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Reviewed July 2002

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Contents

• Conserving soils
• Improving soils
• Special soil situations
• Soil amendment materials

Related publications

• G6956, Making and Using Compost
• G6985, Raised-Bed Gardening

Use our feedback form for questions or comments about G6955.

Figure 1
Mound planting.

Improving Lawn and Landscape Soils

Christopher J. Starbuck
Department of Horticulture

Poor soil is a common cause for failure to grow vigorous, attractive lawns, trees and shrubs. Natural topsoils vary greatly from one location to another. Their ability to grow plants can differ from block to block, from farm to farm or from the bottom of a slope to the top.

Conserving soils

When topsoil is eroded away or removed or buried during construction, it becomes more difficult to grow plants in the remaining soil. How well we conserve and improve the soil largely determines whether plants will be vigorous or if they will survive at all.

When building a new home, the desirable topsoil should be stockpiled during construction and replaced after construction is completed.

The lot grade or slope is also important in soil conservation. Whenever possible, provide good surface drainage without creating steep slopes, depressed areas or large level areas. The average lot should be raised at least 6 inches at the center or around the buildings. Soil types that tend to hold water need more slope than those that dry quickly after rains. Soil improvement should be made after grading is completed.

Improving soils

Lawn and gardens
The lawn and gardens will be in place for many years. While a garden may be fertilized later, the lawn becomes difficult and often impractical to dig up after it is established. Therefore, it is important to get the soil well prepared before planting.

Dark color and crumbly texture may indicate good soil but are not a guarantee that the soil contains all the necessary nutrients. Have a soil test run before the soil is prepared so that fertilizer deficiencies may be corrected as well as pH. After the soil test results have been returned, work any recommended materials into the upper 6 inches of soil.

Organic matter is very important to successful plant growth. At least 2 percent organic matter should be present for growing lawn grasses. Other garden plants will thrive in a soil with about 5 percent organic matter. To add this organic matter spread 1 to 3 inches of peat, compost or well-rotted manure over the soil when other nutrients are added. All these should then be worked to a depth of about 6 inches into the existing soil.

Surface applications of organic matter do not provide the soil aeration, moisture regulation and deep root penetration that is possible when organic matter is mixed into the soil.

In addition to incorporation in the soil, a mulch of organic matter is beneficial for many flowers, vegetables, trees and shrubs. Top-dressing of lawns may be done with good topsoil or topsoil-organic material combinations to smooth out irregular areas in the lawn or help decay thatch. Top-dressing does not generally improve the texture or performance of tight, poorly drained soils unless it is done in combination with core aerification.

Trees and shrubs

• Selection
  Select trees and shrubs that are adapted to soil conditions difficult to change. Know whether a plant requires well-drained soil, is drought tolerant, or water loving.
  If the proper plants are chosen for the location, soil conditioning will help improve their transplant survival, hasten resumption of normal growth and increase their lifelong vigor.
  
• Planting
  When planting trees and shrubs, always dig generous planting holes shallow and wide. The holes should be at least twice the width of the root ball or container in which the plant is growing. Do not dig the hole any deeper than the root ball of the tree or shrub to be planted. Soil loosened below the planting ball will settle over time, creating a planting depression that will accumulate excess water. In heavy clay soils, it is preferable to plant trees and shrubs a few inches above the grade of the surrounding soil. In extreme cases, bermed plantings that permit root development above existing compacted soil may be preferable. See Figure 1.
  While surface drainage is easy to observe, internal drainage of soil is often not known. Internal drainage can be checked quite easily. Dig planting holes in advance, and fill them with water. If all the water drains out in the first 18 to 24 hours, drainage is satisfactory. If the water stays for two days or longer, select water-tolerant plants or plant in a raised bed or berm. Placing gravel at the bottom of the planting hole will not improve the internal drainage.
  Amending the soil used to backfill planting holes provides questionable benefits. If any soil amendment is done in the planting hole, provide for a gradual transition of soil types (textures). If the backfill soil is drastically different from the existing soil, roots will not readily penetrate the soil around the planting hole. Strong soil textural differences also affect soil water movement. If the container growing mix is high in peat and surrounding soil is clay, water will tend to move out of the root ball. Excessively wet root zones may result when the tree or shrub's root ball is high in clay content and backfill soil contains a high level of organic matter. See Figure 2.

Figure 2
Effects on water movement when transplanting into clay soil.

1. When the root ball is also composed of clay soil, water tends to move into the clay and saturate the root ball.
2. When the root ball is composed of a light mix, such as peat, water tends to move away from the root ball. (See arrows.)
   

A small amount of organic matter added to the backfill soil is generally beneficial. Incorporate most of the organic matter into the top 6 to 12 inches of soil in a large area around the planting hole. Improving the soil surrounding the planting hole provides a good base for new root growth in areas where most new roots will develop.

Special soil situations

Raised beds
Where soil or drainage is a problem that can not easily or economically be corrected, raised beds can add several inches of good soil in a well drained area. Beds can be constructed of brick edging, masonry or rot-resistant timbers and filled with soil previously combined with organic matter and nutrients. Low mounds of soil without edging can add several inches of desirable soil. See Figure 3.

Figure 3
Examples of raised beds for clays and other soils of poor structure.

Soil for containers
Planters, patio wells, tubs and pots have their own soil needs. Excellent drainage is essential. Good drainage means that watering must be done frequently. All containers should have drain holes. Use a well-drained, light-weight potting mix designed for growing containerized plants. For very large containers or raised beds, mixing topsoil with a mineral material such as calcined clay or expanded shale can help maintain good trainage for many years.

Soil amendment materials

Many kinds of materials are available for soil improvement. Thorough blending of these amendments with the soil is very important. Plant roots may not cross a boundary between distinctly different types or textures of soils or soil amendment materials. Heavy top-dressing, poor mixing or other practices that cause distinct layering should be avoided.

Peat
An assortment of materials generally referred to as "peatmoss" is the most readily available material for soil conditioning. Peat in the soil creates favorable changes. It makes the soil more granular and more easily worked. It hastens the escape of excess water and, at the same time, absorbs and holds more water for plants. Peat allows more air to enter the soil, thus encouraging the growth of roots and microorganisms that help make plant nutrients more available. Most commercial peats are free of weed seeds and plant diseases. Differences between sources of peats will cause variation in acidity (pH), ability to hold water and nutrient content (Table 1).

Although peat contains some plant nutrients, it should not be considered a fertilizer. It is the long-term improvement in the soil's physical characteristics that make it valuable. Peat materials that are black and fine-textured are of little value for improving soil structure.

Manure
Results when using manure are less consistent because manure varies in nutrient value, degree of decomposition, and freedom from weed seeds and disease organisms. It contains more nutrients than peat, and its acidity is usually less. It will not last in the soil as long as peat.

In some areas barnyard manure is readily available. Although nutrients are lost in aging, thoroughly rotted, old manure is much better for plants than the fresh, strawy material. The greatest disadvantage of manure is the weed seeds that are often present. Manure with a high content of bedding materials may rob more nitrogen from the soil than it provides.

Composted or dried manure is more desirable since weed seeds and potential human pathogens have been killed. It is packaged for more convenient handling and often has plant nutrients added to bring it to a standard fertility level.

Fresh manure should not be used except as a light top-dressing on beds. If used in this way it should not touch any plant stems or leaves.

Processed or rotted manure may be used in fairly large quantities as a soil amendment, mulch or top-dressing. Manure that has been rotted and exposed to weather may be used more liberally than the processed, bagged, dried manure. A layer 1 to 3 inches deep may be incorporated when preparing a flower bed or lawn seedbed, while no more than 1 to 2 inches of the processed type should be used.

Poultry manure contains greater amounts of nitrogen with less litter, and therefore should be used more cautiously than other manures. When preparing flower beds and lawn seedbeds only a 1- to 2-inch layer of rotted poultry manure should be added. If processed types are used, only a 3/4- to 1-inch layer should be incorporated.

Table 1
Major characteristics of peats.

Wood products
Sawdust, wood shavings, shredded wood, pulverized bark and wood chips can be used to improve soils. In the raw or fresh state these products are low in nitrogen. When they decay, nitrogen from the surrounding soil is used and the plants become starved for nitrogen. This can be prevented by the addition of nitrogen fertilizer to the material when it is mixed with soil or composted (Table 2). For best results, wood byproducts should be composted first before being added as a soil amendment. Very coarse wood products should be used only for mulching.

Table 2
Nitrogen addition per volume to wood byproducts.

In addition to the incorporation of nitrogen into the soil, two or three more small applications of nitrogen should be made at three- to five-week intervals during the spring and summer to relieve extended nitrogen starvation when wood byproducts are used directly as a soil amendment.

Wood ash is strongly alkaline, containing mostly lime and potassium. It is a soil conditioner only in the manner of limestone and is most useful in treating strongly acid soils.

Sewage sludge
Dried sewage sludge is often available from sewage treatment plants. Its composition and nutrients vary with the source and the disposal process. Most sludge available in bulk form is lower in nutrients than manure. It is similar to peat for improving soil texture and should be used at the same rates as peat or composted manure.

Compost
Compost is very beneficial for soil improvement. It offers many of the same features as manure and may be used at the same rates. It can be used in potting soil, in the preparation of flower beds and gardens, and as a mulch for trees and shrubs. Chopped straw, leaves, grass clippings, weeds and other plant refuse may be composted.

To start the compost heap, place a 6- to 8-inch layer of plant materials in a well-ventilated bin. Moisten these materials, but do not soak them. Use a mix of dried and fresh plant refuse to achieve a good carbon-to-nitrogen ratio. If green (high nitrogen) plant materials are unavailable, use manure or commercial fertilizer as a nitrogen source. Sprinkle 1 cup of a garden fertilizer such as 6-10-4 or 12-12-12 for each 25 square feet. The layer should then be covered with 1/2 to 1 inch of soil.

Use several layers to complete the heap. Keep the heap moist but not soaking wet. Make the top of the heap flat or slightly depressed in the center so that rainfall can soak in. During warm weather the pile should be turned approximately every month, but during the winter months turning will not be necessary. It will take 4 to 12 months for the materials to decompose thoroughly, depending on the frequency with which the compost heap is turned.

Sand
The grades of sand used in construction are most readily available. This sand has little water-holding capacity and no nutrient value. Seventy-five percent or more by volume must be mixed into heavy clay soils before there is any improvement in aeration, internal drainage or texture. Adding lesser amounts of sand can compound aeration and drainage problems. Sand mixed with clay can set up into a bricklike mixture. Peat or compost should be used instead to improve heavy clay soils.

Sand applied as a top-dressing often seems to disappear into the soil, but actually remains as a surface layer. Therefore, sand should not be used alone as a top-dressing.

Limestone
Agricultural limestone is commonly used to supply calcium and to regulate soil acidity but is overlooked for its ability to improve the texture of heavy soils. If soils contain enough limestone, do not add more, because the balance of soil nutrients can be upset by the use of too much lime. For best results, use lime only as recommended based on results of a soil test.

Other agricultural byproducts
Peanut shells, tobacco stems, rice hulls, corn cobs and other fibrous agricultural byproducts are sometimes available for soil additives or mulching. Most of these materials when crushed to a fine texture with nitrogen added (Table 2) will be good soil conditioners. In their coarse state, these materials are suitable for mulching.

Meat meal, tankage, fish meal and cottonseed meal have little soil conditioning ability. However, since they contain from 6 to 10 percent nitrogen, they are useful for side-dressing many ornamental plants.

G6955, reviewed July 2002