Description of the Potassium Model
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. (See below for a 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.
Flow Diagram of the
Potassium Model
REFERENCES
Full
details of the model and experimental evidence on its validity are given in the
published papers cited below.
KARPINETS T.V. & 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 499-500. Ed T. Ando et al. Kluwer Academic Publishers.
KARPINETS T.V. & GREENWOOD,
D.J. 2003
Potassium dynamics. In Handbook of Processing and Modeling in the Soil-Plant
System 525-559. Eds D.K. Benbi & R. Nieder.