The crop is visualised as growing with all its roots in a single layer of soil, 0-30 cm from the soil surface. The soil is considered to be uniform and the roots and water to be uniformly distributed throughout it. The water content is updated each day depending on the initial soil moisture deficit, the soil type, the potential evaporation from an open water surface, the percentage crop cover and plant weight. (See weather data link at the foot of this page.) Starter fertilizer is assumed to occupy a fixed volume of soil throughout. The enriched zones of soil around phosphate fertilizer granules increase in size with time from incorporation until they reach a maximum. Phosphate is assumed to exist throughout the soil as solution, labile soil-phosphate, assumed to be that extracted with a reagent such as 0.5M bicarbonate, and non-labile soil-phosphate which is the remainder of phosphate in soil. An adsorption/desorption isotherm governs the relation between solution and labile soil-phosphate. An exchange reaction, with velocity constants characteristic of the soil, governs the interdependence of labile and non-labile-P.
For each day , the model calculates the increment in root growth and partitions it into segments between the regions of soil enriched with starter fertilizer, those enriched with granular fertilizer, and the remainder of soil. It calculates the maximum possible amount of P that can diffuse through the soil to each root segment in each region. Using this information and the P concentration in the plant, total P uptake is calculated. The increment in plant weight and root growth is calculated from the current plant weight, plant P concentration and air temperature.
The treatment of transport of phosphate through soil to the root surfaces is the same, irrespective of whether the roots are in fertiliser phosphate enriched zones or not. Even so, calculation of phosphate transport through each segment of root is carried out separately depending on its age, and whether or not it is in a granular phosphate enriched zone of soil. Transport of phosphate from soil is by diffusion and takes account of soil type, buffer capacity and soil water content and the dependence of uptake on the phosphate concentration within the plant. Mass flow transport is ignored. The interchange between solution, labile and non-labile forms of phosphate are recalculated for each day in the phosphate depleted regions around each segment of root and in the fertilised and unfertilised regions of soil into which no roots have penetrated. Routines are included for the effects of daily weather on the various processes.
Vegetable species differ considerably in their responsiveness to phosphate. Most of the "species" parameters in the model are considered to be the same for all species. Differences between them are attributed in most cases to differences in an "effective" root radius. The model is calibrated for this parameter by simulating the dry weights and the % P in the dry matter for an experiment with different levels of soil-phosphate and finding which value gives the closest agreement with the experimental measurements. For some species it was found necessary to calibrate the model for differences in both the "effective" root radius and the minimum possible % P that can occur in the plant dry matter. The validity of the calibrated model was tested against the results of independent experiments on the same soil type. There was quite good agreement between predicted responses of plant dry weight and % P and those measured experimentally.
Click these links for the Phosphate Model flow diagram page and a description of the weather data processing.
( DJG, Oct. 2000, modified Feb. 2002 )
(source: Description of the Phosphate Model)
GREENWOOD DJ, KARPINETS TV and STONE DA (2001) Dynamic model for the effects of soil and fertililzer-P on crop growth, P-uptake and soil-P in arable cropping: model description. Annals of Botany. 88, 279-291.
GREENWOOD DJ, STONE DA and KARPINETS TV (2001) Dynamic model for the effects of soil and fertililzer-P on crop growth, P-uptake and soil-P in arable cropping: experimental test of the model for field vegetables. Annals of Botany. 88, 293-306.
source: Description of Phosphate Model