Changes in P bioavailability induced by the application of liquid and powder sources of P, N and Zn fertilizers in alkaline soils

Alkaline and/or calcareous soils provide significant challenges for management of crop P nutrition, due to rapid fixation of P into sparingly soluble forms not readily accessible by plants. Three powder products, mono- and diammonium phosphate (MAP, DAP) and triple superphosphate (TSP), were compared to three liquid products, liquid MAP, ammonium polyphosphate (APP) and phosphoric acid (H3PO4), as sources of P for wheat grown in four alkaline soils (grey and red calcareous soils, a Vertosol and a Sodosol) sampled in Eyre Peninsula (South Australia), Wimmera and Central Mallee areas (Victoria) of southern Australia. Soils were labelled with P-32 and the labile P pool and P derived from the fertilizer (P-dff) determined. Residual value of the fertilizers was determined after a 4 month wet/dry incubation. Liquid formulations outperformed powder products in the grey calcareous soil and in the Vertosol, as measured by wheat growth, P uptake, plant-labile P pools and P-dff. These increases in P efficiency were not related to differential acidification of the bulk soils. In the most calcareous soil, large proportions of the added DAP, MAP or TSP were rapidly converted (fixed) into non plant-labile pools, likely through precipitation of Ca-P compounds not accessible to plant roots, while conversely, liquid formulations minimized P fixation. Our results suggest that one of the most likely hypotheses to explain difference in efficiencies between powder and liquid forms of fertilizers was that in relatively dry conditions and in high P fixing soils, the dissolution and diffusion of P outwards from the powder is limited. This induces localized areas in the soil with high concentrations of P leading to precipitation of insoluble Ca-P solid phases. In the field, where powder fertilizers are applied as granules rather than as fine powder, differences between the two forms of fertilizer are likely to be larger. The residual value of liquid formulations was equal or superior to powder products. Liquid fertilizers injected into soil may therefore have potential to improve P nutrition in a wide range of calcareous soils under dryland agriculture throughout the world. Field trials have actually been performed in southern Australia to confirm this important issue.