|dc.description.abstract||The platinum microelectrode technique for measuring
oxygen flux in soils has been reviewed. Shortcomings in the
existing technique and instrumentation have been discussed.
The new instrumentation, electrode standardization and
measurement techniques developed enable the method to be used
with confidence in unsaturated soil systems. Measurements
of oxygen flux index in four soil samples showed a very highly
significant regression relationship between oxygen flux index
and air space within the range 3 - 15% air space on each soil.
There was no significant difference in the regression relation
A field penetrometer, designed to measure the presence
and strength of subsurface pans in field soils has been
described. Measurements with the penetrometer on three depth
of ploughing treatments (100, 230 and 355 mm) on tillage trials
at four sites with different clay contents showed that hard
layers were present on all the treatments. Except on the
shallowest ploughing depth treatment on the fine-textured
soil, where the pan was 225 mm below the nominal ploughing
depth, the hard layers were present between a few mm and 150 mm
below the nominal ploughing depth, and had strengths of between
16 and 24 bars.
The theory and factors affecting measurement of soil
strength with needle penetrometers have been investigated.
The design and operation of a laboratory penetrometer used to
measure soil strength under closely controlled laboratory
conditions has been discussed.
Physical factors likely to affect root growth, viz. soil
texture, air space, bulk density, soil strength and available
moisture, have been measured in a comprehensive range of
undisturbed cores taken from the four tillage trials. High
soil strength is considered as being the soil physical factor
most likely to restrict root growth in these soils. Physical
factors affecting soil strength have been investigated. Soil
strength is shown to be highly dependent upon bulk density, matric potential and soil texture.
The hard pans shown to exist in all the tillage trial soils
exhibit many of the characteristics of tillage pans, but their
existence cannot be attributed exclusively to the ploughing
depth treatments imposed in the tillage trials. Rather, the
pans have resulted from a combination of interacting factors,
including the previous history of the soils, the imposed
tillage treatments, crop, and climatic factors.
A study of some of the data from the literature on root
growth and soil strength shows that root growth is severely
restricted by soil strengths of the order of 20 to 30 bars.
In order to determine whether root growth was being restricted
in the tillage trials, root profiles were extracted from one
of the trials. These showed that the pans severely restricted
Analysis of maize yield data from the tillage trials
showed that on the three coarse-textured sites yield increased
with increased depth of ploughing, and that there was a marked
seasonal effect, ploughing depth having a relatively greater
effect on maize yield in dry seasons than in wet. On the
fine-textured site, however, where no pan existed near the
surface in the shallow ploughed treatment, the ploughing depth
effect was not significant, nor was there any marked seasonal
effect of ploughing depth on maize yield. Evidence presented
shows that the pans, by restricting root growth are reducing
the amount of water available to the plant. This effect is
greater in dry seasons, and in soils with low available water .||en