Long term effects of fertilizers on grassland: III. Effects on some soil properties

Effects of N as ‘Nitro-chalk’, P as superphosphate, and K as KCl and K₂SO₄ on ‘available’ P, K and Mg were examined over 14 years. The area was under grass/clover regularly cut 6 times a year. Soil pH was maintained by annual applications of ‘Nitro-chalk’ supplying an average of 4 cwt CaCO₃ and 174 lb N per acre. Superphosphate supplying 43 lb P per acre per annum increased available P (modified Morgan method) appreciably even on plots where more than this quantity was taken off in herbage. Applied N also consistently increased available P. It was thought that this was because P in the root system was rapidly mineralized. Applied K consistently reduced available P because of increased uptake in cut herbage, with no apparent increase in root P for mineralization. The amount of available K in soils which received no added K was fairly constant over the period for a given N treatment. About 40 lb/acre/annum of K was released by weathering. There was a very marked interaction effect between rates of N and K fertilizers on available soil K, the large increase in available K where K fertilizer but no N was applied being considerably reduced with increasing N rates. These effects directly reflected uptake of K in cut herbage. Residual values of applied P, K and Mg on the respective ‘available’ nutrients were considerable and persistent. Available Mg was reduced by increasing rates of fertilizer N and by fertilizer K.