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Fluid Journal : Fluid Journal 1999-2001
Fall 2001 Fluid Journal Summary: Reports covering nearly 40 years of research present strong evidence of the rapidity of phosphate hydrolysis. Whether hydrolysis is complete in a few days or weeks, the process is fast enough to supply plants and roots with sufficient orthophosphate. Phosphorus is required for life. It is the main component of ATP--- the compound essential for energy transfer. It is part of a myriad of functions. Plants are generally thought to consume only the phosphate in the ortho form. Why then are our modern- day high polyphosphate fertilizers effective in overcoming soil phosphorus (P) deficiencies when they contain large portions of their phosphate in the condensed forms -- principally pyrophosphate and tri- polyphosphate? It's a question we get asked more and more, especially with the increased interest in the use of in- furrow starter fertilizers. Liquid fertilizer began its growth with orthophosphates. Early in the sixties, the Tennessee Valley Authority (TVA) researched methods to make a more concentrated liquid phosphate. Growers appreciated many of the benefits of liquid fertilizers but there was a desire to provide more plant food per gallon of fertilizer. TVA found that removing bound water from phosphoric acid boosted the phosphate content from 54% to 70%. This 30% increase reduced freight costs and made a whole new series of products possible. Super acid was born. Reacting super acid with ammonia under controlled conditions resulted in highly concentrated, easy to handle, neutralized liquid ammonium polyphosphates (APP). The polyphosphates (PP) sequestered magnesium, iron, and aluminum which pose problems in orthophosphates. These new phosphates could accept non-chelated zinc. Soil reactions There is a normal hydrolysis of concentrated APP that is strongly by Dr. Raun Lohry Ortho Vs. Poly Author takes a look at the history and behavior of ortho and polyphosphates. temperature related. Many know first hand the problem of hydrolysis that occurs if APP is left to "cook" all summer in a tank. Poly content is reduced and sequestered metals may fall out leaving residue in the tank (and cloudy product). And, once the polys are hydrolyzed, the product may not be able to sequester added micronutrients such as zinc. Dilute APP solutions may hydrolyze to orthophosphate but water dilution does not appear to accelerate the normal hydrolysis process. Adding APP to soil is quite a different matter. Research studies examining the conversion of condensed phosphates to orthophosphate report half-lives of less than one day to as long as 100 days. A half-life is the time it takes to convert half of the polyphosphate to orthophosphate. Some conditions that influence conversion rate are temperature, pH, aerobic status, biological activity, and minerals. Liquid polyphosphate converts more quickly than dry. Water-soluble polys convert quicker than acid-soluble. Researchers have had to take extra care with soil sample storage since polyphosphates convert more rapidly in field-moist soils than air-dried. Sodium phosphate research Sutton and Larsen (1964) studied the hydrolysis rate of radioisotope-labeled sodium pyrophosphate in pot and water cultures. They surmised that hydrolysis to orthophosphate was largely enzymatic and reported half-lives ranged from 4 to 100 days with the average being 18. Rates were higher at higher soil pH values. Hydrolysis proceeded more quickly with higher biological activity (as measured by CO2 evolution). Pyrophosphate was not converted rapidly in the water culture and plants absorbed 2.4 times more orthophosphate. Subsequently, Sutton, et al. (1966) found that pyrophosphatase level, CO2 evolution, temperature, and uptake were loosely correlated. Low temperatures restricted hydrolysis and therefore P uptake in barley. Gilliam and Sample (1968) studied hydrolysis rates in soils with different chemical properties to assess the relative importance of chemical and biological influences. They found a significant chemical contribution to hydrolysis rate. All the observed changes could not be attributed solely to biological factors. Coarse-textured soils appeared to hydrolyze PP faster than fine. Hons et al. (1986) also found texture to significantly interact with other factors to influence rate. Significant interactions expressed were: texture x organic matter content, texture x pH, texture x time, organic matter x time, pH x soil moisture, pH x time, and temperature x time. Dick and Tabatabai (1986) demonstrated hydrolysis rate differences in four soils at three temperature regimes (Figure 1). Rates were lower at 50° than at 68° or 86° F. The amount of P hydrolyzed in the three acid soils (Clarion, Webster, Muscatine) decreased with increasing chain length although there were no significant differences between pyro- (P2) and tri-polyphosphate (P3). Chang and Racz (1977) quantified temperature effects on sodium pyrophosphate hydrolysis (Figure 2). Rates increased linearly and increased about two- to three-fold from 68° to 95° F. Tri-polyphosphate hydrolysis was greater than pyrophosphate and both rates were higher in the non-calcareous soil. About 40-70% of the phosphate hydrolyzed in 120 hours at 68° F whereas about 80-95% hydrolyzed in 120 hours at 95° F Minerals may also affect hydrolysis rate. Dick and Tabatabai (1987) showed Ca2+, pH, and non-buffered phosphatase activity to be positively correlated with hydrolysis rate while percentage of clay, extractable Al3+, and water soluble Mg2+ were negatively correlated. APP The most commonly applied polyphosphate is ammonium
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