Since Native Americans taught the first European settlers to supplement the soil when growing corn, American farmers have been studying how to encourage more abundant harvests. Modern vegetable gardeners have a vast array of products available to help them coax the most out of their plots. Knowing what's important to plant growth and development can help gardeners make the right decisions about what to use and when.
All plants need 12 elements from the soil. The three elements used in the greatest abundance, nitrogen, phosphorus and potassium, are referred to as the macronutrients. Calcium, magnesium and sulfur, which are needed in smaller quantities, are known as secondary macronutrients. Boron, copper, iron, manganese, molybdenum and zinc are vital as well, but in small quantities. These are the micronutrients. Soil rarely has sufficient quantities of the macronutrients to raise a vegetable garden, and most certainly would not have enough for a second season.
Since the last half of the 19th century, agricultural practice has focused on the macro-nutrients, nitrogen, phosphorus and potassium. Synthetic fertilizers marketed in the United States are all labeled with their percentage by weight of the three elements. For example, a bag of fertilizer with 13-13-13 on the label has 13 percent of each nitrogen, phosphorus and potassium. The elements are commonly abbreviated on packages as NPK, with the percentages always presented in the same order. It's common for fertilizers to have unbalanced amounts of the elements, for example 6-1-12 has twice as much potassium as nitrogen and six times more nitrogen than phosphorus. The balance of the material in a synthetic fertilizer is inert ingredients.
Using compost, animal manures and cover crops to improve soil fertility isn't new. But, what has come onto the gardening market since the beginning of the 21st century is widespread commercial distribution of fertilizer composed of natural ingredients. Organic fertilizers blend a variety of sources to create a product that delivers secondary macronutrients and micronutrients as well as nitrogen, phosphorus and potassium. All of the elements are slow release in nature, making them safe to use. They won't "burn" your plants by providing too much of any one input if you accidentally over-apply, which can happen with synthetics. On the other hand you won't see the vigorous growth when using organics that you can see with synthetics.
Certain vegetables need higher amounts of some elements. Leafy plants, like lettuce and cabbage, for example, need more nitrogen. Also, plants may produce better if some elements are more prominent at certain stages of development. For example tomatoes need phosphorus when they're planted to establish a strong root system; nitrogen when they're developing from seedling to mature plant; and potassium when they're developing fruit. You can tailor treatments to the needs of individual species at specific times in their development. Blood meal, fish emulsion and aged herbivore manure deliver nitrogen. Bone meal and rock phosphate provide phosphorus. Greensand, wood ashes and granite dust supply potassium. You can apply these individually or use them to "bump" the needed element when applying a balanced fertilizer.
Don't guess when applying fertilizer to your garden plot. Most states in the U.S. have an extension service that offers soil analysis at a reasonable cost. Your local extension service office can explain the procedure for sampling the soil from your garden. The extension service will analyze your soil and give you recommendations for supplements to make your soil productive.