Farmyard manure affected the response of the potato crop to fertilizers similarly in seven series of experiments (sixty-two in all) at Rothamsted, and in the West Midlands and north of England.

In the absence of f.y.m. there were large interactions between each of the fertilizer nutrients, but with f.y.m. the interactions were small. These interactions appear to have been ignored by previous workers, who claimed that N responses ar unaffected by dressings of f.y.m. In fact, f.y.m. increased the response to N applied alone, but decreased the N response where basal P and K fertilizers were also applied. For P the results were similar to those for N.

Most of the experimental sites were low in K, and K responses were very large, especially in the presence of basal NP fertilizer. Farmyard manure greatly reduced the response to K, particularly in presence of basal NP fertilizer.

A few Rothamsted experiments with f.y.m. and fertilizers and several times of planting showed that the large responses to f.y.m. alone and to fertilizers used in combination (but without f.y.m.) were not reduced by late planting. The result conflicts with previous reports of these data, in which interactions between nutrients were ignored. The combined effect of f.y.m. and fertilizers at late planting was little more than their separate effects, presumably due to delayed maturity, the influence of which would be exaggerated by the early burning-off of haulm in this series of experiments.

Most of the experiments were limited to testing fertilizers at two levels (presence and absence), so the effective quantities of nutrients provided by f.y.m. for the crop to which it was applied could only be estimated approximately; the average amounts suggested for normal f.y.m. are 0·3 cwt. N, 0·4 cwt. P2O5 and 0·75 cwt. K2O in 10 tons f.y.m.

These results lead to the practical conclusion that, in manuring potato crops on average land, much the same plant food ratios will be appropriate whether or not F.Y.M. is given, and the amount of fertilizer applied can be decreased to allow for the nutrients contained in the f.y.m.