Plants must have light, moisture and nutrients to grow. The sun provides light. Moisture comes from rainfall or irrigation. Nutrients come from fertilizers, compost or manure.

If plants are not growing well, fertilizing them will help only if a lack of nutrients is the cause of the problem. Plants grown in poorly drained soils, in excessive shade, or in competition with tree roots will not respond to fertilizer.

Fertilizers are either organic or inorganic. Examples of organic fertilizers include manure (poultry, cow or horse), bone meal, cottonseed, or other naturally occurring materials. Inorganic fertilizers are man made products. They usually have a higher nutrient content.

Buying Fertilizers

The three numbers on fertilizer containers are the fertilizer analysis (Fig. 1). They indicate the percent of nitrogen, phosphorus and potassium in the fertilizer, respectively.

These figures are always listed in the same order. So, a 100-pound bag of 10-20-10 fertilizer contains 10 pounds of nitrogen, 20 pounds of phosphorus, and 10 pounds of potassium. This equals a total of 40 pounds of nutrients. The rest of the fertilizer, or 60 pounds in this example, is a carrier or filler such as sand, perlite, or rice hulls. A complete fertilizer is one that includes all three elements.

All parts of a plant need nitrogen for growth—the roots, leaves, stems, flowers and fruits. Nitrogen gives plants their green color and is needed to form protein. A lack of nitrogen causes the lower leaves to turn yellow and the whole plant to turn pale green. On the other hand, too much nitrogen kills plants.

Phosphorus is needed for cell division and to help form roots, flowers and fruit. Phosphorus deficiency causes stunted growth and poor flowering and fruiting.

Plants need potassium for many of the chemical processes that allow them to live and grow. A potassium shortage shows up in various ways, but stunted growth and yellowish lower leaves are common symptoms in many plants.

When you buy fertilizer, consider the cost per pound of the nutrient(s). Generally, higher analysis fertilizers and larger containers are less expensive. For example, a 50-pound bag of 10-20-10 may not cost any more than a 50-pound bag of 5-10-5 fertilizer, but the 10-20-10 bag contains twice the nutrients.