Abstract
Summary In sixteen factorial field trials with N, P, K, Mg, trace element and farmyard manure treatments, yields declined during the first four years of cropping following a resting period under grass, responses to K and farmyard manure increased, and other treatments showed no significant changes. After a few years of continuous cropping potassium deficiency may seriously limit yields, especially of sweet potatoes. N, P and K fertilizers maintained yields as well as the farmyard manure treatment.

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