Plant Fertilizer Explained

Proper fertilizer use supports healthy plant vegetation, flowers and crops.

Plants need 16 nutrients to grow and reproduce. The most abundant of these--carbon, oxygen and hydrogen--are freely available in air and water. The rest come from fertilizer or soil. Begin any fertilization program with a good soil analysis, to determine soil pH and find out about your soil's nutrient needs. A comprehensive test will inform you about micronutrient levels. Soil makeup will change over time--especially for organic gardeners--so test your soil every two or three years.

Ingredients

Commercial fertilizers feature a guaranteed analysis of their primary ingredients--the relative percentages of nitrogen (N), phosphorus (P) and potassium (K), all needed in large amounts. These proportions are expressed as a numerical ratio. A 100-pound bag of 5-10-10 fertilizer, for example, contains 5 pounds of N and 10 pounds each of P and K. "Complete" fertilizers contain all three nutrients; "balanced" ones contains all three in equal parts. Secondary nutrients are calcium (Ca), magnesium (Mg) and sulfur (S), equally essential but required in smaller quantities. Plant micronutrients--needed in very small quantities--are zinc (Zn), manganese (Mn), iron (Fe), boron (B), copper (Cu), molybdenum (Mo) and chlorine (Cl).

Nitrogen

Nitrogen makes up part of chlorophyll, the green pigment involved in photosynthesis. It's included in every living cell, needed to make proteins and enzymes and to support metabolic processes that synthesize and move energy. Fertilizing with nitrogen encourages rapid plant growth, improves the nutritional quality of leaf and forage crops and increases seed and fruit production.

Phosphorous

Important for encouraging root growth and abundant blooming, phosphorous also supports rapid plant growth. It is essential to photosynthesis too, helping to transform solar energy into chemical energy. Phosphorous also helps plants withstand stress and encourages proper plant maturation. It is central in developing all sugars, starches and oils.

Potassium

Also an aid in photosynthesis, potassium is used by plants in larger quantities than any other nutrient except nitrogen and sometimes calcium. Potassium helps build proteins, enhances fruit quality and reduces susceptibility to diseases.

Organic Fertilizers

An organic fertilizer is derived from animal or plant sources. Popular choices include bone meal, blood meal, cottonseed meal, fishmeal or emulsion, sewage sludge and composted chicken or cattle manure. Organic fertilizers are "slow release" by definition, because nutrients are bound up in organic matter. Microbial soil activity, which is affected by soil temperature, moisture, pH and other factors, releases nutrients from organic fertilizers--a slow process. Most organic fertilizers contain micronutrients too. People think of organic fertilizers as "natural," but they can also be synthetic, as in urea.

Inorganic Fertilizers

Chemical fertilizers are those mostly widely used in commercial agriculture and home garden products. Manufactured chemical or synthetic fertilizers consist almost entirely of nitrogen, potassium and phosphorus--sometimes alone, often in combination--in forms readily available to plants. Slow-release synthetic fertilizers, usually more expensive, release nutrients at a slower rate, in keeping with a plant's typical use. Some inorganic fertilizers--such as pulverized phosphate rock and greensand--are slow-release natural fertilizers used primarily by organic gardeners and growers.

Excess Fertilizer

Too much fertilizer--including organic fertilizers--can damage plants and inhibit growth. Excessive or improper application of both inorganic and organic fertilizers can cause salt burn in plants, induce nutrient toxicity or deficiencies and cause pollution. Nitrogen not needed by plants, for example, remains in the soil as soluble nitrate ions that can leach into groundwater. Phosphorous can contaminate lakes, streams and rivers, and is the primary cause of algae overgrowth, which can lead to major fish kills and other ecosystem damage.

References

• Clemson University Extension: Fertilizers
• North Carolina Department of Agriculture: Kids World - Plant Nutrients
• Utah State University Extension: Selecting and Using Inorganic Fertilizers
• Utah State University Extension: Selecting and Using Organic Fertilizers
• University of Delaware Extension: Fertilizer Basics