Agrochemicals

Fertilizers are substances that are added to agricultural lands to alleviate nutrient deficiencies, allowing large increases in the rates of crop growth. Globally, about 152 million tons (138 million metric tons) of fertilizer are used in agriculture each year. In the United States, the average rate of fertilizer application is about 218 lb per 2.5 acres (99 kilograms per hectare), and about 21 million tons (19 million metric tons) are used in total each year.

The most commonly used fertilizers are inorganic compounds of nitrogen (N). Under conditions where agricultural plants have access to sufficient water, their productivity is most often constrained by the supply of available forms of nitrogen, especially nitrate (NO 3 ), and sometimes ammonium (NH ⁺ 4 ). Farmers commonly increase the availability of these inorganic forms of nitrogen by applying suitable fertilizers, such as urea or ammonium nitrate. The rate of fertilization in intensive agricultural systems is commonly several hundred pounds of nitrogen per acre per year, but it can be as high as 440 lb per acre per year (500 kg per hectare per year).

Phosphorus (P) and potassium (K) are other commonly applied nutrients in agriculture. Most phosphorus fertilizers are manufactured from rock phosphate, and are known as superphosphate and triple-superphosphate. Some other phosphorus fertilizers are made from bone meal or seabird guano. Potassium fertilizers are mostly manufactured from mined potash.

Often, these three macronutrients are applied in a combined formulation that includes nitrogen, in what is known as an N-P-K fertilizer. For example, a 10-10-10 fertilizer would contain materials equivalent to 10% each of nitrogen (N), P₂O₅ (source of phosphorus [P]), and K₂O (source of potassium [K]), while a 4-8-16 fertilizer would contain these nutrients in concentrations of 4%, 8%, and 16%, respectively. The desired ratios of these three nutrients are governed by the qualities of the soil being fertilized, and by the needs of the specific crop.

Sometimes other nutrients must also be supplied to agricultural crops. Sulfur, calcium, or magnesium, for example, are limiting to crop productivity in some places. Rarely, micronutrients such as copper, molybdenum, or zinc must be applied to achieve optimum crop growth.