DYNAMIC-MODEL FOR THE EFFECTS OF K-FERTILIZER ON CROP GROWTH, K-UPTAKE AND SOIL-K IN ARABLE CROPPING - 1 - DESCRIPTION OF THE MODEL

To provide a practical aid to improving fertilizer practice a mechanis tic model was developed that can be readily calibrated for widely diff erent crops. Most of the inputs are easy to obtain and the others, the amounts of fixed soil-K and the velocity constants for fixation and r elease of soil-K, can be readily measured by a novel procedure which i s described. The model calculates for each day the potential increase in plant weight and the increment in root length, from the current pla nt mass, its %K and pan evaporation. It calculates the maximum amount of K that could be transported through soil to the root surfaces. It m odifies this potential uptake by taking account of the 'feedback' of p lant K on root absorption to give the actual uptake and a new %K in th e plant. It calculates the radii of the depletion zones around each ro ot increment and the interchange between the solution, exchangeable an d fixed-K in these zones and also in the undepleted regions of soil. R outines are included for the effects of weather on the various process es. Differences between species are accommodated by selecting one of t hree algorithms for root growth and by adjusting the values of two cro p-K parameters that define the decline in a critical and a maximum pos sible %K with increase in plant mass per unit area. A simplified versi on of the model runs interactively on the Internet at: http://www.qpai s.co.uk/moda-djg/potass.htm.