The three main nutrients vegetable plants require are nitrogen, phosphorus and potassium. Certain amounts may be present in the soil, but it is often necessary to supply additional nutrients to ensure an abundant harvest. This can be achieved by having a soil test done at an extension office or soil lab prior to planting, and every three or four years thereafter.
The first test result to check is the pH level. This shows the acidity or alkalinity of the soil. If the test shows a reading 6.0 to 6.8, the garden is in good shape for most vegetables. A pH above 7 is considered alkaline and requires sulfur to be mixed into the soil. Lime can be added to raise the pH if it reads below 7.
Nitrogen promotes strong, healthy growth and gives plants such as spinach, lettuce and Swiss chard their dark green leaves. Sweet corn, onions and potatoes also need plenty of nitrogen. If the soil is low in nitrogen, the leaves will turn a pale yellow. When purchasing nitrogen fertilizer, refer to the chemical symbol (N) found on the product label. This states the percentage of nitrogen in that particular nutrient and gives directions for quantities.
Phosphorus encourages root growth. It also provides energy for potatoes, sweet potatoes and root crops such as carrots, beets and turnips. If the soil is low on phosphorus, a plant will be slow growing and the lower leaves may turn purple, dark green, dark blue or reddish. Refer to the product label for directions and quantities.
Potassium, or "potash," helps vegetables fight off disease and bear fruit. It also helps plants endure stress from heat and drought. Plants that are low in potassium have weak stalks and are poorly developed. Refer to the product label for directions. All fertilizers come with instructions, and if followed properly, will prevent damage to plants.
Compost is broken-down organic matter and is a good soil amendment. It improves soil nutrient and texture. Compost also breaks up clay, helps sandy soil retain moisture and encourages beneficial earthworms. Good items for composting are lawn and yard clippings, leaves and manure. Kitchen scraps--other than meats--add to a good compost.