Managing Orthophosphate in Conventional and Organic Agriculture

Plant Uptake of Phosphorus (P)

In my previous FieldLink article on phosphorus, we covered how local soil factors (e.g., soil pH) govern the availability of plant available P.  For this issue, we cover the preferred uptake form of P and how it is related to your fertilizer or compost choice. Remember, plant roots can only take up P that is dissolved in solution (inorganic orthophosphate - HPO4 2- or H2PO4).  Moreover, the role of phosphorus in crop production is well established, and P is considered one of the primary macronutrients, along with nitrogen and potassium. Improving phosphorus delivery to plants can help you meet your crop yield and quality goals.

Poly- vs. Ortho- Phosphate

Many conventional phosphate fertilizers are derived from mined mineral ore which is then treated to generate a more familiar P product (e.g., MAP, APP, etc.). Close inspection of Figure 1 reveals two terms, ortho and poly. What exactly does this mean? These two terms refer to the chemical structure of the phosphate ions in the fertilizer source (Figure 2). For example, ortho- refers to a chemical fertilizer that is made up of single phosphate ions. Poly- refers to a fertilizer source that is made up of long chains of chemically bound phosphate ions. Polyphosphate fertilizers must have the bonds that link the phosphate ions broken (e.g., hydrolyzed) in the soil system, which produces orthophosphate, before the P can be used by the plant. Remember, plants only take up the ortho- form of P in the soil solution.

Fertilizers, whether they have ortho- or poly-phosphate, have been shown to be excellent sources of P across a variety of cropping systems. Keep in mind the ortho- form is available immediately for plant uptake, and polyphosphates are available later since time is needed to hydrolyze the chemical bonds. Understanding your P fertilizer source can help meet the P demands and growth timeframe of a crop. For example, supplying an immediately available orthophosphate P source can help match growth demands on seedlings and transplants. This is especially true when plantings occur under cool and/or very moist soil conditions where hydrolysis of polyphosphates can slow considerably. Hydrolysis also slows in very dry, hot soils.

Thoughts on Organic P Sources (e.g., P bound to a carbon chain)

Animal and plant-based P fertilizers are used to improve soil quality and tilth and to provide nutrients for plant growth in both conventional and organic cropping systems.  Organic sources of phosphate include various manure products (e.g., poultry, swine and cattle manure) and animal-based products (e.g., bone meal). Remember, organic P sources (e.g., P bound to a carbon chain) must be actively decomposed or mineralized to the ortho- form before plants can use the nutrient. In this sense, P availability from manure, animal and plant sources must undergo a number of temperature, moisture, metabolic and enzymatic-based reactions before the P nutrient can contribute to crop yield (Figure 3).  Thus, organic-P availability is heavily influenced by a variety of factors such as soil microbial populations and recent weather activity. Therefore, P release from manure and compost sources can be slower, relative to inorganic P (e.g., MAP, 10-34-0, etc.), given the multiple controls on its supply to the cropping system. Again, supplying an immediately available, organic orthophosphate P source can help match growth demands on seedlings and transplants. This is especially true when plantings occur under cool and/or moist soil conditions where microbial activity slows considerably.  Microbial activity can also slow in very dry, hot soils.

Conclusion

Understanding and improving the phosphate nutrition of your crops requires the integration of several avenues of thought across your farm, including weather conditions, soil type, soil test results, crop-specific P needs, available nutrient sources, etc. Below are some tools you can use to help better understand the P requirements of your crop.

Tools Used to Better Understand On Farm P Management

• Soil and Plant Tissue Test Results
• P Uptake Curves
• Plant Signs of P Deficiency
• Nutrient Removal Rates
• Soil Series Information
• Soil Parent Material

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

Ortho vs. Poly – Dr. Raun Lohry – Fluid Fertilizer Foundation – http://tinyurl.com/zt5x6wh

- Dr. Karl Wyant, Agronomist