The Composition of Fertilizers
Soil that is fertile contains all necessary nutrients in amounts sufficient for plant growth. However, most garden soils do not contain adequate amounts of nutrients, and the addition of fertilizer is generally required to promote proper growth. Fertilizers vary widely depending on the plant, its needs and the circumstances under which it grows.
While all nutrients are essential for balanced growth, elements are categorized according to required amounts. Primary nutrients, or macronutrients, are measure as a percentage (parts per 100) of dry weight.
Nitrogen, phosphorus and potassium are the three primary nutrients. Nitrogen deficiency results in chlorosis. Excess vegetative growth and a dark green color indicate nitrogen excess. Lack of phosphorus results in slow growth and a purplish color. Excess phosphorus interferes with micronutrient absorption. Slow growth, leaf tip burn and weak stalks are indicators of potassium deficiency, while light green foliage is an indicator of an excess of the mineral.
Calcium, magnesium and sulfur are the secondary nutrients. Secondary nutrients are also measured as parts per 100.
Calcium deficiency results in reduced growth of buds and roots, and results in plant death. Excess calcium interferes with micronutrient availability. Magnesium is an integral component of chlorophyll. A lack of magnesium results in necrosis and chlorosis, while an excess interferes with calcium uptake.
Though no less important, micronutrients are measured in parts per million. Boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc are the micronutrients.
Deficiencies include chlorosis, poor pigmentation, stunted growth, and curled or wilted foliage. Fruit, flowers and vegetables are usually affected by such insufficiencies. An excess of a micronutrient may affect the uptake of another nutrient. Certain micronutrients in excess may mimic a deficiency of another nutrient.
A wide variety of fertilizers are sold commercially. Complete fertilizers contain the three primary nutrients: nitrogen, phosphorus and potassium. It should not be assumed that a complete fertilizer will provide all the nutrients a plant requires. Complete fertilizers may also contain additional nutrients.
Fertilizer bags are labeled according to a system that shows the percentage of nutrients included. The first number is the percentage of nitrogen, the second, a percentage of phosphorus and the last, a percentage of potassium. A 100-pound bag of 15-15-15 grade fertilizer contains 15 pounds of each primary nutrient and 55 pounds of filler. Filler allows for the even spread of the fertilizer.
By law, any nutrient added to the bag must be listed as well. Incomplete fertilizers contain only one or two specific nutrients and are used when only one or two nutrients are found to be deficient in the soil.
Manure and plant remains, when decomposed, are used as organic fertilizers. Organic composts vary in nutritional content. Manures are often more complete than inorganic fertilizers. Other fertilizer sources include bonemeal, cottonseed and fish emulsion. Organic fertilizers add nutrients as well as improve water infiltration and soil tilth.
- University of California David: Soil Fertility Management for Organic Crops
- California Master Gardener Handbook; University of California Agricultural and Natural Resources; 2004; Publication 3382