Calibration of quefts, a model predicting nutrient uptake and yields from chemical soil fertility indices

The model quefts (QUantitative Evaluation of the Fertility of Tropical Soils) was calibrated using data from maize fertilizer trials in Kenya. quefts describes, in four steps, relations between (i) chemical soil test values, (ii) potential NPK supply from soils and fertilizer, (iii) actual NPK uptake, and (iv) maize grain yield, acknowledging interactions between the three macronutrients. All steps were calibrated separately, and yield a modified version of quefts. Major changes were the inclusion of ambient temperature and clay content in explaining potential nitrogen supply, and the replacement of the parabolic relation between potential supply and actual uptake by an exponential relation. The goodness of fit (r2) between measured and calculated yield was improved from 0.66 in the original version to 0.78 in the modified version of quefts and, when including a boundary condition for harvest index, to 0.88. A satisfactory validation was conducted with input data from fertilizer experiments in other parts of Kenya. Sensitivity analysis revealed that changing the parameters pH and organic N by 20% caused yield differences of at least 10%. The basic thinking and theoretical concepts underlying the original version of quefts still apply to the modified version. Agronomists in tropical environments are encouraged to collect the relatively few input data to further validate the two versions of the model. As a consequence, quefts can contribute to a more efficient procurement and use of mineral fertilizers at both regional and farm level.