This paper is aimed at assessing the comparative importance bf constraints in land and labour endowment for energy balance in agriculture, when assessed at the level of national crop production systems. The relation between output/input energy ratio of agriculture (output: food energy in crops; input: commercial energy embodied in technical inputs), average labour productivity (food energy (in Joules) produced per hour of labour allocated to agriculture) and land productivity (food energy (in J) produced per hectare of cropped land) has been studied on a 75-country sample using a cluster analysis procedure. A cross-section equation has been developed, explaining output/input ratio in terms of intensity of land and labour-food-energy throughputs. The results suggest that land constraints, with respect to the total population size, rather than labour constraints, tend to be associated with comparatively higher energy requirements in agricultural production. Therefore, the 'emancipation' of agricultural production from land shortages implies two 'biophysical costs.' That is, for the production of the same amount of food, a consistent increase in demographic density implies both a larger consumption of fossil energy input and a larger environmental impact. (C) 1997 Elsevier Science B.V.
