Making efficient use of nitrogen (N) for annual crops is a challenge. If insufficient N is available to meet the needs of a crop, yields suffer. Applying too much N wastes money and threatens water quality. Optimizing the use of N fertilizer means applying only that which is needed, at the right time, in the right place. The following strategies comprise good N management:
1. Know the nutrient needs of the crop. The amount of N removed from soil by vegetable crops varies widely. A 250 bushel snap bean crop requires about 30 lb of N/acre, while a 30 ton tomato yield will require around 200 lb of N/acre. The New England Vegetable Management Guide lists the nutrient needs of many vegetables. Plan to give the crop what it needs and no more.
2. Estimate non-fertilizer N contributions. Not all of the N needed by a crop has to be supplied by chemical, or inorganic, fertilizers. Soil organic matter, animal and green manures, and crop residues (known as organic N sources) also supply N to a crop. Conservative estimates of organic N contributions are: 10 lb/acre from each 1% soil organic matter, 5 lb/ton/acre from incorporated manure, and 100 lb/acre from a vigorous legume sod.
3. Consider the timing of crop N uptake. Crops don't use much N early in their growth. Because fertilizer N is soluble, pre-plant applications are vulnerable to leaching before being taken-up by roots. Generally, it is more efficient to delay application of some fertilizer N until the time of maximum crop uptake. This is usually several weeks after crop emergence or transplanting. Bulky organic N sources, like manures, are of necessity incorporated before planting. If applied in sufficient quantity, these can meet the N needs of a crop. However, N from organic sources is released rather slowly, and is most available in warm rather than cool soil. Therefore, some starter N fertilizer is advisable for early crops to supplement organic N.
4. Place fertilizer where it is needed. Although organic residues are usually spread over the entire field due to their large volume, soluble fertilizers should be placed near plant roots to facilitate nutrient uptake. Water your starter fertilizer in with transplants, band it near the seed, or incorporate it into beds. Sidedressed N should be placed a few inches from the plant rows.
5. Use cover crops. At the end of the season, much soil N may be available when no crop is present to use it. A winter cover crop such as rye can capture this residual N before it is lost to leaching. Cover crops also add organic matter to soil, improving its physical condition, or structure, thus promoting crop root growth and fertilizer recovery. In addition, legume cover crops such as clovers, hairy vetch and alfalfa are able to fix "free" N from the air, some of which can be used by a subsequent crop.