Interpreting Analyses of Fossil Energy Use in Agriculture: What Does Efficiency Mean?.

There is recently renewed interest in research reporting the quantity of non-renewable energy embodied in fertilizer, fuel, machinery, and other synthetic inputs used in crop production and ratios of this input energy to crop yield as a measure of cropping system energy efficiency. Such methods are a component of Life Cycle Assessment approaches to quantifying greenhouse gas emissions associated with non-renewable energy use in agriculture. Critics of energy analysis have long recognized a lack of consistent terminology and methodology in this field of study, and some scientists have questioned the utility of this approach altogether, or for comparison among different crops or environments. A review of the critiques of agricultural energy analysis is presented here, along with a look at some paradoxes that arise when comparisons are made. Nitrogen fertilizer is often found to be the largest component of energy input, and in some systems it is realistically the only variable input. Optimization of energy input per unit of crop output by variation of N fertilizer level or N source is shown to be sensitive to the system boundaries chosen. It has been proposed by others that consideration of energy use efficiency in agriculture is useful only in the broadest sense: for comparing systems producing the same crops in the same environment. We add to this the suggestion that consideration of N fertility rates and sources should be considered on a marginal basis rather than on the basis of system level analyses, and should include consideration of changes in soil organic pools. Parsimonious approaches to methodology are preferred, and an effort should be made to standardize terminology, system boundaries, and methods.