Published online by Cambridge University Press: 27 March 2009
In a long-term experiment on a boulder-clay soil Ralph & Ridgman (1981) found that the response of potatoes to freshly applied K fertilizer was independent of the amount of K that had been applied during the preceding 10 years. The greatest yield was obtained from plots which had received the greatest amount of K during the preceding 10 years together with the optimum amount of freshly applied K (120 kg K/ha in this case) and the addition of an extra 60 kg K/ha to plots which had received less K previously did not increase the yield. Similar benefits from residual K had been reported by Johnston, Warren & Penny (1970). In view of the continuing disagreement about the need for applying K to cereals on boulder-clay soils it was decided to attempt to quantify these effects more precisely.