Caroline Johnston

Mobility, solubility and lability of fluid and granular forms of P fertiliser in calcareous and non-calcareous soils under laboratory conditions

Despite a long history of application of phosphorus fertilisers, P deficiency is still a major limitation to crop production on calcareous soils. Recent field research conducted in highly calcareous soils in southern Australia has demonstrated that both grain yield and P uptake of wheat (Triticum aestivum L.) is greater when fluid forms of P are used compared to granular forms. To improve our understanding of the mechanisms underlying this response to P in the field, we compared the lability, solubility and mobility of P applied as either a fluid (3 products) or granular (3 products) form to two calcareous and one alkaline non-calcareous soils in the laboratory. Over a five-week period, between 9.5 and 18 % of the P initially present in the fertiliser granules did not diffuse into the surrounding soil. The degree of granule dissolution was independent of the soil type. In contrast, P solubility, lability and diffusion were significantly greater when fluid products were applied to the calcareous soils, but not to the alkaline non-calcareous soil. These findings are discussed in relation to field trials results where fluid products outperformed granular fertilisers.

Chemical behavior of fluid and granular Mn and Zn fertilisers in alkaline soils

The grain yield benefits of applying micronutrient fluid fertilisers over conventional granular products in calcareous sandy loam soils have been agronomically demonstrated. An understanding of the fundamental mechanisms and reactions occurring following application of these products to soils is critical to improve fertiliser management. We therefore examined the diffusion, solubility, and potential availability of manganese (Mn) and zinc (Zn) from both granular and fluid forms of Mn and Zn fertiliser in an alkaline calcareous and alkaline non-calcareous soil using laboratory incubation experiments in conjunction with an isotopic dilution technique with 54Mn and 65Zn. Enhanced mobility, solubility, and/or potential availability of Mn and Zn from fluid fertilisers were observed in comparison to Mn or Zn from granular fertilisers in both soils after 5 weeks of incubation. Differential behaviour of fluid and granular fertilisers for Mn and Zn appeared to be independent of their effects on soil pH. Most (~90%) of the Mn in granular fertiliser dissolved and diffused out of the granule but was retained within 4 mm of the point of granular placement, whereas most (~85%) of the Zn in the granular Zn fertiliser source remained in the granule. Our data suggest that the superior agronomic effectiveness of fluid Mn and Zn fertilisers observed in calcareous soils under field conditions may have resulted from the enhanced diffusion (Mn) and/or solubility/availability (Mn, Zn) of these micronutrients in soil when applied in fluid form.