Most complete commercial fertilizers include an assortment of micronutrients in small quantities and an abundance of three macronutrients in much larger quantities: nitrogen, phosphorous, and potassium, with phosphorous being the second most important of the three. What’s so special about phosphorous? Why is it a macronutrient and not a micronutrient? For what biological processes is it essential? How does it occur in nature and in what forms can it be artificially provided?
Chemistry of Plants
Plants are made up of cells that contain many organic and some inorganic compounds. Among the former are proteins, carbohydrates such as starches and sugars, phospholipids, ATP, DNA, plant hormones (such as auxins and cytokinins), as well as-generally-chlorophyll and other pigments. Most of these compounds do not contain any phosphorous. The catch is, a few of them do contain phosphorous in abundance. In fact, phosphorous is important to a host of chemical processes within the cell.
ATP stands for adenosine triphosphate, a multi-purpose nucleotide involved with the intracellular transfer of energy. Adenosine triphosphate contains a multitude of nitrogen atoms, but it also contains three phosphorous atoms. This means ATP contains approximately 18 percent by weight of phosphorous.
Phospholipids are a major constituent of cell walls. Although there are a variety of these compounds, and thus the percentage by weight varies, and although the phosphorous content is not large, the abundance of membranes in plant tissues assures the importance of phosphorous uptake for this purpose, as well.
DNA, or deoxyribonucleic acid, is the chemical substance that determines identity and heredity. Although the four nucleotides within DNA are phosphorous free, the outer backbone of DNA consists of phosphate-deoxyribose. Thus there is one phosphorous atom for every nucleotide contained within the DNA structure. This corresponds to a moderately small but respectable percentage of phosphorous.
Phosphorous, unlike nitrogen, does not occur in elemental form. Rather, it is an abundant constituent of many minerals. It also occurs in nature also through the decay of plant matter. Although most phosphorous is provided in the form of phosphates derived from rock, or organic phosphorous derived from animal remains (bone meal), sometimes soil amendment is made through the application of sewer sludge.
References and Resources:
Welcome to Plant-Hormones
University of Minnesota, Extension – The Nature of Phosphorous in Soils
Plant Physiology – “Phosphorous Uptake by Plants: From Soil to Cell,” by DP Schachtman, RJ Reid, and SM Ayling.