SOME EFFECTS 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.