Productivity of intensified crop-fallow rotations in the Trenbath model

Management of crop-fallow rotations should strike a balance between exploit ation, during cropping, and restoration of soil fertility during the fallow period. The 'Trenbath' model describes build-up of soil fertility during a fallow period by two parameters (a maximum level and a half-recovery time) and decline during cropping as a simple proportion. The model can be used to predict potential crop production for a large number of management optio ns consisting of length of cropping period and duration of fallow. In solvi ng the equations, the model can be restricted to 'sustainable' systems, whe re fallow length is sufficient to restore soil fertility to its value at th e start of the previous cropping period. The model outcome suggests that th e highest yields per unit of land can be obtained by starting a new croppin g period after soil fertility has recovered to 50-60% of its maximum value. This prediction is virtually independent of the growth rate of the fallow vegetation. The nature of the fallow vegetation (natural regrowth, planted trees, or cover crops) mainly influences the crop yield by modifying the re quired duration of fallow periods. Intensification of land use by shortenin g fallow periods will initially increase returns per unit land at the likel y costs of returns per unit labor. When fallows no longer restore soil fert ility to 50% of the maximum, overall productivity will decline both per uni t land and per unit labor, unless external inputs replace the soil fertilit y restoring functions of a fallow.