By Ron Alexander

Q: I find the labels and regulations around phosphorus confusing. What can I expect from the phosphorus in Bloom?

A: The short answer is you’re not alone in finding this confusing, and it’s complicated.

Phosphorus is one of the 16 essential plant nutrients and one of the three major plant nutrients (along with nitrogen and potassium). Phosphorus is important in encouraging root growth during establishment of turfgrass and many other plants, improving flower and seed development, and hastening maturity in food crops.

Phosphorous or phosphate fertilizers are sold in the marketplace by their amount of “pentavalent phosphorous” in the material, which is calculated as phosphorous pentoxide or P 2 O 5 . On a fertilizer label, P 2 O 5 is listed as Available Phosphate. However, some of this “available phosphate” is not readily taken up by plants, but is bound to the soil, awaiting release through microbial activity as orthophosphate. Orthophosphoric acid (orthophosphate) is actually the form in which phosphate is taken up by plants, and inorganic forms of phosphate (chemical fertilizer products) are essentially derived from orthophosphate. This form of phosphate is more highly mobile, containing a large percentage in a water extractible form. Today, the amount of water extractible phosphorous in a product can actually be measured, helping to determine the portion of phosphorous that is water soluble. Again, these water soluble (extractable) forms of phosphorus are more available to the plant, but it also pose a greater risk to water quality through unwanted migration.

Inorganic forms of phosphorus can become unavailable when they react with specific elements, such as iron, aluminum, manganese (in acid soils), and calcium (in alkaline soils). This makes the phosphorous less available for plant uptake, but also makes it less likely to leach through the soil profile. Bloom contains significant amounts of iron, aluminum and calcium and it possesses very little orthophosphate, so its supply of phosphorus is made available over time with the help of soil microbes. Microbes release
the phosphorous as they degrade the organic matter in Bloom.

In fact, testing through the Pennsylvania State University illustrates that only 1% of the phosphorus in Bloom is readily water extractable. So the phosphorous in Bloom is less likely to leach (dissolved in water), especially in heavier silt and clay soils. It will also only be available to plants on a more slowly releasing basis (likely over three to five years), depending on the soil type and its chemical properties. With that known, hopefully state nutrient planners will consider the water extractability of phosphorus when developing nutrient management plans. Further, this means that nutrient drift from Bloom should be avoidable simply by controlling erosion (soil movement to surface waters). Of course, greater risk of nutrient leaching exists in sandy soils, so smart soil management practices should be used.

However, the other side of the coin is that Bloom cannot always be counted on as the primary source of phosphorus for a crop. The application of additional amounts of phosphate fertilizers may or may not be required on a year-to-year basis where Bloom has been applied. Of course, Bloom may supply enough phosphorous, depending on the amount released from it (and other soil-bound sources of phosphorus) by microbial activity. Therefore, crop or plant growth should be monitored, or a Mehlich 3 soil test be completed, in order to determine if additional phosphorous should be applied where Bloom has been applied.

Have a question for the expert? Send us an email to submit a question for consideration.