Soil Management in Yards and Gardens
Soils Information for Gardeners - WSU Puyallup Research Center
Gardener's Guide to Soils and Fertilizers" - WSU Publication EB1971E
- University of New Hampshire
Information - WSU Research and Extension Center, Puyallup
Testing and Soil Improvement- WSU King Co. Community Horticulture Fact Sheet #6
Soil Testing - Agriculture and Natural Resources Fact Sheet #508
Gardeners manage the environment of plants to help them grow and develop normally. Gardeners
should select plants adapted to the over-all climate, plant them in the exposure (sun or shade)
preferred by the plant, and provide an adequate supply of air, water, and mineral nutrients in the
Soil management is the major gardening task since the soil is the most easily controllable part
of the plant's environment. Soil provides the mineral nutrients and water used by plants to
manufacture their food supply and structural components. Soil also supplies oxygen and structural
support for plant roots.
Few garden soils provide ideal conditions for plant growth. Sandy soils provide plenty of oxygen
but do not hold water and mineral nutrients well. They must be irrigated and fertilized more
frequently and in lesser amounts than fine-textured soils. Clay soils hold minerals well but can
hold too much water, excluding air from the soil. They are sticky when wet and become hard when dry.
Gardeners should work to improve both the plant environment and the greater environment. Sound
soil management helps reduce runoff, erosion, chemical leaching, arid weed problems, and helps
This bulletin describes garden soils and their management, including organic matter, efficient
irrigation, nutrients and fertilizers, soil testing, and soil amendments.
- Organic Matter Conditions Soil
Adding organic matter is the best way to improve the plant environment in nearly all soils. Organic
matter improves soil structure and aeration, creating a better environment for garden plants. Organic
matter helps sandy soils hold water and plant nutrients. It makes clay soils easier to work by
helping water enter and move more rapidly, providing better aeration for plant roots.
Increase the organic matter content of garden soils by tilling in organic residues, such as manures,
compost, sewage sludge (biosolids), or sawdust, or by growing and turning under green manure crops
such as rye, oats, or clover. You can use both methods in any garden soil.
- Add Small Amounts Each Year
Add no more than 2 inches of organic matter to garden soil each year. Use much less if the organic
matter is rich in nutrients, such as fresh animal manure. The organic matter of soils contains
organic residues and living organisms. Adding undecayed organic residues stimulates a rapid increase
in microbial activity in the soil. This can temporarily tie up the available nitrogen (N) supply
until the soil organisms have attained a new equilibrium. Fresh animal manures usually contain large
amounts of N, but often, organic matter alone will not supply adequate or balanced fertility. Some
organic residues require additional nitrogen fertilizer to compensate for the increased microbial
activity in the soil.
ORGANIC MATTER SOURCES
- Manure is a Good Humus Source
Manures are a good source of humus or organic matter for yard and garden soils. They also provide
some nutrients if there is not too much litter (straw, sawdust, or shavings) mixed in, and if they
have not been stored outdoors where heavy rains wash out the nutrients. Poultry or rabbit droppings,
taken from beneath roosts or hutches, are high in nitrogen. They may actually burn plant roots if
used too generously.
Average Nutrient Content Table
This table is based on the average nutrient content in one yard of undiluted animal and poultry excrement. Divide these numbers by 40 to estimate the nutrient content per 5-gallon bucket of fresh, undiluted manure. The value of manure as fertilizer depends on how much it has been diluted or leached by water and the proportion of bedding, such as straw, sawdust, or shavings, that is mixed in undiluted manure.
(Units in lbs./yard)
Add manure in the early spring. Do not spread manure in fall or winter if rainfall or snowmelt will run off your garden into a stream, lake, or drainage ditch. Do not dig too close to trees, shrubs, and perennials, you may cut off feeder roots. Manure containing hay or straw bedding may bring seeds or new weeds into your garden
- Composting Recycles Plant Refuse
Composting is the biological breakdown of organic matter. Composts are a good source of humus and a good way to recycle plant refuse from the yard or garden. However, backyard composting may not kill weed seeds, disease organisms, or underground stems or roots of such plants as quackgrass, morning glory, or Canada thistle. Do not put diseased plant materials in your compost. Fruit and vegetable wastes are good compost materials, but do not compost meat and fatty food.
- Composting Methods
Many gardeners compost using a slow, low-temperature method. Yard wastes are piled in a heap and allowed to compost in place. This method requires little effort, but it does not generate enough heat to kill weed seeds or disease organisms. Often a year or more is needed to produce compost by the slow method.
- To compost quickly and generate the high temperatures needed to kill most weed seeds, you must maintain conditions favorable for the high-temperature compost bacteria. This requires:
- a pile of organic material at least 3' high x 3' wide x 3' deep
- balancing high-carbon and high-nitrogen plant materials (Table 1)
- chopping materials to increase their surface area
- keeping the pile aerated by turning, and
- maintaining adequate moisture.
- If you do not have enough nitrogen-rich plant materials to compost rapidly, you may need to add nitrogen, using fertilizer or animal manure. No other additives are necessary. The high-temperature method produces high-quality compost quickly, but it requires much more effort and care than the slow method.
- You may dig organic materials directly into fallow (unplanted) areas of your garden. This is a good way to improve your soil using food wastes.
- Commercial Composts
Composts are also commercially produced on a large scale from yard debris (woody prunings and grass clippings) and food waste. They vary widely in nutrient availability. Composts that look woody are usually low in nitrogen. Like sawdust, they require additional nitrogen when mixed into the soil.
Commercial composts are screened to separate particle sizes. Mix fine compost with soil to increase organic matter. Use coarse compost for mulching trees and shrubs.
Wastewater Sludge (Biosolids)
- Types of Wastewater Sludge
Sludge, also called biosolids, is a by-product of municipal wastewater treatment. It includes materials separated from the wastewater stream and microbial matter produced during wastewater treatment. It is a good source of plant nutrients and organic matter.
Two types of biosolids are available. Class A biosolids have been treated to be free of disease-causing microbes. It is often sold as a fertilizer for turf. Class B biosolids may still contain disease-causing organisms and must be used with caution. Do not use Class B biosolids where root crops or low-growing food crops will be harvested within the next 3 years. Also, avoid using it where children are likely to play.
Most biosolids currently available from local wastewater treatment plants are Class B, although some plants will be upgrading to Class A. Both types of biosolids usually contain substantial amounts of nitrogen and phosphorus (2-6% N, 1-3% P2O5), but are low in potassium (less than 1% K2O). Relatively small amounts are needed to meet plant needs for nitrogen and phosphorus. Applying 1 pound of biosolids (at 80% moisture content) per square foot of soil is enough to supply the nutrient needs of most plants for a growing season. Do not overapply. Biosolids are also used as an ingredient in some commercial composts. Biosolids composts meet Class A standards. They are usually low in nutrients, but make a good source of organic matter for soil.
- Fresh Sawdust Calls for Extra Nitrogen
Sawdust is often readily available, but is the least desirable source of organic matter. Sawdust ties up nitrogen as it decomposes in the soil, causing plants to suffer or even die from nitrogen deficiency. The nitrogen deficiency from sawdust decomposition often lasts 2-4 years, requiring additional applications of nitrogen to supply plant needs. Mix slow-release forms of nitrogen, such as manure or composted biosolids, with sawdust because they will supply nitrogen for many months as the sawdust decomposes. Sawdust and biosolids or manure should be mixed in a roughly 2-3:1 (sawdust:nitrogen source) ratio. Additional nitrogen fertilizer is often needed in the second and even third year after sawdust addition to maintain plant growth. Watch plants for nitrogen deficiency (pale green leaves and slow growth) and add nitrogen in doses of 1 pound per 1000 square feet of garden as needed. Nitrogen deficiency is usually most apparent during the summer when sawdust decomposition is fastest and plant demand for nitrogen is greatest.
- Fast-Growing Plants Make Green Manure
"Green manure" is a cover crop of plants tilled into the soil. Fast-growing plants, such as wheat, oats, rye, vetch, or crimson clover, can be grown as cover crops between garden crops and then tilled into the garden as it is prepared for the next planting. Green manures absorb and use nutrients from the soil that might otherwise be lost through leaching and return these nutrients to the soil when they are tilled under. The root system of cover crops improves soil structure and helps prevent erosion. Nitrogen-fixing crops such as vetch, peas, and crimson clover add some nitrogen to the soil as they are turned under and decompose. Cover crops also help reduce weed growth during the fall and winter months.
MANAGING AIR AND WATER
- Plant roots need both air and water in the soil. Air fills all pore spaces not occupied by water.
- Garden Soil Type Affects Irrigation
The type of soil in your garden determines how frequently you should irrigate and how much water to apply. Coarse-textured soils (high sand content) contain mostly large pores that cannot hold water and the soil drains very quickly.
Fine-textured soils (high clay content) have mostly small pore spaces and can hold up to one-third their volume of water.
Sandy soils will hold only about 1 inch of water in the top foot of soil. This is usually enough water for three to five days, depending upon the crop and the temperature. It is of little value to apply more than 1 inch of irrigation water at one time to this type of soil, but it must be applied every few days.
Clay soils can hold as much as 4 inches of water in the top foot of soil. Therefore, these fine-textured soils should be watered thoroughly but not too frequently.
- Tips on Managing Air and Water in Soil
Most garden soils fall somewhere between these two extremes. To maintain a desirable balance of air and water in your soil, follow these recommendations:
- When you irrigate, apply enough water to wet the soil to a depth of 12 inches.
- Do not irrigate again until the top 2-3 inches of soil are dry to the touch.
- Use a soil probe or shovel to examine your soil before and 1 day after irrigation to determine how dry the soil is and how deep the water has penetrated.
- Mulches Affect Moisture and Weeds
Use mulches to conserve moisture and help reduce weeds. Mulches can be plastic film, woven plastic geotextiles, or layers of organic refuse spread over the soil surface. Almost any material can be used to mulch around plants providing it allows air and water to penetrate through to the soil below. Only a thin layer is needed to conserve soil moisture; however, for organic mulches use at least 2-4 inches for weed control.
Coarse-textured materials, such as stemmy hay, straw, and wood shavings or chips, are good mulches. Fine or flat materials such as leaves or sawdust must be loosened occasionally to prevent sealing the soil surface. Perforate plastic film to allow water and air to pass through. Woven geotextiles are designed to be permeable to water and air.
Organic materials used as mulches in the summer can be spaded in or plowed down in the fall to build the organic matter content of the soil. If sawdust or shavings are used, add extra nitrogen fertilizer.
MANAGING GARDEN FERTILITY
For more information, see the WSU Extension publication, "How Much Fertilizer Shall I Use - A Gardener's Guide for Converting Tons or Pounds Per Acre into Pints, Cups, Tablespoons, or Teaspoons per Row or Plant"
- Plants obtain mineral elements (plant nutrients) from air, water, and soil. Carbon (C) is taken from the air as carbon-dioxide (CO2) absorbed through the leaves. Hydrogen (H) and oxygen (O) are derived from water. All other elements used by the plant come mainly from the soil and are absorbed into the plant as minerals dissolved in water.
Soils contain variable amounts of nutrient elements available for plant use. If the quantity of one or more nutrient elements is too small, plant growth will be limited. Plant removal of elements, leaching, and erosion gradually reduce the level of available nutrients in most soils. Additional supplies must be returned to the soil as fertilizer to maintain adequate plant growth.
Fertilizers may come from either organic or inorganic sources. The source is not as important as making sure the fertilizers contain the mineral nutrients needed to replace those in low supply in your soil.
- Plants Need Thirteen Mineral Elements
- Plants need many elements for normal growth. Three elements--nitrogen (N), phosphorus (P), and potassium (K)--are used in larger quantities than the others and are called the primary plant nutrients. Plants use large quantities of these nutrients, requiring frequent replacement in soils from which crops are regularly harvested.
Three secondary elements--calcium (Ca), magnesium (Mg), and sulfur (S)--are used in modest amounts by plants and may require periodic replacement in some soils. These elements are replaced by adding minerals containing them, such as ground limestone (calcium carbonate), gypsum (calcium sulfate), or powdered sulfur, to the soil. These minerals are called soil amendments. A single application usually supplies enough nutrients to last several years. Sulfur is also added to soil in ammonium sulfate fertilizer.
A number of other elements, such as boron (B), manganese (Mn), copper (Cu), zinc (Zn), iron (Fe), molybdenum (Mo), and chlorine (Cl), are used by plants in very small amounts and are called micro-nutrients. Most soils contain enough micro-nutrients, so they are added only if some abnormal plant response indicates a deficiency.
- Fertilizers Increase Nutrients in Soil
Fertilizers are materials, either mineral or organic residues, that contain one or more plant nutrients. Fertilizers are added to soil to increase the supply of these nutrients. The term "complete" fertilizer describes a mixture that contains all three primary ingredients: nitrogen (N), phosphorus (P), and potassium (K).
Fertilizers are often called "plant food." Strictly speaking, these materials are not plant food but the raw materials or minerals used by green plants in their growth processes. However, many people use the terms synonymously.
Fertilizers may be either liquid or dry mixtures. The labels on fertilizer containers tell the amount of each of the three primary nutrients expressed as a percent of the total weight of the contents. Nitrogen (N) is always listed first; phosphorus (P) as phosphate (P2O5) is second; and potassium (K) as potash (K2O) is third. For example, fertilizer with a 5-10-10 analysis contains 5% N, 10% P2O5, and 10% K2O. (Footnote: The terms phosphate (P2O5) and potash (K2O) are standard units for expressing the concentration of phosphorus and potassium in a fertilizer. Fertilizer labels, soil test results, and extension bulletins all express amounts of phosphorus and potassium in terms of P205 and K2O. If you need to convert units from P and K to P205 and K2O, the conversions are: 1 pound P = 2.3 pounds P205, and 1 pound K = 1.2 pounds K2O.)
For more information on fertilizer labels, see the Rutgers WSU Extension publication, "Understanding Fertilizer Labels", at http://njaes.rutgers.edu/pubs/publication.asp?pid=FS871
- Spring Is Best Time to Apply Fertilizer
As a general rule, the best time to apply fertilizer is in the spring just as the plants are ready to begin their new season of growth. On lawns, which have a long growing season, you may split the fertilizer application into two - four smaller applications spaced across the growing season.
- Fertilize Root Zones of Trees and Shrubs
Fertilize vegetable gardens and annual flowerbeds when the soil is being tilled and prepared for planting. Broadcast the desired amount of fertilizer evenly over the garden space and till it into the top 6-8 inches of soil. This distributes the fertilizer evenly throughout the root zone of the plants.
Fertilize trees, shrubs, and perennial plants by spreading the desired amount of fertilizer over the root zone (approximately the spread of the branches). Incorporate the fertilizer by shallow cultivation or irrigation. Fertilize lawns by spreading the fertilizer over the lawn surface and irrigating the lawn to wash the fertilizer into the soil. Aerating or power-raking the lawn before fertilizing helps the fertilizer penetrate into the soil faster.
To estimate how much fertilizer to use, we rely on soil tests or information in extension bulletins. Too much fertilizer, as too little, can cause many problems. Plants may be affected by nutrient imbalances, and excess fertilizer can cause groundwater or surface water contamination.
In the absence of a soil test, we assume an average level of native fertility for the region. The following general recommendations are for 1,000 square feet of garden for eastern and western Washington. Recommendations for rates and timing of fertilizer applications for specific crops are in the appropriate extension bulletins.
- Western Washington Fertilizer Formulas
In western Washington, for vegetables and flowers: I pound nitrogen (N), 2 pounds phosphate (P2O5) and 2 pounds of potash (K2O). These amounts of actual nutrients can be supplied by applying 20 pounds of
5-10-10 analysis fertilizer or 10 pounds of 10-20-20 fertilizer. If leaves become pale green in color or growth is slow, the plants need additional nitrogen. Apply this adjacent to the plants and irrigate into the soil.
For woody plants (trees, shrubs, and vines): If you desire more vigorous growth, apply 1/2 pound nitrogen (N) only. This amount of nitrogen can be supplied by applying 2 1/2 pounds ammonium sulfate 21-0-0 or 1 1/2 pounds ammonium nitrate 33-0-0.
If the plants are as large as desirable, and have healthy-looking foliage, you don't need to apply fertilizer.
- Fertilizing Lawns
Lawns generally require nutrients in a 3:1:2 ratio. Apply lawn fertilizer at a rate of 1-pound nitrogen per 1,000 sq. ft. of lawn up to four times each year. Apply it in early May, late June, September and November. Lawn fertilizers containing slow-release or organic forms of nitrogen furnish a steadier supply of nitrogen to the grass. Leaving clippings on the lawn after mowing can reduce the need for fertilizer because some of the nutrients in the clippings are recycled back into the soil. See EB0482, Home Lawns, for details on lawn fertilization.
- Organic Versus Processed Fertilizers
Fertilizers can be either organic or processed. Both types provide the same plant nutrients. Organic fertilizers are used in a near-natural form, with little processing beyond grinding, sterilizing, or composting. Processed fertilizers are produced from raw materials in a manufacturing plant, involving either chemical or physical processes. Organic fertilizers tend to be less concentrated in nutrients than processed fertilizers, and the nutrients in organic fertilizers are usually more slowly released. This means that larger amounts of organic fertilizers are needed, but their effects will last longer. Organic fertilizers are not always balanced in nutrients, and it is more difficult to estimate how much to use compared to commercial fertilizers. But, when we use organic fertilizers we often benefit the environment by recycling materials that would otherwise be wastes, and by saving the energy and natural resources required to make processed fertilizers.
Using Soil Tests
- Soil Tests Reduce Fertilizer Guesswork
Soil tests reduce the guesswork involved in deciding what kind and how much fertilizer to use. Your soil may contain an adequate level of one mineral nutrient and be quite low in another. A test of your soil will enable you to determine the correct amount of each mineral nutrient to apply.
Soil tests are done by laboratories at Oregon State University, the University of Idaho, and by private labs. A soil test result will give you the levels of nutrients in your soil and the recommendation for how much fertilizer your plants will need each year. You don't need to test your soil every year--testing every three to five years is adequate. For more information, see the WSU Extension King County publication, "Soil Testing - Fact Sheet No. 508".
To get useful results from a soil test, take a soil sample that represents your garden and provide information about your crop on the form provided by the soil test lab. When collecting a soil sample, take samples to a depth of 6-8 inches from at least ten spots in your garden. Avoid sampling areas that are not typical of the garden. Mix the samples together well and take a sub-sample to send to the lab.
If only one or two fertilizer nutrients are needed, you may prefer to purchase fertilizers containing only the needed minerals and apply them separately. Some common fertilizer sources are:
Common Fertilizer Sources
|ammonium nitrate||33-0-0||33 % Nitrogen|
|ammonium sulfate||21-0-0||21 % Nitrogen|
|bloodmeal (organic)||16-0-0||16 % Nitrogen|
|sulfer-coated urea (slow release)||36-0-0||36 % Nitrogen|
|treble superphosphate||0-45-0||45 % Phosphate|
|bonemeal (organic)||4-17-0||17 % Phosphate|
|muriate of potash||0-0-60||60 % Potash|
|woodashes (organic)||0-0-?||6 % Potash|
Soil amendments adjust the pH (soil acidity or alkalinity level) and supply calcium (Ca) and magnesium (Mg) in the soil. Most home garden plants grow best in moderately acid soil. Soils with a pH from 5.5-7.0 do not need soil amendments to adjust the pH level.
Lime is calcium carbonate. It will raise soil pH (reduce acidity) and supply calcium. Dolomite lime is calcium-magnesium carbonate. It will supply both calcium and magnesium as well as raise soil pH. Sulfur is used to lower soil pH. Apply soil amendments based on the recommendations from a soil test. The quantities of lime or dolomite recommended should supply enough calcium or magnesium to last several years.
- Gypsum Seldom Improves Soil Structure
Gypsum (calcium sulfate) has been promoted as a soil amendment to improve soil structure. In the vast majority of cases it will not work. Gypsum only improves structure when the problem results from excess sodium in the soil, a rare condition in Washington.
Apply lime and other soil amendments in the fall. On vegetable gardens and flowerbeds, till the lime into the soil. On lawns and perennial plantings, fall application allows the winter rains to dissolve and carry the amendments into the soil. Do not apply lime and fertilizer at the same time. Lime, if needed, should be applied at least 30 days before any fertilizer application.
- Salinity Test Shows Soluble Salts in Soil
The salinity test measures the total soluble salts in the soil. In arid regions of Washington, the total amount of soluble salts can accumulate to problem levels. The following table shows how to interpret a salinity test.
Salt in millimhos per centimeter (mmhos/cm)
4 or above
Severe accumulation of total salts in the soil which may restrict growth of many ornamental plants. Salt level should be reduced by leaching.
Moderate accumulation of salts. Will not restrict the growth of ornamental plants, but may require more frequent irrigation to prevent wilting.
2 or less
Low salt accumulation. Well below the tolerance of all ornamental plants.
Leaching is the practice of applying irrigation water in excess of the water-holding capacity of the soil. This excess water must drain downward through the soil, and in so doing, it carries with it the excess soluble salts. Three inches of excess water will remove about 50 percent of the soluble salts in a soil. Five inches of excess water will remove approximately 90 percent. When leaching, apply water at a rate that does not exceed the permeability of the subsoil.
SEASONS AND SOIL
Soil Management Practices by Season
Spade or plow down cover crops.
Add fertilizers as needed.
Prepare seedbeds for planting.
Cultivate shallowly to control weeds.
Irrigate as needed.
Irrigate as needed.
Mulch to conserve moisture and control weeds.
Use shallow cultivation as needed to control weeds. Build compost pile with organic refuse (grass clippings, weeds, vegetable trimmings, etc.)
Add soil amendments if needed.
Spade or plow down organic refuse (compost, leaves, grass clippings, cornstalks, etc.)
Plant cover crops.
Add manure to soil if runoff is not a problem.
For more information contact your local WSU Extension Office.
Revised by Craig Cogger, WSU Extension Soil Scientist. Washington State
University Extension Bulletin 1102. Link update 06/11, Dave Pehling, WSU
Extension Snohomish County