Different Sources of Phosphorus

To determine the right source of phosphorus, there are several factors that need to be taken into consideration.  First, a background of this macro-nutrient and how it is utilized in the plant.  Phosphorus, or P on the periodic table, has various states in soil.  It can be fixed, active and/or part of the soil solution.  Fixed P is the largest of pools and is not available to the plant.  The conversion of fixed to active is very slow and has little impact on soil fertility.  Active P in solid phase can be released into soil solution easily and replenishes the soil solution as the plant removes.  Phosphorus in a soluble state, or solution, is the only form for plant uptake. 

The role of phosphorus in the plant starts with being the elemental component of ATP-ADP, DNA and RNA.  These are the energy sources for photosynthesis, and transfer energy/nutrients. P plays an important role in fruit set/development and an essential component of cell membranes. 

Manure and biosolids can supply large amounts of P and is usually a direct reflection of their diet.  Before applying these sources, a laboratory analysis must be conducted for proper rates.  Handling, storing and transport of manure sources can be challenging.  This usually creates more inconsistencies from one sample to another.  There are, however, several commercial fertilizer options that are consistent.

Monoammonium Phosphate (MAP; 11-52-0) is water soluble and makes the pH surrounding the granule reasonably acidic at 4-4.5 pH.  This makes MAP one of the more suitable fertilizers in neutral to high pH soils. 

Diammonium Phosphate (DAP; 18-46-0) is one of the most used P fertilizers in North America.  It has a high nutrient content and is easy to handle/store.  Unlike MAP, an alkaline pH of 7.5-8 is formed around the granule.  Ammonium is released from the fertilizer and becomes volatile.  This is only a concern if placed too close to germinating seeds, is not incorporated, or pH is higher than 7.  As bacteria convert the nitrogen to nitrate, the pH is lowered.

Ammonium Polyphosphate (APP; 10-34-0 or 11-37-0) is comprised of orthophosphate and polyphosphate.  50-75% of the P is in the form of polyphosphate.  This makes the other 50-25% readily available for plant uptake.  What is not readily available, will eventually convert to available in a short period of time depending on the environment. 

Triple Superphosphate (TSP; 0-45-0) has one of the highest P concentrations out of all the fertilizers not containing nitrogen.  Over 90% of the total P is water soluble, making it a good choice for fast plant uptake.  This fertilizer will make the soil solution acidic (pH 2-3) as the granule dissolves.  It is a great choice for blending custom applications.

There are several options for phosphorus fertilizers on the market, and these are just a few of the commercial ones available.  When deciding which one to utilize, always apply based on soil test data, and be mindful of the 4Rs (Right Source, Right Rate, Right Time and Right Place). 

Reference: International Plant Nutrition Institute. (2012). 4R Plant Nutrition: A manual for improving the management of plant nutrition (pp. 40-41). Norcross, GA, USA: International Plant Nutrition Institute