Know Your Soil and How to Manage It

by Linda J. Bartelli, David F. Slasher, and Kelton L. Anderson.

Linda J. Bartelli retired in December 1976 as Director, Soil Survey Interpretations Division, Soil Conservation Service. David F. Slasher is Assistant Director of the Division. Kelton L. Anderson is Leader, Extension Agronomy, Mississippi State University.

Essential elements of successful fruit and vegetable gardens are suitable soil, an adequate water supply, enough sunlight, and climatically adapted plants. These, of course, are supplemented by the gardener's know-how and good management. Water, sunlight, and suitable plants are available to most home gardeners. Well suited soils may not be.

The soil must permit adequate root growth to support the plant and must supply water and oxygen. It should be free of toxic elements. An ideal garden soil is nearly level or gently sloping and has favorable air and water movement. It is medium acid to neutral, pH 5.5 to 7.0, and has a good supply of organic matter in the surface layer.

How then does the home gardener, except by trial and error, determine how suitable the soil is or what properties need special attention to make it as productive as possible? A good guide is a soil survey.

Soil surveys have been made and published for more than a third of the counties in the United States by the U.S. Department of Agriculture in cooperation with State and other Federal agencies. Each year about a hundred additional soil surveys are made and published. Copies for a county are available in most public libraries. They include maps on which the gardener can locate his garden plot and determine the name of the soil.

Text of the soil survey describes the soil in terms of properties important to plant growth, such as depth of root zone, available water capacity, soil texture, wetness or soil drainage, acidity or alkalinity, and the rate of air and water movement. General statements of the organic matter content, fertility level, and the need for lime and fertilizer are often given. Plants suitable for certain soils are also listed.

If the land area contains two or more soils, you can select the most desirable for your garden. The soil survey can be very useful in selecting the most desirable sites if such alternatives exist.

Section of soil survey map showing kinds of soil by symbols within each outlined soil area. County soil survey report describes characteristics of soils. Besides soil areas, this map section includes a town name (Gladys), a railroad, road intersection, and powerline.

The major soil type in each area outlined on the soil map is given a name. The soil description gives the properties of each soil layer, usually to a depth of several feet. From the description you can learn about the soil properties important to plant growth and some of the practices that may be helpful in good soil management.

Many soils in urban areas have been disturbed to some degree during excavation for utilities and foundations for buildings and during construction of roads and walks. In areas of naturally steep slopes, cuts and fills may have altered the soils drastically. However, the soil survey can be helpful because it states the properties of underlying material in cuts or can be used to predict the composition of fill or soil moved only a short distance as a result of site preparation.

Most grading operations around homesites result in a greater degree Of soil compaction than that in natural soils, and water or root penetration may be restricted.

If you have an opportunity to supervise grading during site preparations, you should see that the natural topsoil is stockpiled and used as top dressing in the final grading and smoothing operations.

Examine soil to depth of two or three feet.

Soil is a mixture of mineral particles, organic matter, air, and water. It typically occurs as a series of horizontal layers from deposition of geologic material or the result of soil-forming processes. The surface layer of an undisturbed soil normally is a more desirable medium for plant growth than underlying layers.

Relative composition of desirable soils is about 45% mineral matter, 5% organic matter, 25% air, and 25% water. Also important are depth to water table, permeability to air and water, and available water capacity. These are all given in the soil survey.

If no soil survey is available, you should examine the soil to a depth of 2 or 3 feet to evaluate its properties. It's best to do this when the soil is moist but any time is O.K. Use an auger or spade, dig down a few inches at a time, and examine each layer for thickness, color, stickiness, texture, and content of stones. If the soil is dry, moisten a small amount with water and rub it between your fingers to note the sand content and stickiness. The importance of these and other properties in gardening are discussed separately.

Texture A soil contains different sizes of particles called sand, silt, or clay. Clay is the smallest and sand the largest. A loamy soil has a desirable amount of each. For example, a mixture containing about 20% clay, 40% silt, and 40% sand is a loam a desirable texture for a garden soil.

Slightly different composition results in textures of silt loam, sandy loam, or clay loam, all of which are desirable for garden soils. Texture of soil layers of each soil is given in the soil survey.

Too many clay particles result in a soil that feels smooth and plastic when moist and retains its shape when molded. Clays have very small pores, and air and water movement is too slow. They are sticky and plastic when wet, and hard when dry.

Too many sand particles make a texture that is loose and feels gritty between the fingers. Sands are easy to work but do not have the capacity to store water for plants. Some soils contain rocks and stones that interfere with cultivation unless removed.

Surface texture can be changed by adding silt, sand or clay and working it into the surface layer. This is quite difficult and seldom practical on a good-sized garden. A better alternative is to cover the garden with 4 to 6 inches of desirable topsoil from some other source.

Five cubic yards (a common truck load) will cover a 400-square-foot garden (20x20) with 4 inches of soil. The same amount will cover a 270-square-foot garden (16x17) with 6 inches of soil.

Your soil survey also will rate the suitability of the different soils in the area as a source of topsoil.

Some effects of unfavorable surface soil texture can be overcome by adding organic material. That and other important functions of organic matter will be discussed later.

Soil drainage, permeability Garden plants grow best in soils with good drainage and aeration. The drainage property is given in the soil survey in terms such as well drained or somewhat poorly drained. Permeability is the rate of water movement through the soil. The rate is given in the soil survey as inches per hour or in terms such as moderate permeability or slow permeability.

Another way to find out about drainage is to dig a hole a few inches across and about 3 feet deep. Observe the hole for presence of water, especially during the wettest part of the season. If the water table (free water level) is near the surface during the growing season, most plants can't be expected to do well unless a drainage system is installed.

For technical assistance, contact your soil conservation district office, Extension office, or a drainage contractor.