Apparent recovery of fertilizer nitrogen

Attention is paid in the present paper to the conflicting information in the literature about the recovery of fertilizer nitrogen by crops. One reason for this inconsistency is suggested to be the fact that in several cases the difference in the nitrogen content of the crops receiving all the main nutrients and that of the unfertilized crops is supposed to represent the amount of nitrogen taken up from the fertilizer. This has been done even when results of the same experiment distinctly show that an application of potassium or phosphorus fertilizers has markedly increased the amount of nitrogen in the crop as compared with the unfertilized one. In some Danish and Finnish longterm field experiments the uptake of fertilizer nitrogen by the crops of the whole rotation has been reported to be 60 to 104 per cent. When the apparent recovery of fertilizer nitrogen is computed on the basis of results from treatments which differ only in regard to the nitrogen application, values ranging from 11 to 55 per cent of the amount applied are obtained from these experiments. In two American longterm rotation experiments with grains the recovery of fertilizer nitrogen varies from 18 to 42 per cent. The present of root crops in the rotation appears to increase the recovery of fertilizer nitrogen. It seems that the residual effect of fertilizer nitrogen is worth some more consideration. Results of Finnish pasture experiments are in accordance with the statement by COOKE (3) that on the average two thirds to three quarters of the applied fertilizer nitrogen is taken up by grass. In field experiments carried out with oats by the present institute the apparent recovery of fertilizer nitrogen by grains and straw ranged from 24 to 76 per cent of nitrogen applied as calcium nitrate or ammonium nitrate limestone. In general, the recovery was related to the nitrogen conditions of the soils characterized by the contents of total nitrogen and mobilizeable nitrogen. The relative recovery did not seem to depend on the rate of application. No significant difference could be found between the uptake of nitrogen from the two fertilizers, although some tendency towards a slightly better utilization of calcium nitrate may exist. The variation in the uptake of fertilizer nitrogen is large, but obviously, the apparent recovery by arable crops, except by root crops, is usually more likely to be less than 50 per cent than to exceed 70 per cent. Therefore, studies on the improvement of the efficiency of nitrogen fertilizers seem to be necessary