Phosphates
Phosphorus is one of the three most important elements required for plant growth because it regulates protein synthesis and is essential for cell division and development. Phosphorus is added in a high abundance because phosphorus fertilisers have a low initial efficiency. Available phosphorus quickly binds to iron and aluminium in the soil, meaning it is unavailable for plants to absorb through their roots. Due to this, phosphorus in fertiliser is added in more abundance and more often than needed, but this means plants can absorb both phosphorus just added and any residual phosphorus from previous years. Extreme weathering and tropical acidic soils accelerate the binding of phosphorus to other metals, and is the reason why Australian farmers need to add so much more phosphorus to their soils than their European or North American counterparts.
Phosphorus deficiency can be difficult to diagnose, and if left too late added phosphorus may not alleviate the issues. The symptoms of less severe phosphorus deficiencies include slowed plant growth, stunted roots, red pigmentation in dying leaves and dull grey-green colour in all leaves. Phosphorus is commonly added as an ammonium compound, and MAP and DAP are produced by treating phosphoric acid with ammonia.
Mclaughlin, M. (2022). Technical Bulletin: Phosphorus fertilizer use efficiency in soils [Ebook]. The University of Adelaide. Retrieved 10 February 2022, from https://sciences.adelaide.edu.au/fertiliser/system/files/media/documents/2020-01/factsheet-phosphorus-fertilizer-use-efficiency-in-soils.pdf.
Why phosphorous is important. Department of Primary Industries. (2022). Retrieved 10 February 2022, from https://www.dpi.nsw.gov.au/agriculture/soils/more-information/improvement/phosphorous