NUTRIENT LOSSES IN RUNOFF WATER FOLLOWING APPLICATION OF DIFFERENT FERTILIZERS TO GRASSLAND CUT FOR SILAGE

Studies were made to assess the losses of nutrients in runoff from pas tures, and to ascertain whether application of N as NH4 rather than NO 3 both reduced N losses and enhanced yields of herbage. Two experiment s were conducted during consecutive springs in which surface lysimeter plots were established on sloping grassland with soils of low permeab ility, and different forms of fertiliser applied for first-cut silage. Despite weather conditions conducive to large nutrient losses, less t han 10% of the N applied was found in runoff. However, the peak concen trations of nutrients were large relative to standards for good water quality. The concentrations of NO3-N were greater in water from plots that received NH4 NO3 than from those that received either (NH4)(3) PO 4 or urea. Large concentrations of NH4-N were measured in runoff in re sponse to all treatments, with the largest in water from plots that re ceived urea. The mechanisms that may account for the observed resistan ce to loss in runoff are discussed. In the first experiment there was no advantage (or disadvantage) in applying an (NH4), PO4 formulation 2 weeks earlier than applying either NH4 NO3 or urea. In the second exp eriment, where all treatments were applied on the same dates, herbage mass accumulated more slowly on plots that received NH4 NO3 compared t o those that received more NH4-rich forms. However, in neither experim ent did the fertiliser treatments influence the yields of herbage cut for silage.