The reactions taking place in the soil and the properties of. various phosphate materials combine to determine the effectiveness of any source of phosphorus under any set of soil and cropping conditions. The behavior of some phosphatic fertilizers is discussed below.
When fertilizer materials containing a high percentage of monocalcium phosphate (SSP or TSP) are applied to the soil, water vapor moves rapidly into each fertilizer granule. The solution formed is saturated with monocalcium phosphate and dicalcium phosphate dihydrate, and the residue of dicalcium phosphate remains at the site of the
original granule. The saturated solution is extremely acid (pH 1.8) and is concentrated withrespect to Ca and P. When it moves out of the granule, the P can be absorbed by plant roots, and the remainder will be converted to various forms. In calcareous soils, the moving phosphate front is precipitated as dicalcium phosphate on the surface of eae03 particles. Further reversion to hydroxy-apatite may also occur. Phosphate ions also may react directly with calcium ions present in the soil solution to form dicalcium phosphate and, under some circumstances, small amounts of hydroxy-apatite.
Ammonium phosphates move out from the fertilizer granule as the monocaIcium phosphates do. However, no residue of dicalcium phosphate remains at the granule site because there is no calcium in the fertilizer. The solution moving out of the granule will react with Ca (in soils containing a large amount of exchangeable Ca), forming
The pH of a saturated solution of DAP is 8.0, whereas that-of MAP is about 4.0. This difference in the pH of the saturated solution of the two salts affects the type of reaction products formed in different soils by the two compounds. In calcareous soils, a greater proportion of insoluble reaction products will be formed with DAP than with MAP. Although the initial pH of DAP is alkaline, the net impact of its application in soils is acidic. This is so because NH4 + present in DAP yields H+ ions when nitrified by soil microorga?ism~. Thelet~re, it is safe to use ~~ in alkaline soil~. .
When a fertlh~er granule contams a high proportion of watr–insoluble material, such as dicalcium phosphate, no soil solution is formed, and the dispersion of the reaction products into the soil surrounding the original fertilizer particle does not occur. Therefore, plant availability of phosphorus is reduced, particularly in calcareous soils.
Effect of adding nitrogen and phosphorus. When ammonium sulfate is mixed with a water-soluble phosphatic fertilizer and applied in a band, there is a great proliferation of roots in the band resulting in greatly increased uptake of phosphorus by the plant. This effect does not occur when the nitrogen is applied in the form of nitrate instead of ammonium. This differential uptake of P could result from the decreased activity of calcium in the fertilizer zone, brought about by the presence of the S04 ions which could in turn precipitate Ca2+ ions in the form of gypsum. It could possibly also result from the acidifying effect that accompanies the bacterial oxidation of ammonium to nitrate. The acidity produced would enhance the formation of the more soluble H2P04 – ion and allow greater uptake of phosphorus by the plant.
Key Reference Soil Science By A. Rashid