Vegetables require nutrients from the soil to grow and develop properly. If any one of the nutrients a vegetable plant needs is too low, it will not produce well, no matter how abundant the rest of the nutrients are in the soil. Adding vegetable fertilizers and organic matter can help amend the nutritional content of garden soil and make nutrients available to the plants in a form they can easily use. Not all fertilizers are appropriate for all gardens. A soil test is available at plant nursery centers and through state extension offices will give a gardener a detailed look at exactly what nutrients are missing so that the proper vegetable fertilizer can be chosen.
Although there are 16 essential nutrients to plant growth, soils are most often deficient in nitrogen, phosphorus, or potassium. Commercial fertilizers usually contain all three of these in differing amounts. The numbers on the bag refer to the percentage of nitrogen, phosphorus, and potassium contained in the fertilizer, in the same order. A bag labeled 24-8-4 has much more nitrogen than phosphorus or potassium and would be appropriate if a soil test showed a nitrogen deficiency.
Vegetable fertilizers may be organic or chemical (often called inorganic). Each has its advantages and disadvantages and may be appropriate for differing situations. An organic vegetable fertilizer has lower percentages of each primary nutrient, but contains many other nutrients and trace elements. Organic fertilizer nutrients are bound up with other organic matter which can help amend soil conditions in the long-term. Nutrients break down in soil more slowly, making them available for a longer period of time, but can take too long to become available if the nutrient is already in very short supply.
Chemical, or inorganic, fertilizers contain higher amounts of nutrients mixed with an inert carrying material. There are no additional nutrients in the fertilizer and no organic matter. Nutrients are immediately available for use but can be depleted quickly and do nothing to amend poor soil conditions in the long-term. They may also burn plants if applied too heavily. Some experts believe that chemical fertilizers may alter the balance of essential micro-organisms in the soil and cause more damage than good.
Regardless of which type of vegetable fertilizer is used, plants cannot use the nutrients if the soil pH is too high or too low, no matter how abundant the nutrient is in the soil. A soil pH test can be done at the same time as a soil nutrient test. Most vegetables prefer soil pH levels to be around 6.0 to 6.8. Outside of this range, the plants cannot free the nutrients in the soil to use them. Soil pH can be lowered by applying sulfur and raised by applying lime.