1. General Model Information

Name: WWW On-line Model of Crop Response to Potassium Fertilizer

Acronym: POTASS

Main medium: terrestrial
Main subject: agriculture, biogeochemestry
Organization level: Not applicable
Type of model: not specified
Main application:
Keywords: potassium, fertilizer, response, model, on-line, web, interactive


Potassium Response Model:
Duncan J. Greenwood
Horticulture Research International,
Wellesbourne, UK
email: duncan.greenwood@hri.ac.uk

Online WWW-Installation:
Ian E. Currah
Agronomic Computing Consultant,
Stratford-on-Avon, UK.
email: ian.currah@ibm.net
homepage: http://www.qpais.co.uk/homes/currah.htm


Duncan J. Greenwood, Horticulture Research International, Wellesbourne, UK. and
T. B. Karpinets, Institute for the Protection of Soil from Erosion, Kursk, Russia.


The Potassium model calculates the effects of incorporating potassium fertilizer on the yield and potassium uptake of 13 different vegetable and arable crops. It also calculates the changes in exchangeable and in fixed soil-potassium and in the activity ratio of potassium in soil solution.
The model updates the variables for each day during growth but only the values at harvest are shown in the results.
A user-friendly interactive version of the model runs on-line from a World Wide Web page and immediately returns both numerical and graphical results to the user.

Model description:
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

This model is part of the NPK Web System
see also:

II. Technical Information

II.1 Executables:

Operating System(s): World Wide Web CGI Server
Model of Crop Response to Potassium Fertilizer Model Startpage

II.2 Source-code:

Programming Language(s):

II.3 Manuals:

Self-explanatory WWW input page

II.4 Data:

The input page is fully completed with default data which can be altered by the user.

III. Mathematical Information

III.1 Mathematics

Links to documentation and references are provided on the model's input page.

III.2 Quantities

III.2.1 Input

See WWW input page.

III.2.2 Output

Numerical and graphical.

IV. References

Full details of the model and experimental evidence on its validity are given in the three published papers cited below

Greenwood, D.J. & Karpinets, T.V. 1997. Dynamic model for the effects of K fertilizer on crop growth, K-uptake and soil-K in arable cropping. 1. Description of the model. Soil Use and Management 13, 178-183.
Greenwood, D.J. & Karpinets, T.V. 1997. Dynamic model for the effects of K fertilizer on crop growth, K-uptake and soil-K in arable cropping. 2. Field test of the model. Soil Use and Management 13, 184-189.
Karpinets, T.W. & Greenwood, D.J. 1997. Modelling and measurement of the effects of K-fertilizer on yield, crop uptake and soil-K. In: Plant Nutrition for sustainable food production and environment. p 499-500. Ed.: T. Ando et al. Kluwer Academic Publishers.

V. Further information in the World-Wide-Web

VI. Additional remarks

This is a sophisticated research-based model which has been made so easy to use that it can be started with a single mouse click.
Last review of this document by: Dr Ian E. Currah Fri Oct 17 1997;
Juergen Bierwirth Mon Nov 9 1998
Status of the document:
last modified by Joachim Benz Wed Mar 19 14:51:09 CET 2003

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