Phosphorus in agricultural soils: the role of biofertilisers and their derivatives: POSTER
Tompkins, D., Aqua Enviro, UK
(free)
• The majority of UK biosolids are applied to agricultural land as an ‘organic fertiliser’, and make an important contribution to phosphorus fertilisation (Figure 3).
• Biosolids and other bioresources contain a combination of organic and inorganic forms of phosphorus. The latter may be insoluble or barely soluble in water (Table 1) and exhibit different soil behaviours to conventional fertilisers.
• There are no simple tests to determine the fertilising potential of different phosphorus compounds, although research efforts are increasingly focussed on DGT (Diffusive Gradients in Thin films) techniques applied to controlled incubations of fertiliser and soil mixes [8].
• Experimental data show that struvite can out-perform conventional fertilisers, despite its relative insolubility in water (see Figure 4) [12].
• Future requirements for predictable P fertilisers that can be used as seed dressings, accurately placed within soils or applied as foliar feeds will provide new opportunities for recovered P products such as struvite, but may require significant changes to current P removal approaches and a move away from broadcast application of cake or liquid digestates.
Figure 4. The agronomic efficiency (in terms of dry matter yield and phosphorus use efficiency) of struvite and calcium phosphate relative to conventional P fertilisers. Results are presented as weighted means (square) and 95% confidence intervals (error bars). The numbers of contributing research papers are listed alongside each parameter. Redrawn from [9]
• Soil P dynamics are complex, due to combinations of biotic and abiotic interactions that include sorption and precipitation reactions with soil minerals (Figures 1 & 2) [1]. • Crops take up phosphorus from soil solution as orthophosphate (H2PO4- and HPO42-) [1]. This labile phosphorus pool is normally very small compared with other pools [2].
• Conventional fertilisers such as Triple Super Phosphate (TSP) are highly soluble in water, and readily available for crop uptake. However, recent research has shown that crop recovery of P from such fertilisers is extremely poor – on average only 4%. The remaining proportion is adsorbed or precipitated and added to soil P reserves, but is used inefficiently by subsequent crops [4].
• Meeting crop demand for phosphorus currently relies on an ‘insurance-based’ approach to fertiliser applications (see Figure 5), which means that UK soils exhibit a net phosphorus surplus (see Figure 3). This increases risks of diffuse pollution [4].
• A combination of resource security, environmental pollution and cost associated with the current ‘feed the soil’ approach is forcing a shift to ‘feed the plant’. This means changing forms and application methods of phosphatic fertilisers as well as breeding crops with different rooting traits (Figure 5).
