Ian E. Currah
Agronomic Computing Consultant,
The crop is visualized as growing with all its roots in a single layer of soil , 0 - 30 cm, from the surface. The soil is considered to be uniform and the roots and water are considered to be uniformly distributed throughout it.
The model recalculates, the plant weight and K content and the exchangeable and fixed
soil-K for each day.
(Click here for a simplified flow diagram.)
It first calculates the increment in growth from the current weight of the plant, the corresponding critical %K and the current %K in the plant. The increment in root length is calculated from the most recent plant weight and increment in weight using whichever of three species related relationships is most appropriate to the crop.
The model calculates the potential maximum amount of K that can be transported by diffusion through soil, with a small correction for mass flow, as the sum of the amounts transported to each of the "active" increments of root formed during the life of the crop; segments are considered to be fully "active" for 20 days after their formation and then cease to function. The actual uptake is usually less than this potential maximum because of the suppressive effects of plant %K on uptake, which in the model is calculated from the %K in the plant and the maximum possible %K for a plant of that size. Both this maximum possible %K and the critical %K decline with increasing plant mass and the way they do so varies with the species by relationships that are embedded in the model.
The model also calculates for each day the maximum radial distance in soil round each increment of root where root absorption has induced a significant decrease in exchangeable soil-K. This distance is assumed to be the radius of the depletion zone around the segment. There is a continuous interchange between the fixed and exchangeable soil-K in these depletion zones. Also when fertilizer is applied there is a continuing interchange between fixed and exchangeable soil-K in the remainder of the soil that has not been affected by root uptake. The model re-calculates for each day the interchange between the exchangeable and fixed soil-K for the depletion zone round each root increment and in the body of the soil. It also calculates mean values of exchangeable and fixed soil-K in the rooting layer.
No correction is made in the model for loss of plant-K during senescence which can be
considerable especially from cereals. Nor does the model include any relationships for
uptake of K from the subsoil or for the substitution of K by Na.
The initial activity ratio of the soil [K]/sqrt([Ca] +[Mg]) and its gradient with exchangeable soil K are calculated very approximately from the value for a soil containing 15 % clay, dominated by mica smectite, in the UK West Midlands and the percentage clay in the input to the model.
Full details of the model and experimental evidence on its validity are given in the three published papers cited below (see IV. References).
The Web version:
The Web version of the model is a simplification of a more sophisticated one. The simplifications include calculating the important relationships between K activity ratio and exchangeable soil-K from the clay content and assuming a constant soil water content instead of calculating it for each day from from daily inputs of weather.
The program POTASS (version 1.0) was compiled and linked from one large source module
and two smaller ones. The large module embodies the potassium mathematical model itself.
The two smaller modules support the model on a World Wide Web server.
They incorporate the Common Gateway Interface (CGI) routines and they are connected to
a system of Hypertext Markup Language (HTML) files. These HTML files appear as the
'Web pages' on the remote screens of users running the model.
There is an input page, there are two output pages and there are four pages of associated information.
Source of abstract:
QPAIS Potassium Crop Response Model pages:
Potassium Crop Response Model discription,
Potassium Model Startpage