The Role of Phosphorus in Crop Production

The Roles of Phosphorus in Plant Growth

The role of phosphorus (P) in crop production is well established, and P is considered one of the primary macronutrients, along with nitrogen and potassium. Phosphorus is a crucial component for converting solar energy into food, fiber and other plant products. Phosphorus also plays a key role in the metabolism of sugars, energy storage and transfer, cell growth and the transfer of genetic information (IPNI Nutri-Fact Sheet).  For example, growing plants use P to form cell membranes for organelles (e.g., chloroplasts), pay for biological work (ATP) and create genetic material (e.g., DNA and RNA) as shown in Figure 1. Plant roots can only take up P that is dissolved in solution (inorganic phosphate - HPO4 2- or H2PO4). This uptake preference is where soil phosphate supply often fails to meet plant P demand.

Phosphorus Availability in Western U.S. Soils

In a perfect world, phosphate would diffuse out of soil complexes (e.g., organic matter, weathered minerals, etc.) and into solution at the exact time the plant needed the nutrient. However, this isn’t quite the case.In Western U.S. soils, calcium competes with plant roots for phosphate in solution. When calcium and phosphate come together, they form a hard mineral called apatite, which is made up of the same mineral in your bones and teeth. Once this mineral fixation happens, the bound phosphate is now unavailable for plant uptake. Figure 2 shows just how much soil phosphate is made unavailable by the calcium complexes. Cold temperatures and dry conditions can also draw down the plant available phosphate pool by slowing diffusion rates. In fact, due to calcium tie-up and weather conditions, the plant available supply might only represent <5% of total soil phosphate measured in Western U.S. soils.This small fraction is what you are amending when you apply P fertilizers to help meet crop yield and quality goals.

Sources of Phosphorus – Inorganic and Organic Fertilizers

Phosphorus fertilizers are recommended to help supply growing crops with available phosphate (HPO4 2- or H2PO4). Figure 3 illustrates the myriad of choices for inorganic phosphate sources, including dry and liquid forms of P fertilizer. The common denominator of the most popular inorganic P fertilizers is they all start with mined phosphate-rich ore. Then, the various fertilizer products go through different treatments to arrive at their final form (e.g., MAP, DAP, 10-34-0, Nucleus O-Phos, etc.).  Organic sources of phosphate include various manure products (e.g., poultry, swine and cattle manure), animal products (e.g., bone meal) and plant compost mixes. Always obtain an NPK analysis of  a manure sample before spreading it in order to plan your application according to crop nutrient needs. This will help prevent the over application of other nutrients such as nitrogen and potassium.

Conclusion

Questions about phosphorus soil fertility? Please contact your nearest Helena branch for more information about the best choice for your operation, crop portfolio and field logistics. You can also consult the resources listed below for detailed discussion on the topics presented above.

Suggested Reading

Understanding Phosphorus Fertilizers By George Rehm, Michael Schmitt, John Lamb, Gyes Randall, and Lowell Busman http://www.extension.umn.edu/agriculture/nutrient-management/phosphorus/understanding-phosphorus-fertilizers/

Phosphorus Fertilizer Production and Technology – International Plant Nutrition Institute - http://tinyurl.com/p5jjl57

Phosphorus - International Plant Nutrition Institute – Nutri-Fact Sheet –http://tinyurl.com/pdutwr9

Phosphorus, Food, and Our Future Eds.: Karl A. Wyant, Jessica E. Corman, Jessica R. Corman, James J. Elser.  Oxford University Press – USA. 2013 - 224 pages

- Dr. Karl Wyant, Desert Division Agronomist