Description of the Phosphate Routine
(derived from PHOSMOD)
This dynamic
model calculates the effects of soil-phosphate and granular fertiliser
phosphate on daily crop growth, phosphate concentration in the plant, and the
changes in the different forms of soil phosphate (See below for a flow diagram) for conditions when growth is not
limited by deficiency of either P or K. It is mechanistic and largely based on
well-known equations for key processes. The inputs are generally easy to
obtain.
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.)
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 region of soil 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.
Diagram of the Phosphate Model
( DJG, April 2008)
Flow Diagram
of the Phosphate Model
The meanings of the symbols are as follows:
- Boxes. Variable quantities
or in some cases, such as 'Soil properties', groups of quantities.
- Lines and arrows. The interdependence
of the variables.
Brief Explanation
The flow diagram
represents a simplification of the model. Soil phosphate is very immobile and
moves only short distances. The model treats the processes associated with
broadcast granular fertilizer and soil phosphate separately but in the diagram
they are combined.
The model
assumes that root absorbtion of phosphate is from the
upper 30 cm of soil and there is no leaching.
The different
components of weather are represented by a single box on the right hand side of
the diagram and the various soil properties by a single box at the top of the
diagram. The different types of fertilizer-phosphate are also represented by
another single box at the top of the diagram. The various below ground
processes are presented in the upper section of the diagram and the above
ground processes in the lower section.
The numerous
processes in the model are represented by equations. These are solved for each
day of the simulation and the variables updated accordingly. The main inputs to
the equations used for calculating each variable are given by the sources of
the arrows in the diagram, For example, the % P in the plant is calculated from
the total P in the plant and plant dry weight.
Moving from the
top of the diagram downwards, the model calculates for each day new values for
the increment in root length, the soil water content, the diffusion coefficient
and the different forms of soil-P. From this information and the weather
conditions it calculates the maximum possible P-uptake by the plant. This is
reduced depending on the % P already in the plant to give the actual P-uptake,
and a new % P in the plant which is then used with the weather conditions and
the existing plant weight to calculate a new plant weight and also a new
increment in root length and the cycle of calculations begins again.
( DJG. April 2008 )
GREENWOOD,
D. J., STELLACCI, A. M., MEACHAM, M. C., BROADLEY, M. R. & WHITE, P. J. (2004) Brassica cultivars: P response and fertilizer efficient cropping. Italus Hortus 11: 17-19.
[Special issue ISHS symposium towards ecologically sound fertilizer
strategies for field vegetable production
GREENWOOD,
D.J., STELLACCI, A.M., MEACHAM, M.C., BROADLEY M.R., & WHITE
, P. J.
(2006).Brassica cultivars: P response and
fertilizer efficient cropping. Acta Horticulturae 700: 97-102.
GREENWOOD, D.J.,
STELLACCI, A.M., MEACHAM, M.C., MEAD, A., BROADLEY M.R., & WHITE , P. J. (2006).Relative values
of physiological parameters of P response of different genotypes can be
measured in experiments with only two P treatments. Plant and Soil 281:
159-179
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TV,
KARPINETS,
T. V.,
KARPINETS , T.V.,
KRISTOFFERSEN,
A O.,
WHITE,
P.J., BROADLEY,
M.R.,,
WHITE,
P., BROADLEY,
M., BURNS,
WHITE,
P.J., BROADLEY,
M.R.,,