Influence of Soluble Salts on the Solubility of and Plant Response to Dicalcium Phosphate

Abstract

A considerable part of the phosphorus in most mixed ferti lizers is carried as dicalcium phosphate produced as a conse quence of ammonia neutralization of the mixture. Agronomic experience with such mixtures suggests that the phosphate pro duced in situ has a higher nutritive value than separate prepara tions of dicalcium phosphate. Any increased nutritive value of the dicalcium phosphate must, it would seem, stem mainly from enhancement of phosphorus solubility in the presence of soluble salts. Accordingly, the solubility of a laboratory preparation of dicalcium phosphate was measured in water and several salt solutions. Greenhouse techniques were designed to minimize re sponses to nutrient constitutents of the salts, change in available soil phosphorus status and physiological effects due to the salts, so that differences in plant response to binary mixtures of di calcium phosphate and several salts were indicative of differences in solubility of dicalcium phosphate. Alfalfa was used as the test crop on Nunn, Carrington, and Chester soils. In general, salts such as ammonium sulfate, which in laboratory tests markedly increase the solubility of dicalcium phosphate with increasing ionic strength, also gave the greatest crop responses in the greenhouse. Those salts such as the ammonium nitrate type, which increase the solubility only moderately and level off at ionic strengths between 2 and 4, only occasionally gave increased crop response, while salts such as the calcium nitrate type, which decrease the solubility sharply with increasing ionic strengths, usually gave a negative crop response.

The chemical nature of P accumulation in agricultural soils—implications for fertiliser management and design: an Australian perspective

“…It has been known for some time that co-application of ammonium salts increases the solubility and effectiveness of P fertilisers. This has been attributed to both stimulation of root P uptake and to increases in solubility of the P (Starostka and Hill 1955;Bouldin and Sample 1958;Miller et al 1970;Leikam et al 1983). In glasshouse experiments Miller et al (1970) showed that co-application of ammonium sulfate ((NH 4 ) 2 SO 4 ) with monocalcium phosphate (MCP) increased the recovery of fertiliser by maize, and these authors were able to demonstrate a reduced precipitation of Ca-P compounds at the root surface when ammonium salts were co-located with the MCP, largely as a result of a rhizosphere pH reduction.…”

Ammonium Effects on Phosphorus Absorption through pH Changes and Phosphorus Precipitation at the Soil‐Root Interface1

“…reported by many workers to increase the P absorption by the plant. This has been attributed to increased root growth in the vicinity of the band (Duncan and Ohlrogge, 1956; Grunes, Viets, and Shih, I 958; Miller and Ohlrogge, 1958), increased solubility of the fertilizer P (Bouldin andSample, 1958, 1969;Starostka and Hill, 1955), and increased metabolic acti'rity of the plant (Cole et al, 1963;Leonce and Miller, E66;Minshall, 1964). While nitrogen has been shown to affect each of these, no one mechanism has been found to satisfactorily explain all the observations.…”

Effect of Adsorbed Cations on Phosphorus Absorption by Various Plant Species1

“…N-containing compou nels, particularly those containing NH 4 +, frequently t.ave the greatest effect (2,4), but other salts such as KN0 3 , K 2 C0 3 , and Ca(N0 3 )2-4H 2 0 when intermixed with dicalcium phosphate increased P uptake (10). Several factors are believed to be effective (4) including fertilizer solubility (10), root development in the fertilizer band (7,8), and physiological factors (6). In 1ddition to all these factors perhaps the cation effe:ts which are known to affect P uptake from solution should be considered.…”