Chemical Behavior of Monocalcium Phosphate Monohydrate in Soils

Abstract

To gain an insight into the phenomena that are initiated when superphosphate is placed in soil, tablets containing monocalcium phosphate monohydrate were placed in several soils and observed directly. Petrographic examinations and supporting chemical studies gave information on chemical reactions associated with the dissolution process and on the transport of materials to and from the tablet—factors that bear upon the distribution and chemical combination of the phosphorus and thus upon agronomic response. The results suggest that capillary flow was a principal mechanism in the movement of phosphate solution away from the tablet. Vapor‐phase transport of water to the tablet apparently controlled the rate of dissolution. A significant part of the P remained at the tablet site as dicalcium phosphate, anhydrous or dihydrated. The dissolution process was affected by soil type, soil moisture content, humus content, microbiological activity, capillarity of soil and tablet, time, and temperature.

Dissolution of granulated CuZn-superphosphate in soils

“…More than 50% of initial P was lost within 7 days whereas dissolution of S occurred more uniformly throughout the period of the experiment. This behaviour is consistent with the laboratory observations of several workers [12,14,20], and may be explained as follows. Superphosphate granules rapidly become wet after application to moist soils by the mechanisms of capillarity and vapour-phase transport induced by the presence of hygroscopic MCP.…”

“…MCP). These amounts correspond to the congruent dissolution of MCP rather than to incongruent dissolution with precipitation of DCP and/or DCP dihyrdate as has been observed in laboratory studies of MCP and OSP dissolution [14,16]. This interpretation of the chemical data is supported by the XRD patterns of residual pellets which at no stage showed evidence for the formation of any DCP or of significant amounts of DCP dihydrate (Figure 3).…”

“…The chemical reactions occurring within granules of micronutrient-superphosphate applied to soils are generally understood [13]. However some inconsistencies are apparent from the published literature on this topic and require further investigation.Various proportions (18-33%) of the initial P content of superphosphate granules remained in soils in laboratory experiments as the poorly soluble compounds DCP (dicalcium phosphate) and DCP dihydrate [12,14,16,20,21].Residual OSP granules recovered from beneath farmers' crops after periods of 5-20 months in the soil contain about 25% of the initial P [3]. However this P is not as DCP but as MCP (monocalcium phosphate) and the waterinsoluble constituents of OSP, i.e.…”

Eficiência agronômica de fertilizantes fosfatados com solubilidade variada

“…SSP, Superfosfato simples; FAS, Fosfato acidulado sulfúrico; FSR, Fosfato de solubilidade reduzida.. A presença de P nos grânulos de fertilizantes mesmo depois de decorrido muito tempo de sua aplicação no solo é um fato relatado há muito tempo. Mehring et al (1937) e posteriormente Lehr et al (1959) relataram que grande parte do P permanecia nos grânulos ou na região ao redor deles, mesmo após 100 dias de incubação, devido à formação de precipitados. Espantosamente, desde então pouquíssimo avanço foi obtido para buscar alternativas a este respeito.…”

“…Como o MCP constitui cerca de 95% do P -total em SSP comerciais (LEIKAN, ACHORN, 2005), sabe-se que o mesmo absorve até 16% de seu peso em água quando um solo com -300kpa de tensão de umidade (LEHR et al, 1959). Mesmo em solos muito secos, a solução saturada de sais fosfáticos forma uma pseudo esfera ao redor do grânulo, onde até 55% do grânulo pode se dissolver e o P deixando o mesmo em ate 24h (HEDLEY, McLAUGHLIN, 2005).…”