Rock Phosphate Availability and Long Term Effectiveness

DissolutionSoluble phosphate is an indicator of plant available phosphate in the soil (Chien and Hammond, 1978). Dissolution of rock phosphate is dependent on soil proton supply as well as calcium and phosphate concentration gradients in the soil solution. In acidic soil, phosphate may become soluble within the first year of application however in pH neutral soils, 50% dissolution occurred within the first 2 years (Robinson et al., 1994).
Rhizome and SolubilityVarious crops are also capable of increasing rock phosphate use efficiency by lowering the pH in the rhizosphere through secretion of organic acids; thus improving the solubilization of phosphorus from rock phosphate e.g. Canola (Bekele et al., 1983).
Solubility by Organic SubstancesHumic substances, applied with rock phosphate effectively increased phosphate availability in Nigerian soil by 233 % (Adesanwo et al., 2013). P solubility is enhanced in rock phosphate enriched compost due to the formation of weak carbonic acid during decomposition of organic matter CO2 (Meena and Biswas, 2013).
Solubility by SulphurNumerous studies have shown that the agronomic effectiveness of rock phosphate can be improved when admixing or cogranulating with sulphur (Zapata and Roy, 2004). These early studies found that rock phosphate blended with sulphur at rates between 1:1 and 5:1 were as effective Single Superphosphate (Kittams and Attoe, 1965; Attoe and Olson, 1966; Swaby, 1975)
Residual Effects MICROBIOMESoil bacterial and fungal species improve soil health and are important for the solubilisation of plant- available phosphate in soil, through various methods.
• Phosphate availability from rock phosphate can be increased over years of cultivation due to the action of soil microbiota (Coutinho et al., 1991 and Silva et al., 2017). Soil microbes also produce organic acids, which is involved in P solubilisation (Mendes et al., 2014). Additionally metabolic reactions such as cellular respiration by soil microbes also contribute to P solubilisation (Illmer and Schinner, 1995). Lastly, microbes help supply P by hydrolysis of organic P through the action of phosphatases, especially phytases (Richardson and Simpson, 2011).
• Long-term effects of rock phosphate on the maize rhizosphere were studied by Ubiana et al., 2017. They found that phosphate from rock phosphate may favourably alter microbial communities to improve P availability.
EnvironmentThe environmental benefits received through the substitution of highly soluble commercial fertilizers with rock phosphate are also important to consider. Commercial phosphate sources are highly soluble leading to surface and ground water contamination through leaching and run-off shortly after application. Rock phosphate is solubilized slowly through soil processes, made available to the crop as required and mitigates loss of phosphate from the field (Chien et al., 2011).

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