Soil Amendments and Cover Crops

Most soils benefit from the addition of organic matter. In addition to improving soil structure, water retention and drainage, aeration, and the quality of nursery stock grown, digging is usually easier in mineral soils that have been amended with organic matter. Also, some nursery species develop a more fibrous root system as the amount of organic matter is increased.

Unfortunately, costs may prohibit transporting significant quantities of bark, compost, or other organic amendments to any but the most intensively cultivated sites like seed beds or transplant production beds. Animal wastes such as cattle manures or poultry litter can be applied to fields, but only in light applications of 1/4 to 1/2 inch over surface areas, or better yet, incorporated into the soil after application. If wastes are incorporated, 75 to 100 percent of the nitrogen in the waste may be available the first year. Rate of application should be based on nutrient analysis of animal wastes. Waste analysis on samples is available for a $4.00 fee from the Agronomic Division of the North Carolina Soil Testing Lab ( 4300 Reedy Creek Road, Raleigh, N.C. 27607; 919-733-2655). Foliar tissue analysis of fully expanded leaves collected early in the growing season can provide valuable information about the efficiency of the animal waste application and determine if any supplement is required. (For more details refer to the North Carolina Agricultural Chemicals Manual, Livestock Manure Production and Nutrient Content Section or contact your County Extension Center for soil fact sheets on manures as fertilizer sources).

Composts from municipal yard wastes may become an affordable organic source for amending fields. Application rates of stabilized composted wastes can be 50 to 200 tons per acre since composted yard wastes may have only 0.2 to 0.5 percent nitrogen content and nutrient loss is of less concern. The 50 tons per acre application rate represents approximately 1/2-inch coverage over a 1-acre area, while the 200 tons per acre would be approximately a 2-inch depth.

An alternative to applying organic materials over the entire field is to incorporate the organic matter in planting rows only. If rows in the field are spaced 12 feet apart and the root zone area of plants is considered to be 2 feet on each side of the stem, a 4-foot strip would receive the organic matter, thus reducing the amount of organic matter applied in the field by two-thirds.

Traditional methods to increase organic matter in fields include green manure crop rotation. Since the primary concern with a green manure program is increasing organic matter levels in the soil, grasses and small grains are generally used in a double cropping system. Small grains are sown in the fall then killed with herbicide or plowed in before they produce seed in the spring (Table 1). Sorghum-sudan hybrids are commonly used as summer cover crops sown in April or May. Sorghum-sudan hybrids should be mowed at least twice to prevent seed formation, then they are generally plowed under in the fall. Buckwheat should be plowed down after the first major bloom and allowed to reseed.

Field Preparation

Whether or not you decide to use a green manure program, it will be necessary to mix in previous crop stubble, fertilizer, lime, and soil amendments. Tall weeds should be mowed before seed dispersal then disced or plowed under. This will permit more effective soil mixing during plowing and minimize the problem of long, coarse stems becoming entwined in equipment.

A chisel plow will efficiently mix materials in the soil profile, ensuring an even distribution of materials and rapid root development during the growing season. Rotavating can also prove effective but generally does not mix the soil as deeply as a chisel plow. Avoid the use of a mold board plow, because it tends to deposit a layer of material beneath the soil rather than distributing it evenly throughout the soil. For best results, chisel plow twice. If erosion is not a problem, plow first across the field in one direction then plow at right angles to the first path. Terraces and contours should not be destroyed during chisel plowing.