The first commercial phosphorus fertilizer was superphosphate and triple superphosphate (table 2). Ordinary superphosphate is no longer used in crop production.

The most common phosphorus fertilizers currently used are monoammonium phosphate (MAP, 11-48- 0), diammonium phosphate (DAP, 18-46-0), and polyphosphate.

Monoammonium phosphate contains 82 percent water- soluble phosphate. It rapidly releases phosphorus in moist soils and is most commonly used as a starter fertilizer.

Diammounium phosphate contains 90 percent water-soluble phosphate. The application of MAP creates a temporary acidic zone (pH of 3.5‒4.5) surrounding the granules. Conversely, application of DAP creates a temporary alkaline condition (pH of 7.8‒8.2) around the granules. Application of DAP in calcareous soils or high pH soils may cause seedling damage if used as a starter fertilizer or banded in high concentration near roots. Although these soil pH changes occur after application of MAP or DAP, the effects lessen over time and do not affect plant growth and yield.

Polyphosphate is a liquid fertilizer and contains two forms of phosphorus: orthophosphate and polyphosphate. Orthophosphate is readily available to crops; polyphosphate needs to convert into orthophosphate before plants can take it up. Enzymes produced by soil microorganisms break down the polyphosphates into the usable form. Common polyphosphate fertilizers are the crystal-free fluids 10-34-0 or 11-37-0.

Organic Phosphorus Sources

The most common sources of organic phosphorus for crops are animal manure, compost, or sewage sludge. Phosphorus content of organic sources varies, and the release of P is slow and variable.

The phosphorus content of manure varies with diet of the animal, animal species, handling and storage of manure, type of bedding material, and, in the case of poultry litter, the number of flocks between cleanout. For example, broiler litter may contain 60 pounds P2O5 per ton whereas beef cattle manure may contain only 30 pounds P2O5 per ton. For a description of factors affecting the nutrient composition of poultry litter, refer to “Nutrient Content and Composition of Poultry Litter” (Extension publication ANR-2522).

Phosphorus in manure is mostly in organic form and must break down into plant-available form. This process is slow and is accomplished by soil microbes. Manures also are excellent sources of other nutrients, such as calcium, magnesium, zinc, and sulphur.

Manure application should be carefully managed since the relative amount of nutrients required by each crop is different compared to the relative amount of nutrients present in the manure. For example, applying manure as the main source of nitrogen for corn will result in three- to four-fold excess application of phosphorus over the amount typically required by the plant. Additionally, if manure is applied annually, soil phosphorus will build up quickly in a few years.

From a nutrient management standpoint, the source of phosphorus does not matter; however, what separates different sources is their ability to release phosphorus and increase the availability of phosphorus in soil solution for plant uptake. Two tons of chicken litter (3-3-2) will provide the same amount of phosphorus as 260 pounds of DAP; however, the phosphorus availability will differ between these two sources. Because this is a biological process, chicken litter P will be slowly available as compared to DAP.

Debolina Chakraborty, Research Fellow, and Rishi Prasad, Extension Specialist, Assistant Professor, both in Crop, Soil, and Environmental Sciences, Auburn University

Reviewed October 2021, Phosphorus Basics: Deficiency Symptoms, Sufficiency Ranges, and Common Sources, ANR-2588