Abstract:
As
plant roots grow through the soil, the hydraulic properties of the adjacent soil may
change and be altered through the release of exudates and mucilage or through the re-
arrangement of soil particles. Though these two possible mechanisms have been
identified in the literature, the published work has not distinguished whether the
change in water release and sorption are due to the mucilage affecting surface tension
and/or contact
angle or if the growing root re-arranges the soil particles so that there is
less
pore space that can hold water. In this thesis the effect of mucilage and particle
re-arrangement on rate of inltration and water release has been studied.
First, the effect of mucilage and particle re-arrangement on rate of inltration of water
was
investigated. Wheat, maize and barley plants were grown in silty loam cores. The
results illustrated
significant differences between these plant species grown in the soil
cores. More
importantly when maize seedlings were grown in sandy soil rhizoboxes
the
rhizosphere and bulk soil were significantly different, with the sorption rate being
greater for the bulk soil. To explain these differences the impact of both soil density
changes and mucilage on inltration in soil were studied. The lower density was
significantly different compared with the higher density, with the lower having a
greater sorption rate. There was no effect of mucilage on inltration but there were
significant differences between the sorption rate at different time intervals.
The effect of
mucilage and particle re-arrangement on water release was investigated
using rhizosphere and bulk aggregates and soil from plastic rings. The water content
of
rhizosphere soil was not significantly different from that for the bulk soil at
saturation, suggesting that roots had no effect on the porosity of the soil. However the
water content of the
rhizosphere soil for maize and barley was significantly lower
(P<0.05) at a pore water pressure of -0.15 kPa. The capillary fringe in glass capillaries
using deionised water and natural mucilage was measured and it was found that
mucilage decreased the capillary fringe to 0.7 of that for pure water. However
application of this value to the bulk soil water release characteristic did not account
for the
changes observed in the water release characteristic for the rhizosphere soil.
Possible
explanations for the results from the sorption and water release experiments
include
wetting and drying, change in contact angle and surface tension by mucilage,
particle re-arrangement increasing soil density, and microbes altering mucilage
concentrations. It is concluded that the
re-arrangement of soil particles by roots was
found to be more
important than the changes caused by the release of mucilage in
affecting the hydraulic properties of the soil.