The manuring of maincrop potatoes on warp soils.

The effects of 6 rates of N, P and K on the yield of ware potatoes was studied at 32 sites in N. and S. Humberside in 1972-7. The variations in yield between sites were of similar magnitude to the increases given by opt. fertilization. 73% of the variation in the overall mean ware yield between sites was due to yr and K index. Responses to N were generally large and positive at 25 sites and were good when the OM content of the soil was low, poor when it was high and variable for intermediate values. The av. opt. rate of N was 143 kg/ha. At 14 sites there were low positive responses to all rates of P and the net response to P was good for soil P index 0 but was variable for P index >0. There were small and linear positive responses to all rates of K at 8 sites and interactions between nutrients at 5 sites. No opt. rates were determined for P or K. RESP-826