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Land degradation in Mexican maize fields Sancholuz, Luis Alberto


This study seeks answers for two simple yet elusive questions: a) is land degradation a real threat to the productivity of the Mexican maize field? and b) can the application of fertilizer compensate for the losses of production due to fertility depletion by soil erosion? The thesis is based on an integrated examination of empirical evidence. International literature, national statistics, regional surveys, and greenhouse and field experiments are pursued in order to answer the above questions. In each case conclusions are drawn, but these conclusions vary with the level of analysis. Statistics of maize production in Mexico show net gains in productivity in the last thirty years. After correcting for the technological improvements in that period, it appears that the intrinsic productivity of the land has not declined. This is contrary to predictions in the literature on soil erosion and soil fertility depletion, particularly in the tropics. According to this literature, these maize fields are not only threatened, but they should already exhibit significant losses in productivity. To examine this conflicting evidence, a case study on three contrasting soil types was conducted in central Veracruz. Greenhouse experiments with erosion and fertilization of these soils suggest that fertilizers can compensate for losses of productivity resulting from erosion. Field experiments leave no doubt that the opposite is true: erosion dramatically reduces maize productivity and fertilizers do not compensate. In conclusion, the thesis offers an explanation for this paradox. As levels of analysis are abstracted from the field to the national level, or projected from the greenhouse to the field, critical information is lost. Measures of land productivity are too aggregated at the national level and too disaggregated in the greenhouse. This confuses the assessment of land degradation which requires the detection of small changes in land productivity. When land is properly considered, as in the literature reviewed and the field experiments included in this thesis, the result is clear. The productivity of the Mexican maize field will suffer from continuous land degradation, and this notwithstanding better management inputs.

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