The three macronutrients most critical for vegetable development, and therefore the nutrients included in greatest concentration in most fertilizers, are nitrogen, phosphorus and potassium. Although the ways plants use these nutrients are complex, each one plays a different role in plant growth. In general, nitrogen helps plants develop healthy foliage; phosphorus helps with flower and fruit development; potassium contributes to healthy root development.
On fertilizer labels, the three macronutrients are often referred to by their chemical symbols: N for nitrogen, P for phosphorus and K for potassium. The label also includes three numbers that indicate the percentage of each macronutrient in the fertilizer by weight. For example, a 10-10-10 fertilizer contains 1 pound of each nutrient in every 10 pounds of fertilizer, and a 5-10-5 fertilizer contains 1/2 pound of nitrogen, 1 pound of phosphorus and 1/2 pound of potassium per 10 pounds of fertilizer.
Different plant species have varying needs for each macronutrient depending on the plant's growth habit and its role in the garden. For example, leafy greens, cabbage (Brassica oleracea), corn (Zea mays) and other plants that need to put much of their energy into foliage production can benefit from higher levels of nitrogen, while tomatoes (Solanum lycopersicum), root vegetables and plants that channel their energy into fruit or root development do better with relatively higher levels of phosphorus and potassium.
In general, moderate application of a balanced fertilizer provides basic nutrition for a wide range of vegetables. The application of 1 pound of 10-10-10 fertilizer per 100 foot row, or 2 pounds of 5-10-5 or 5-10-10 fertilizer for plants with low nitrogen needs, is a common rule of thumb.
It is important, too, that your soil's texture allows for good drainage and that the soil contains plenty of organic matter, because excess water can injure plants, and organic matter can help make nutrients more available to plants.
Problems With Overfertilization
Applying too much fertilizer can cause as many problems as applying too little. The wrong balance of nutrients can encourage undesirable growth habits in plants, and excessive concentrations of nutrients in the soil can inhibit plant growth.
For example, when vining plants such as tomatoes, cucumbers (Cucumis sativus), squash (_Cucurbita)_or melons (Cucurbitaceae) are given too much nitrogen, they divert their energy toward producing longer vines at the expense of fruit development.
Applying too much phosphorus, typically included in fertilizers in the form of phosphates, interferes with plants' ability to take in micronutrients such as iron and zinc, and it may also kill beneficial microorganisms in the soil. Too much potassium can increase the level of salt in the soil to the point that it becomes damaging to plants.
Because native nutrient levels in soil vary from location to location, the only reliable way to know precisely what type of fertilizer your garden vegetables need is to have your soil tested to determine what nutrients are already present and in what amounts. Tests conducted by your local extension service can determine the level of macronutrients and micronutrients in your soil, as well as provide data on the soil's organic matter content, sand and clay content, and pH level.
Testing your garden soil every two to three years will alert you to any deficiencies and guide your choice of an appropriate fertilizer.