Recently, I was asked, 'How do I fertilize my vegetable garden?' There isn't really one answer. There are several factors to consider.
A gardener needs to understand what's in fertilizer, whether it's a chemical fertilizer or an organic fertilizer. The main ingredients measured in fertilizers are nitrogen, phosphorous and potassium - called macronutrients because they are required in large quantities for plant growth. Nitrogen is responsible for good leaf color and foliage growth. Phosphorous helps plants get started and develop a strong root system. Potassium, or potash, encourages plants to flower and set fruit.
A fertilizer label will read as a series of numbers (12-12-12). The first number represents nitrogen, the second phosphorous, and the third potassium. The numbers are the percent of the element in the container. Using the 12-12-12 ratio, a 10-pound bag of fertilizer would contain 1.2 pounds of each of the macronutrients. For organic gardeners, composted cow manure contains about a 1-1-1 ratio, so you would need 100 pounds of manure to apply one pound of nitrogen.
Picking the correct ratio of fertilizer depends on the type of plant and the growth stage. Most garden vegetables will benefit from a balanced fertilizer similar to the 12-12-12 when they are getting started from seed or seedling. As the plants begin to develop, it's important to decide what you want from the plant. Lettuce and other vegetables grown for foliage require more nitrogen to produce a good yield. Legumes, such as beans and peas, which produce nitrogen from their roots, need very little nitrogen. Vegetables grown for their roots need a higher phosphorous percentage. Potassium is important to vine crops, so they'll bloom and fruit.
Choosing a fertilizer
A gardener also has to choose a fertilizing method. Most often, I'll use a granular fertilizer applied in bands along the outside of the seeded or transplanted row as a starter fertilizer. Then I'll use a liquid fertilizer every couple of weeks through the season to feed the plants through the foliage. This is a great method if the garden is small enough for time to permit, or if there are not large plants, such as sweet corn, that would make it difficult. In those cases, 'side dressing' is a good method. Side dressing is done by applying dry fertilizer down the side of the row or around the plant base.
Soil types will also make a difference in fertilization. Sandy soil will let nitrogen move rapidly down through the soil and away from the root zone, requiring smaller amounts to be applied more periodically, whereas heavier soil will not allow nitrogen to leak through as fast. Phosphorous and potassium do not move readily through any soil.
Nutrients in fertilizers that are used by plants in small quantities are called micronutrients. They include iron, zinc and manganese. Normally these micronutrients don't need to be applied to garden soil since they are there naturally. But, they are necessary in some of the soil-less potting mixes. I cheat sometimes and use a fertilizer with iron to give grass or other plants that dark-green foliage color, even if it's not benefiting the plant that much more.
Stop by the county extension office nearest you to pick up information on exact fertilizer amounts recommended for the crop in your particular area.