Timing incorporation of different green manure crops to minimize the risk of nitrogen leaching

Seven field trials at four research sites were carried out to study the eff ect of incorporation time of different plant materials on soil mineral N co ntent during two successive seasons. Annual hairy vetch (Vicia villosa Roth ), red clover (Trifolium pratense L.), westerwold ryegrass (Lolium multiflo rum Lam. var. westerwoldicum) and straw residues of N-fertilized spring bar ley (Hordeum vulgare) were incorporated into the soil by ploughing in early September, late October and the following May, and by reduced tillage in M ay. Delaying incorporation of the green manure crop in autumn lessened the risk of N leaching. The higher the crop N and soil NO,-N content, the greater t he risk of leaching. Incorporation in the following spring, which lessened the risk of N leaching as compared with early autumn ploughing, often had a n adverse effect on the growth of the succeeding crop. After spring barley, the NO3-N content of the soil tended to be high, but the timing of incorpo ration did not have a marked-effect on-soil N. With exceptionally high soil mineral N content, N leaching was best inhibited by growing westerwold rye grass in the first experimental year.