Vegetable Garden Soils

Vegetable plants grow best in a fertile, well-drained, loamy soil. Sandy loam soils, well-supplied with organic matter, are easily worked and are quite productive. Clay and sandy soils must be amended for successful vegetable gardening. To obtain a desirable soil for gardening, consider the following: water drainage, moisture condition at thetime of working, erosion, texture and structure, fertility, and pH.

Water Drainage and Erosion

Satisfactory vegetable production cannot be expected on poorly-drained soils. In home gardens, improving subsurface drainage is not practical, therefore, surface drainage is very important. The surface should be graded so that excess water will be removed promptly but in a controlled manner to prevent erosion. Leveling will eliminate  pockets  and low spots where water tends to stand for long periods. Adding organic matter to clayand clay-loam soils will also improve drainage and aeration.

Growing vegetables in raised beds or on planting ridges is another option for controlling drainage. However, keep in mind that proper water management during periods of drought may be more difficult in raised beds.

Moisture Conditions at Time of Working

Many anxious gardeners work the soil in early spring when it is still too wet. The soil should not be tilled or spaded until it is sufficiently dry to crumble when worked. Use the "squeeze test” to determine if the soil is ready: use a spade to turn over a slice of soil about 6 inches deep, then pick up a handful of soil and squeeze it. If the soil remains in a tight ball when pressure is released, wait several days (without rainfall) before spading or tilling. If the soil crumbles when pressure is released, it is ready for working.

Some clay soils benefit from fall tilling or spading. Such soils will be loose and fluffy in the spring as a result of the conditioning effect of winter freezing and thawing. Fall-worked soils often require only leveling and a light raking in the spring before planting. Fall-worked soils can be compacted quite easily, however, if worked when too wet.

Texture and Structure

Soil texture and structure can be modified by using soil conditioners which act to improve soil aeration, drainage, moisture-holding capacity, and tilth, or workability, of thesoil. Commonly used soil conditioners include compost, peat moss, sawdust, wood chips, composted animal manures, green manure crops, coarse sand, and perlite. By incorporating coarse sand and organic matter into a garden soil, the gardener can, over time, produce a desirable loamy-type soil. The addition of fine sand, however, into some soils, especially clay, will be detrimental to the soil structure.

A common mistake made when attempting to improve garden soil is failing to use enough soil conditioning material. For soil that is mostly clay or sand, large amounts of conditioners must be used to effectively improve texture and structure. Condition the soil by applying 3 to 4 inches of organic matter and, if available, 1 to 2 inches of coarse sand uniformly over the surface of the garden. Till or spade the material thoroughly into the top 8 to 10 inches of soil.

Garden soils benefit by being protected by a winter crop such as winter rye. Cover crops prevent soil erosion and add organic matter when turned under in the spring. Cover crops should be turned over before growth is so tall they are difficult to handle. Cover crops may be seeded in August or September between garden rows before vegetables are harvested.

Fertility and pH

Vegetable plants require several nutrient elements for growth, development, and productivity: carbon,  hydrogen, and oxygen are available from the atmosphere and from water; nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are supplied from the mineral portion of the soil. In addition, micronutrients, or trace elements,  used in small quantities, include boron, chlorine, copper, iron, manganese, molybdenum, and zinc. These 16 plant nutrients, whether used in large or small quantities, are absolutely essential for plant vigor and productivity. A deficiency of any of these nutrient elements can limit plant growth and development and, ultimately, yield. Most soils contain sufficient amounts of the micronutrients needed to support plant growth. However, soils may be lacking in nitrogen, phosphorus, and potassium. The gardener must ensure the presence of all the essential elements supplied by the soil in the right quantities and the right chemical forms for plant use. This is done by supplying organic matter and by the judicious use of fertilizers to maintain or increase soil fertility.

Most vegetable crops prefer a slightly acid pH of 6.0 to 6.8, although this varies with each crop. Nutrient elements in the soil may be chemically tied up or bound to soil particles and unavailable to plants if the pH is outside of this range. Soil pH can be raised by applying ground agricultural limestone containing calcium and some magnesium. Lowering soil pH is achieved by adding sulfur, either in its elemental form or as a component of some fertilizers.

The gardener cannot assume fertile soils will not require a periodic fertilizer application. Therefore, it is essential, in order to determine the fertility level and pH of garden soil, to do a soil test every two years.Nitrogen is the plant nutrient most often in short supply in the soil. It is usually necessary to supply additional nitrogen  each season. Phosphorus and potassium are also required in large amounts. Nitrogen is essential for vigorous vegetative growth and development. Phosphorus is necessary for good root development and for fruit and seed production. Potassium is important for overall plant development. These are the three nutrients that are almost always present in commercial fertilizers. Organic fertilizers contain nutrients in forms that must be chemically changed in the soil before the plants can use them. These nutrients, although not immediately available, are usable to the plants over a longer period of time. Some chemical fertilizers will contain nutrients in both readily available and slow-release forms. Organic fertilizers are less likely to burn plants and often contain several micronutrients in addition to nitrogen, phosphorus, and potassium.

Lead in Soils

Garden soils contaminated with lead can pose a health risk if vegetables and fruits from the garden are consumed. Soil can be contaminated with lead from several sources—paint residues, industrial sites, industrial sludge with heavy metals, old lead plumbing pipes or even old orchard sites in production when lead arsenate was in use. From a gardener’s viewpoint, lead contamination is forever. Without remedial action, soil lead levels may never return to normal. The effects of lead contamination can raise the lead level in the body and thus poses a substantial health risk. Young children under the age of six and pregnant women are at the greatest risk.

Testing for lead will help to evaluate the lead hazard level. Ultimately, the risk is based on exposure. Soil samples should be taken from several areas to determine the  location of the contamination. The greatest lead concentration is in the top 1 to 2 inches of soil. Children’s play areas or vegetable gardens should be sampled separately. Avoid mixing several sites into one sample. Sample high-risk areas to locate potential problems.

Adapted from: Marianne Riofrio and E.C. Wittmeyer, Ohio State University Extension, 2000; Denise D. Sharp and David L. Clement, Maryland Cooperative Extension Service, 2001