Grain mineral concentrations and yield of wheat grown under organic and conventional management

On the low-P soils in southeastern Australia, organic crops differ from conventional ones primarily in the use of relatively insoluble, as opposed to soluble, P fertilisers and in the non-use of herbicides. As organic management, particularly elimination of soluble fertilisers, is often claimed to enhance grain mineral concentrations, we examined grain from wheat on paired organic and conventional farms in two sets of experiments: (1) four pairs of commercial crops (1991-1993); and (2) fertiliser experiments on one farm pair where nil fertiliser was compared with 40 kg ha(-1) of P as either relatively insoluble reactive phosphate rock or more soluble superphosphate (1991 and 1992). All wheat was grown following a 2-6 year legume-based pasture phase. Both conventional management and the superphosphate treatment greatly increased yields but reduced colonisation by mycorrhizal fungi. While only minor variations occurred in grain N, K, Mg, Ca, S and Fe concentrations, conventional grain had lower Zn and Cu but higher Mn and P than organic grain. These differences were ascribed to: soluble P fertilisers increasing P uptake but reducing mycorrhizal colonisation and thereby reducing Zn uptake and enhancing Mn uptake; dilution of Cu in heavier crops; and past lime applications on the organic farm decreasing Mn availability. These variations in grain minerals had nutritional implications primarily favouring the organic grain; however, organic management and, specifically, elimination of soluble fertilisers did not induce dramatic increases in grain mineral concentrations. In addition, organic management was coupled with yield reductions of 17-84 per cent due to P limitation and weeds. The impact of large regional variations in the characteristics of organic and conventional systems on the general applicability of the results from this study and other similar studies is discussed. (C) 2004 Society of Chemical Industry