Fertilizer

Adding nutrients to agricultural systems is essential to enhance crop yield, crop quality, and economic returns. Commercial fertilizers are typically used to supply needed nutrients to crops. Nitrogen (N), phosphorus (P), andpotassium (K) fertilizers are used extensively. Other secondary and micronutrient fertilizers are generally required in small quantities to correct plant nutrient deficiencies.

An Amish farmer uses a horse team to spread fertilizer on a field in Lancaster County, Pennsylvania.

Commercial fertilizers contain a guaranteed quantity of nutrients, expressed as fertilizer grade on a label showing the weight percentage of available N, P₂ O₅, and K₂ O equivalent (N-P-K) in the fertilizer. Additional nutrients in fertilizer formulations are listed at the end of the fertilizer grade with the nutrient identified. Commonly used commercial fertilizers include ammonium nitrate (fertilizer grade 33-0-0), urea (45-0-0), urea-ammonium nitrate (28-0-0), anhydrous ammonia (82-0-0), diammonium phosphate (18-46-0), monoammonium phosphate (10-52-0), ammonium polyphosphate (10-34-0), ammonium thiosulfate (12-0-0-26S), potassium chloride (0-0-60-45Cl), potassium sulfate (0-0-50-18S), and potassium-magnesium sulfate (0-0-22-22S-11Mg). The secondary plant nutrients sulfur (S) and magnesium (Mg) are often contained in the nitrogen, phosphorus, and potassium fertilizers as shown.

Fertilizers are available in several forms (solids, fluids, and gases), which makes their handling and precise application very compatible with planting and fertilizer application equipment. Fertilizers are applied in several ways; they can be broadcast over the soil surface or in narrow bands on or in the soil, as foliar applications to plants, or through irrigation systems. For more efficient use, fertilizer should normally be applied just prior to the time of greatest plant nutrient uptake. In contrast, organic sources, such as animalmanures, need to be applied and incorporated into the soil prior to planting the crop to be most effective.

Management of crop nutrient requirements is easier with commercial fertilizers than with organic fertilizers such as animal manures, bio-solids, byproducts, and other organic waste products. Release of many of the plant nutrients from these sources requires the breakdown of organic material by soil microbes and release of plant nutrients through a process called mineralization. Many of the nutrients from organic sources are not available to plants until this process has occurred. Release of plant nutrients from organic sources may not correspond with the period of greatest crop need.

Organic fertilizers and legumes are good sources of nutrients for crop production. Balancing the quantity of nutrient application with organic sources to match crop need is more difficult than with commercial fertilizers. Application of sufficient animal manure to meet crop nitrogen needs will likely result in an overapplication of phosphorus. Conversely, application of sufficient manure to meet the phosphorus needs of crops could result in the under application of nitrogen. Nutrient content of most organic sources is highly variable and needs to be determined before application to soils to avoid overapplication of some nutrients.

Balancing crop nutrient needs using both inorganic commercial fertilizer and organic sources is an excellent way to avoid overapplication of plant nutrients. Soil and/or plant tissue testing should be used to determine crop nutrient needs before applying nutrients from any source. This will ensure efficient use of plant nutrients while maintaining high crop yields, crop quality and profitability, and preserving or enhancing environmental quality.

Agriculture, Modern; Biochemical Cycles; Compost; Nutrients; Organic Agriculture; Soil, Chemistry Of; Soil, Physical Characteristics Of.

California Fertilizer Association. Soil Improvement Committee. Western Fertilizer Handbook. Danville, IL: Interstate Publishers, 1995.

United Nations Industrial Development Organization. Fertilizer Manual. Norwell, MA: Kluwer Academic, 1998.