Long-Term Influence of Management Practices on Nutrient Supply Potential of a Silt Loam Soil.

A long term trial was established on a Wakanui silt loam in 2000 to quantify the effects of management (tillage intensity, i.e., intensive, minimum and no-tillage; plus or minus winter cover crops) on soil fertility and crop production following the conversion of long term pasture to arable cropping. In 2013, soil samples were collected (0-7.5, 7.5-15, 15-25 cm depths) from five treatments in at Lincoln, Canterbury. Nitrogen (and C) mineralisation potential were determined by incubating samples for 98 d (25^oC, -60 kPa water potential). The effects of the treatments on chemical fertility were evaluated by using standard soil tests (Olsen P, pH, available cations, cation exchange capacity (CEC)). Under arable cropping, N mineralisation potential declined by 22 to 28% compared with long-term pasture. Tillage type had little effect on total mineraliation in the top 25 cm, but it did affect the vertical distribution of mineralisation (more concentrated near soil surface under no-tillage; uniform depth distribution in intensively tilled plots). Mineralisation was least in the long-term fallow (67% less than in pasture), which had negligible inputs of fresh organic matter in the 2000-2013 period.  As a result of a decline in soil organic matter under arable cropping and long-term fallow, there was a decrease in CEC (pH 7) of up to 22% at 0-7.5 cm depth. There was substantial decrease in available cations under arable cropping, with K decreasing by 46 to 73% and Mg by 36 to 52%. Although most indicators of nutrient availability were low in the fallow treatment, it had highest levels of Olsen P, presumably due to accumulation of mineralised P.