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AVAIL® is a fertilizer additive or “enhancer” for use with phosphate fertilizer materials. It is not a fertilizer or fertilizer substitute and by itself has no nutrient value. AVAIL is a polymer material applied with phosphate fertilizer. Nutrisphere® is a similar product used with nitrogen fertilizer.
There are several different formulations, but a common formulation is 40% to 60% maleic itaconic copolymer solution. Specific formulations can be either impregnated with or applied to dry prilled fertilizer or may be mixed with liquid phosphate fertilizers.
Manufacturer literature notes that AVAIL “sequesters antagonistic ions such as iron, aluminum, calcium and magnesium that tie up phosphate in the soil surrounding the fertilizer”, “reduces tie-up of phosphate”, and “makes phosphate more available to the plant”.
The AVAIL mechanism of action is due to the large net negative charge of the maleic itaconic copolymer. This provides the material with a high cation exchange capacity (CEC), about 1800 milliequivalents/100 gram (meq/100g). By comparison, sand-size particles have CEC values of about 1 to 2 meq/100g and soil organic matter has a CEC of 200 to 400 meq/100g.
AVAIL can interact with positively charged ions in soil solution (Ca2+, Mg2+, Al3+, Fe2+, etc.) that typically react with the phosphate ions (HPO4-, H2PO42-) in solution, eventually forming less soluble phosphate minerals. The elevated CEC can potentially remove (or “sequester”) these antagonistic cations, preventing them from interacting with phosphate ions. Theoretically, applying of this material with a phosphorus fertilizer source should improve phosphorus availability, at least in the short term.
The CEC of the AVAIL copolymer is not affected by pH. It tends to affect multivalent cations (more than one positive charge), having a higher attraction for cations with a higher valence. Thus, some of the micronutrient metals, like zinc or nickel, may be highly attracted to the copolymer.
The potential effect of the copolymer on the soil CEC can be calculated from the existing soil CEC (from a standard soil test report), the product CEC, and the product rate using a weighted average approach. The actual rate of copolymer applied to the soil can be calculated from the fertilizer rate, the recommended product rate per unit fertilizer, and the product.
One AVAIL formulation weighs 10.8 lb/gal, has 40% copolymer, and is mixed with dry phosphate at one-half gallon per ton. A 100 lb/ac application of dry phosphate fertilizer would contain 0.27 lb of material or 0.108 lb of copolymer.
Assume the fertilizer is mixed 6 inches deep into a very sandy soil with a CEC of only 5 meq/100g. The copolymer would be diluted with approximately 2 million pounds of soil, resulting in a minuscule increase from a CEC of 5.0000 meq/100g to a CEC of 5.0001 meq/100g.
The copolymer product can also be mixed with liquid fertilizer and band-applied. Band application only mixes the product with a small fraction of the total soil volume. Applying a 1-inch diameter fertilizer band on 30-inch spacings, mixes the copolymer with only 0.4% of the soil volume in the top 6-inch of soil (or 8000 lb of soil). A 40% copolymer product with a density of 9.7 lb/gal is recommended at 0.5 gallons per 99.5 gallons of phosphate fertilizer.
Assuming a liquid fertilizer density of 11.6 lb/gal, each 10 gallons of applied fertilizer would include 0.194 lb of copolymer. Using the weighted average approach, the CEC in the 1-inch diameter band would change from 5.00 meq/100g to 5.04 meq/100g.
Based on these calculations, this small change in CEC would be expected to have little impact on phosphate availability. These calculations were also based on a sandy soil (CEC = 5). The change in CEC would be proportionately less with finer textured soils having higher clay content.
Osmond, D., et. al. 2008. Testing new fertilizers and fertilizer additives. Memo. Dept of Soil Science, North Carolina State Univ. stanly.ces.ncsu.edu/files/library/84/Fertilizer%20Additives.3.7.2008.pdf