Nutrient budgets and status in three pairs of conventional and alternative mixed cropping farms in Canterbury, New Zealand

The major inputs of nitrogen (N), phosphorus (P) and sulphur (S) as fertilizer and biological N2 fixation, and the major removals of these nutrients in harvested products (grain, meat and wool) for each year of the rotation of three pairs of farms were calculated. The farms were under a mixed cropping system of farming, common to the study area, in which grazed grass/white clover pasture and/or grass and white clover seed crops are grown in rotation with arable crops. One of each pair of farms was managed conventionally and the other was under an alternative organic or biodynamic system. The nutrient status of soils, harvested grain and pasture herbage were also measured along with the organic matter content and enzyme activities of the soils. Nutrient budgets (inputs minus outputs) for N, P and S on the conventional farms were generally balanced or positive so that the supply of these nutrients is unlikely to be limiting production. Nitrogen budgets were positive at all three alternative farms with biological N2 fixation accounting for most or all of the N input. One alternative farm had positive P and S budgets because of additions of compost, phosphate rock and elemental S. By contrast, at the other two alternative farms, net removal of P, and in one case S, occurred. In the other case, the S budget was balanced by one application (per rotation) of S fertilizer to the pasture. At these two sites, concentrations of grain N, P and S were lower than those at conventional sites and pasture herbage P and S concentrations were below recommended critical concentrations. Levels of total S and P and available P in soils were also lower on the alternative than conventional farms. Production on these alternative farms is relying on soil reserves of P and S, and additions of P and S will be required in the future to sustain current production levels. Organic C content and arylphosphatase, arysulphatase and urease activities tended to be higher under alternative than conventional pastoral management. This was attributed to the longer pastoral phase under alternative (3-4 year) than conventional (1-2 year) management resulting in a small build up of organic matter. However, organic C content and enzyme activities were similar under conventional and alternative systems during the arable phase suggesting that conventional management had no adverse effect on soil biological activity.