a direct reading shear vane the shear strength of the soil
was measured. The readings for the pasture and cultivated
areas were compared using an analysis of variance.
for Oneway Anova
||Sum of Squares
||Prob > F
Analysis of Shear Vane (Kpa) By Land use
was a significant difference between the soil strength of
the pasture and the cultivated area. The size of the mean
values first brings this to our attention, and this is statistically
backed up by the probability value of .0001. Being less
than 5%, the means of the two areas being looked at, are
statistically significantly different. Visually, when soil
strength is plotted against landuse its also easy to see
the shear strength of the two areas are different.
result is one you would expect to find between these two
areas. The area covered in pasture has had time for the
pasture to become established, therefore has no recent history
of soil disturbance. So from the time when the pasture was
initially sown the soil has remained untouched and has been
given time for a structure in the soil profile to reestablish
itself. Compared to the cultivated area which had no plants
growing on it, because it had recently been cultivated to
keep it in a bare fallow state.
the soil of an area has been disturbed and a pasture is
allowed to develop the structure and fertility of a soil
does improve. This is also seen when incorporating a pasture
component in a cropping cycle. This has a direct impact
on the strength of a soil. In both cases the soil is not
disturbed by cropping and cultivation which means the amount
of disaggregation occurring has been significantly reduced
which allows the levels of organic matter to increase. This
effects the soil in many ways, but looking more precisely
at soil strength, the increasing amounts of organic matter
means the soil can hold more moisture which directly influences
soil strength. The soil we are dealing with is a cracking
clay where soil strength increases with decreasing moisture
content.(Friend & Chan, 2001)
the result we would expect is that the strength of a soil
would be higher in a pasture, than in a cultivated area
for the same soil. For a cracking clay the soil strength
in both situations would increase as moisture content decreases.
Our results confirm this and the results from the dynamic
cone penetrometer further back this up.
Problems with method
direct reading shear vane is probably one of the quickest
ways of getting a value of shear strength for a soil on
site. But its not the most accurate method available. Another
method available is the 'Shear Box'. The shear box would
give a more accurate reading as a sample of soil is used
where the structure is still intact, and its examined under
laboratory conditions. When using the shear vane you physically
push it into the soil. In doing this you are altering the
soil by pushing the soil in immediate contact with the shaft
and vane closer together. This would only change the soil
in a minor way, but it still could slightly change the strength
in some way which could cause the final reading to deviate
from its true value. Also the friction along the shaft itself
is not taken into account. The shaft would experience a
resistance to movement as you are rotating the shear vane.
However small this resistance is, a force is required to
overcome this which would cause the final reading to be
higher than it actually is.
K.O., Bowyer J.W. (1988) The Scientific Basis of Modern
Agriculture. Sydney University Press. Ch 10, pg186-88
J.J.,Chan K.Y. (2001) A simple method for quantifying effects
of management practices on cracking clays under dryland
conditions. Australian Society of Agronomy
A.B.,Minqsny B.(2004) Laboratory and Field Soil Physics.
(Notes and Exercises)
S. ,Jangle A. (1972) A Direct Reading Pocket Shear Vane.