Reference:
F. V. WIDDOWSON, A. PENNY and M. V. HEWITT (1982) Results from the Woburn Reference Experiment, III. Yields of the Crops and Recoveries of N, P, K, and Mg from Manures and Soil, 1975-79 in : Rothamsted Experimental Station Report for 1981 Part 2 - pp 5 - 21