Nutrient levels in the soil and the yield of potatoes following cut grass, grazed grass and arable cropping

Mineralisable N, exchangeable K and the concentrations of K, Na, Mg and Ca in equilibrium soil extracts were determined after 4 years under cut grass, grazed grass and arable cropping with several levels of N and K applied as fertilisers. The yield of potatoes which followed these systems of cropping and manuring ranged from <20 to >60 t/ha. More than 50% of the yield variance was related to differences in the levels of mineralisable N and exchangeable K in the soil, compared with 17% which was related to levels of fertiliser (0 and 112 kg N/ha, 0‐250 kg K/ha) applied direct to the potatoes. Levels of all nutrients measured in the soil and yields of potatoes were higher after grazed grass than after cut grass or arable cropping. Concentrations of Mg and Ca in the foliage of the potatoes were not, however, appreciably affected by previous systems of cropping and the yield of tubers seemed to be determined primarily by differences in the supply of N and K. Application of these two nutrients as fertilisers to the potatoes largely rectified any deficiency in their supply from the soil which resulted from the previous cropping.