Conclusions:
The changes in farming practice which have taken place in the last 30 years include an increase in the use of artificial fertilizers for fertility control and maintenance, and a less close integration of crops and stock on many farms. There is evidence that although most of the farm crops and grass grown are destined to be consumed by stock, most advisors only consider the economics of the primary crop response rather than those of the secondary animal response when they decide fertilizer policy.

Some examples have been given to illustrate the problems encountered when animal production is considered as the output term which is dependent on fertilizer inputs, and the complexities are apparent. The complexities are obviously even more intricate if one considers the whole process with which farming is concerned, namely the relationships between primary inputs of which fertilizers are but one, and the ultimate output, namely the satisfaction and well-being of the people we are trying to feed.

Dr K L Blaxter FRS, The Rowett Research Institute, Bucksburn, Aberdeen, UK.

46 pages, 6 figures, 9 tables, 31 references.