The movement of nutrients within the root zone of orchard crops is important in
determining both fruit yield and quality. Currently much of the research on solute
movement in field soils concerns movement of chemicals to ground water. Little
attention has been paid to smaller scale movement. In this study the movement of
solutes in response to intermittent soil water flow was investigated in columns of
repacked silt loam in the laboratory and in a similar soil in the field.
In the laboratory study a 5mm pulse of a solution of potassium bromide and urea in
tritiated water was applied to columns of repacked soil, left for three or ten days, and
then leached with 30 mm of distilled water. Twelve days after the solute pulse was
applied, the distributions of water, tritiated water, applied and resident nutrients and pH
The bulk of the bromide and tritiated water was moved to between 50 and 1 50 mm
depth in both water treatments. As the nitrogen applied in urea was mainly in the form
of ammonium after three days, the water applied then caused little movement of
nitrogen. But the water applied after 10 days caused the nitrogen, now in the form of
nitrate, to move in a similar fashion to the bromide.
The soil solution anion concentration determined the amount of cations leached.
Calcium and magnesium were the dominant cations accompanying the nitrate and
bromide downwards. The added potassium remained near the soil surface.
Given the soil hydraulic properties, the behaviour of water and solutes could be
simulated by coupling the water flow equations with the convection-dispersion equation,
and by using solute dispersion , diffusion and adsorption parameters derived from the
literature. The model assumed the Gapon relationship for cation exchange, and that
hydrogen ion production during nitrification reduced the effective cation exchange
capacity. It was able to simulate closely the experimental data.
Two field experiments were conducted. The first involved application of a 5 mm pulse
of potassium bromide solution followed by 50 mm of water to pasture plots of
contrasting initial water content. Twenty-four hours later core samples of soil were
collected and the distribution of water and bromide measured. Bromide applied to
initially dry soil was much more resistant to leaching than bromide applied to moist
The second experiment lasted 12 days and was essentially an analogue of the laboratory
experiment. The final nutrient distributions however differed considerably from those
obtained in the laboratory, due to non-uniform flow in the structured field soil.
Coupling a mobile-immobile variant of the convection-dispersion model with a
description of the water flow provided a mechanistic model. When combined with the
submodels developed in the laboratory study describing nutrient interactions and
transformations, this model successfully described the solute movement under the four
different field regimes of water and solute application. Evaporation and plant uptake,
and diffusion between mobile and immobile phases emerged as key processes affecting
nutrient movement. It is suggested some control over nutrient movement is possible
by varying the relative timing of water and fertiliser applications.
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Tillman, R. W., Scotter, D. R., Wallis, M. G., & Clothier, B. E. (1989). Water-repellency and its measurement by using intrinsic sorptivity. Australian Journal Of Soil Research, 27(4), 637-644.