Some effect of sodium application on the yield and chemical composition of pasture grown under differing conditions of potassium and moisture supply

The effects of Na fertiliser (supplied as an NPK-Na compound) on herbage yield and composition were examined at two different sites to see if pasture responses to Na were affected by differences in K and moisture availability. At one site, pasture was grown under conditions of moisture stress and limited K availability, whereas at the other site the pasture was grown under comparatively non-stress conditions.The results were interesting in that Na fertilisation appeared to be detrimental to pasture yield and quality under conditions of moisture stress and suboptimal K supply, whereas under the non-stress conditions it actually increased herbage N offtake, marginally improved the nutritional quality of the pasture and produced appreciable (albeit non-significant) increases in DM yield. It was suggested that an important effect of Na on grass production may have been its ability to enhance the rate of NO₃^– uptake by plants, thereby minimising NO₃^–-N losses from the soil-plant system by denitrification. However, because the amount of N fertiliser used in the experiments (i.e. 390 kg N ha^–1 yr^–1) was close to that normally required for maximum yield production (N max) under Northern Ireland conditions, the scope for yield increases in response to Na-elicited improvements in N offtake were thought to have been very limited at both experimental sites.On the basis of results from both the present study and previous field trials, a unifying theory is presented to explain the differential effects of Na on NO₃^– uptake and herbage growth under different sets of circumstances.