Modeling soil nutrient limitations to crop production in semiarid NE of Brazil with a modified EPIC version : I. Changes in the source code of the model

A series of modifications for the source code of EPIC version 0160 in order to simulate growth, yield, and nutrient uptake by the crops under the pedo-climatic conditions of the semiarid Northeastern Brazil are presented. The modifications included changes in chemical properties of the soil due to liming as manual option, changes in the estimation of soil Al saturation, effects of Na on the soil physical properties, interaction between climatic and nutrient constraints to crop growth, modifications in the estimation of labile P factor for uptake and in the calculations of the N and P stress factors which are now estimated as a function of the ratio between the daily uptake and daily demand for the potential growth. Moreover, the structure of a simple soil and plant K model designed to be incorporated into EPIC is described. The model maintains pools of exchangeable, soluble and fixed K for the mineral form in the soil; of crop residue K for the organic form and of grain, stover and root K for the crop form. The model runs on a daily time step, simulating K uptake by the crops and transformations between the different soil pools as well as several other processes like leaching, runoff, lateral subsurface flow, movement upward through soil evaporation, K recycling from crop residues and the effect of limited soil K availability on crop development. The model is sensitive to soil chemical and physical properties, crop K requirements, crop competition, tillage practices, fertilizer rate, and soil water content. In a second paper of this series, the test of the proposed modifications, including the plant and soil potassium sub-model, for simulations of crop growth, yield and nutrient uptake of a maize × cowpea intercrop in the semiarid NE of Brazil is presented. (Résumé d'auteur)