Can the efficiency of phosphorus fertilisers be increased on highly fixing soils?

摘要

Phosphorus (P) is an essential element for all life forms. However, it is one of the most difficult nutrients for plants to obtain from the soil and often represents a limiting factor to agricultural production. Millions of tonnes of P fertiliser are applied to agricultural soils each year and a high proportion of the P applied in this way is rapidly converted back to insoluble phosphates to which plants have minimal access. This is particularly a problem in highly P-sorbing soils such as very calcareous or strongly weathered acidic soils that dominate large areas of land used for agriculture throughout the world. Despite the importance of P and the large research effort, our knowledge regarding the chemistry of P in these highly P-sorbing soils is far from complete. For instance, until very recently the efficiency of fluid and granular fertilisers was considered to be equal. However, we have shown that in calcareous soils a significant increase in yield can be achieved using fluid fertilisers. We have observed wheat yield responses from 8% to 48% higher when treated with fluids than an equivalent rate of granular P fertiliser. In this work we present results examining the chemistry of different forms of orthophosphate fertiliser in calcareous soils. In particular we have investigated the availability and reaction products of fluid fertilisers using isotopic dilution techniques and a number of spectroscopic techniques including X-ray microanalyses (EDXMA), X-ray diffraction (XRD) and synchrotron-based μ-X-ray absorption spectroscopy (μ-XAS). The results of our study provide an insight into some of the chemical and physical processes occurring when fluid and granular P fertilisers are applied to calcareous soils. Results from μ-XAS confirm that soluble P quickly reacts with Ca ions but different reaction products result from the application of fluid and granular products. The solubility and lability of P derived from fluids appear to be enhanced in comparison to granular products. XRD, EDXMA and chemical data suggest that there is incomplete dissolution of granules in these soils, even after incubation for 5 weeks at a high soil water content. This is due partly to the presence of insoluble crandallite in the granule, and also due to the precipitation in situ of similar minerals resulting from the diffusion of Ca and Al into the granule. In contrast there is significantly less precipitation of soluble P when fluid forms are applied, and hence greater concentrations of labile P available for plant uptake.