Tomato farmers test their soils before planting at various depths in the tomato rooting zone, which runs up to a foot deep, for needed nutrients, according to the University of California at Davis. While tomatoes are growing, whole-leaf tissue testing can tell the plant's levels of nutrients, so that the farm can produce maximum yields by continued fertilization.
Home gardeners can simply apply a complete garden fertilizer to the soil before planting. Avoid over-fertilizing, as it may be detrimental to the crop and create runoff into waterways.
Nitrogen helps the leaves and stems grow vigorously. Too much nitrogen creates strong thick stems, curled leaves in the head of the plant and poor fruit set, according to the University of Vermont Extension. Too little results in light green foliage and pale yellow flowers.
Use a fertilizer low in nitrogen (N), high in phosphorus (P) and medium to high in potassium (K) for tomatoes, recommends the University of Missouri Extension. Look for analyses such as 8-32-16 and 6-24-24. Work 1 pound of fertilizer for every 100 square feet into the top 6 inches of soil about two weeks before planting.
Phosphorus assists early root growth, especially in cool soil, and assists vegetative growth and fruit set. Symptoms of phosphorus deficiency include a purple veins and stem and thin growth and poor cluster development. In greenhouses, the tomato crop requires 45 pounds of phosphorus per acre.
Potassium improves growth and fruit quality and ripening. The ratio between potassium and nitrogen also matters: The lower the ratio, the faster the growth. Blotchy ripening and boxy fruit may trace to low levels of potassium. In greenhouses, the tomato crop requires 600 pounds of potassium per acre.
Calcium deficiency manifests itself as blossom-end rot of the fruit (which appears as a black bottom to the fruit) and as the growing leaf tips die. The plant often is stressed by from inadequate or uneven watering, variations in relative humidity or a high level of salts. Add agricultural lime to the soil and use nitrate nitrogen as the nitrogen source rather than ammoniacal nitrogen, the Ohio State University Extension recommends.
Farmers may check their fields for zinc; amounts in the soil of less than 0.5 parts per million indicate the need for more of this nutrient.
Greenhouse tomatoes may display magnesium deficiency, visible as yellowing between the leaf veins. While this rarely results in yield reduction, according to the University of Vermont Extension, the yellowing leaf tissue can create entry points for diseases.
Correct magnesium deficiency by applying an organically approved epsom salts fertilizer (magnesium sulfate) watered directly onto plant rows. In greenhouses, the tomato crop requires 260 pounds of magnesium per acre and 40 pounds of calcium.