Effect of lime and nitrogen fertiliser on the plant availability of residual copper in pasture soils

摘要

Copper (Cu) is very strongly bound to soil organic matter and hence the plant availability of Cu added through fertilisers is likely to decline with increasing time of contact between soil and the Cu fertiliser. Nitrogen (N) fertilisers and lime are used in pasture soils to encourage out off-season pasture growth and to maintain the soil pH, respectively. Little is known about the effect of lime and N fertiliser on the availability of residual Cu to plants. The aim of the study was to examine the residual effectiveness of two Cu fertilisers, as influenced by lime and N fertiliser application. A large-scale glass house experiment was conducted to examine the effectiveness of a range of Cu fertilisers in a range of soils. The study, indicated that the effectiveness of Cu fertilisers decreases with increasing contact with the soil. A subsequent glass house experiment was conducted using the soil samples from the first experiment to examine the effect of lime and N fertiliser on the plant availability of the residual Cu in the soils. The treatments included: two soils (Manawatu fine silt loam and Ngamoka silt loam) with different fertility status, two Cu fertiliser sources (CuSO_4, and CuO) with different solubility, four levels of Cu (0, 5, 10 and 20 kg Cu ha~(-1)), three levels of lime (0, 1000 and 2000 kg lime ha~(-1)) and three levels of N (0, 50, 100 kg N ha~(-1)). Lime was incorporated with the soil 100 days prior to the sowing of ryegrass seeds and N fertiliser was added 45 days after sowing. Ryegrass was grown for 90 days and the dry matter (DM) yield and the Cu concentration measured. The fractions of Cu in the soil were also measured. Increasing levels of lime increased the DM yield of pasture, but decreased the Cu concentration in pasture at the highest level in both soils. The total uptake of Cu by ryegrass plants, however, increased with increasing levels of lime. Increasing levels of N increased both the DM yields and the Cu concentration initially in both soils. The effect on Cu concentration persisted beyond the first cut only at the highest N addition. There was no difference in the effect of both lime and N fertilisers on Cu uptake between the fertiliser sources. A substantial proportion (>80 percent) of Cu was present in the oxide and organic fractions and only a small amount was present in exchangeable and residual fractions. Increasing levels of Cu significantly increased the concentration of exchangeable Cu. The highest application rates of lime and N fertiliser decreased the exchangeable and free Cu in soil. The present study revealed that application of 1000-kg lime ha~(-1) and 50 kg N ha~(-1) was very effective in enhancing plant availability of residual Cu in soils.