Vegetable plants require a number of nutrients to grow properly. Different types of plants have different specific needs, but, generally, all plants require a basic one part nitrogen to one part phosphorus to one part potassium mixture most of all. The recommendation is to take a soil sampling in to your local agricultural extension office for testing to find out what you need to get the correct proportion for your garden.
Nitrogen is a key component in chlorophyll, the food plants produce for themselves, and is what makes the plant green. Nitrogen is needed by all living things, found in the air, but must be converted to a usable form for plants. If your plants start to turn yellow, they're likely in need of more nitrogen.
If you compost leaves, plants or other organic matter in the garden, your bed is likely lacking nitrogen since microorganisms will strip nitrogen out to decompose matter. Composting in a bin and then spreading your finished product in the garden keeps nitrogen in the soil available for the plants.
Nitrogen is available in a fertilizer form from full-service garden centers or in the manure of herbivorous animals. Avoid touching plants with fresh manure, as the nitrogen levels are so high that they will burn the plant. Manure that has been composted first is best for placing around existing plants; fresh manure works best when tilled into the soil before planting.
Phosphorus, also known as phosphate, is a cell-building block in all living things. Derived from decomposed plant and animal matter, phosphate builds up easily and is not as regularly needed as an amendment in the garden. Phosphorus is paramount to growing your plant to maturity. Adding nitrogen often to the soil keeps the phosphorus levels in check.
Potassium, sometimes known as potash in fertilizer, makes for strong stalks just as the mineral is important to build strong bones in animals. Potassium promotes strong root growth and reproduction. Manure, again, is an excellent source of potash for your garden.
A number of other nutrients are found naturally in good soil including magnesium, carbon, calcium and sulfur among others. Different vegetables may have variations on needs, so having your soil tested as understanding what you wish to grow will help you decide what, if any, amendments are needed. Talk to your local extension office for hints and tips on your fertilizer needs.