a foodsource for disease-causing soil microorganisms.Uncomposted materials also might containplant and human pathogens. Gardeners mayinadvertently introduce plant diseases withfresh organic materials. Allow fresh organicmaterials to decompose in the soil for a monthbefore planting.

Suppression of plant disease by soil organ-isms is a complex biological process that isdifficult to predict. If you have experiencedproblems with common soil-borne diseasessuch as Phytophthora root rot, Verticillium wilt,or damping off, you should not expect to solvethese problems by adding organic matter.

Will adding sand promote drainagein clay soil?

Occasionally, a homeowner tries to changethe basic nature of a clay soil by adding sand.Clays are well-ordered mineral structures thatform extremely fine, flat particles. Theseparticles are layered something like a messydeck of cards. The spaces between clay par-ticles are extremely small, which is why waterand air move between them so slowly.

The diameter of the finest sand is more thana thousand times larger than the diameter of thelargest clay particle. Sand worked into claysoils provides a surface onto which clay par-ticles can adhere. The result is a concrete-likemixture that can be more difficult to managethan the original clay soil. No amount of addedsand will change a clay loam into a sandy loam.

Are herbicide residues a problemin composted materials?

In very rare cases, compost is contaminatedwith herbicides used in turf or agricultural weedmanagement. These residues survive thecomposting process and result in injury tosubsequent plantings. Another source of herbi-cide contamination in compost can be animalmanure, as animals can pass some herbicidesthrough their systems from contaminated feed.

The easiest and cheapest way to tell whethercompost is contaminated with herbicides is toconduct a bioassay. This is a simple test thatallows you to look for the effects of herbicidesin compost. To conduct an herbicide bioassay,mix equal parts of potting soil and the suspectcompost. Fill four pots with the potting soil–compost mix, and four pots with potting soilalone. Plant each pot with the plant you wouldlike to grow. Place pots in adequate light, andwater regularly. Compare the results from thetwo sets of pots. Extension Master Gardenervolunteers can provide more information on thediagnosis of plant injury from herbicides.

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Will adding organic matter correctproblems with soil acidity?

Some plants, including many vegetablecrops, are sensitive to soil acidity (low pH).Soils west of the Cascades are naturally acidic.Plants sensitive to soil acidity may exhibit poorgrowth even when the soil has excellentphysical condition (good tilth).

Adding organic materials to soil generallyincreases soil pH slightly (reduces acidity).However, it is not as effective as lime at raisingthe pH of very acid soils. The best way todetermine appropriate lime application rates isto have a soil test performed by a reputable soiltesting laboratory. If you are considering tillageto incorporate organic matter, it makes sense toadd lime (if needed) at the same time, becauselime needs to be mixed with soil to be effective.

What is the difference between a mulchand a soil amendment?

A mulch is a protective layer of material thatis spread over the soil surface. Unlike a soilamendment, mulches are not dug or rototilledinto the soil. Mulches are commonly used inhome and commercial landscapes to reduce

water evaporation from the soil, suppressweeds, protect soil from compaction, andprovide nutrients to the soil. Generally speak-ing, mulches improve the soil environment andgrowing conditions for most plants, regardlessof whether the underlying soil is amended.

Mulching materials are as varied as thereasons for using them. They include manufac-tured products, such as woven polyethylenesheets, and organic materials, such as compost,leaf mold, manure, barkdust, and nutshells.

Organic materials that make good mulches inperennial plantings usually have larger particlesize than organic materials used for amendingsoil. Woody materials make excellent mulchesbecause they have lots of pore space to trapwater and prevent it from running off, and theyare not conducive to weed growth. Whenwoody materials are used as a mulch, theirdecomposition does not cause nitrogen defi-ciency in plants.

Some materials that are used as soil amend-ments, such as leaf mold, compost, and certaincrop residues, also can be used as effectivemulches.

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For more informationOSU Extension Service publicationsCover Crops for Home Gardens, FS 304

(revised 1994).Fertilizing Home Lawns, EC 1278 (1989).Gardening with Composts, Mulches, and Row

Covers, EC 1247 (1986).A List of Analytical Labs Serving Oregon,

EM 8677 (revised 2002).Practical Lawn Establishment and Renovation,

EC 1550 (2002).Soil Sampling for Home Gardens and Small

Acreages, EC 628 (revised 2002).Soil Test Interpretation Guide, EC 1478 (1996).Using Cover Crops in Oregon, EM 8704

(1998).Water-efficient Landscape Plants, EC 1546

(2001).

Willamette Valley Soil Quality Card, EM 8711(1998).

Willamette Valley Soil Quality Card Guide,EM 8710 (1998).

Many OSU Extension Service publications,as well as additional gardening information,may be viewed or downloaded from the Web(http://eesc.oregonstate.edu).

Copies of many of our publications andvideos also are available from OSU Extensionand Experiment Station Communications. Forprices and ordering information, visit our onlinecatalog (http://eesc.oregonstate.edu) or contactus by fax (541-737-0817), e-mail (puborders@oregonstate.edu), or phone (541-737-2513).

WSU Cooperative Extension publicationsBackyard Composting, WAEB 1784.Soil Management for Small Farms, EB 1895.

Visit the WSU Cooperative Extension publi-cations Web site (http://pubs.wsu.edu).

© 2003 Oregon State University. This publication was produced and distributed in furtherance of the Acts of Congress of May 8and June 30, 1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture,and Oregon counties. Oregon State University Extension Service offers educational programs, activities, and materials—withoutdiscrimination based on race, color, religion, sex, sexual orientation, national origin, age, marital status, disability, or disabledveteran or Vietnam-era veteran status. Oregon State University Extension Service is an Equal Opportunity Employer.

Published May 2003.

AcknowledgmentsThe authors thank the following individuals for reviewing a draft of this publication.

Craig Cogger, Crop and Soil Sciences, Washington State University-Puyallup, Puyallup, WAJack Hoeck, Rexius Forest Byproducts, Eugene, ORJed Colquhoun, Crop and Soil Science, Oregon State University, Corvallis, ORJohn Foseid, Metro, Portland, ORJoyce Jimerson, Washington State University Cooperative Extension, Bellingham, WAJudy Good, JB Good Inc., Corvallis, ORLauren Ettlin, Oregon Department of Environmental Quality, Portland, ORMarti Roberts-Pillon, Oregon Department of Environmental Quality, Portland, ORRita Hummel, Horticulture, Washington State University-Puyallup, Puyallup, WASteve Schueurell, Botany and Plant Pathology, Oregon State University, Corvallis, ORTeresa Matteson, Crop and Soil Science, Oregon State University, Corvallis, OR