Fertiliser responses of maincrop potatoes: A re‐examination of the experimental evidence

The results of recent manurial experiments on maincrop potatoes were re‐examined to determine (i) what general changes in level of response had occurred since the summary of Crowther & Yates in 1940, and (ii) the nature of the interactions between nutrients and their influence on the form of the fertiliser response curve and on optimal fertiliser dressings. Average responses to N 0.8 cwt., P 2 O 5 1.0 cwt. and K 2 O 1.5 cwt. per acre in over 100 experiments done since 1940 were 1.8, 1.4 and 2.0 tons per acre respectively, and these do not differ by more than 0.2 ton per acre from the figures of Crowther & Yates. Most of the recent experiments on mineral soils showed large interactions between nutrients which affected both the magnitude of the response to a given dressing and the degree to which the response fell off with increasing levels of dressing. The concept of a ‘standard’ response curve of exponential form did not adequately describe the response surface revealed in these experiments. Evidence is presented that, for any nutrient, the response curve rises to a maximum and then begins to fall, and that the level of dressing at which the fall begins depends on the amounts of the nutrient present in the soil, on the supply of the other plant nutrients both by the soil and from fertilisers and farmyard manure, and on other factors such as the method of fertiliser application. For fertiliser applied over the ridges at planting, optimal dressings were estimated to be N and P 2 O 5 1.0–1.2 cwt. per acre and K 2 O 1.5–2.0 cwt. per acre, but the optima were ill determined because the fertiliser rates tested in most of the experiments were too low.