Introduction
Moisture content
Moisture Potential
Infiltration & Hydraulic Conductivity
Soil EC, using EM devices
Soil Temperature
Overall discussion
References
Stated qualitatively, soil strength is the capacity of a soil to withstand forces without experiencing failure, whether by rupture, fragmentation or flow. Consideration of the strength of a soil is very important, especially for agricultural purposes. There is an upper and a lower limit that is ideal for a soil, and if the strength exceeds either of these extremes, it has a detrimental effect. If a soil is too weak, it will be unable to adequately anchor the plant, as well as not being ablle to withstand the forces of wind and water. On the other hand, if a soil is too strong, plant roots, as well as organisms such as earthworms, will not have the required strength to penetrate the soil.
"Though easy to define, the measurement of soil strength is far from easy, as it is a highly variable property which often changes during the very process of measurement as the deformed body of soil might either decrease or increase in resistance to further deformation" (Hillel, 1980). Resistance to applied stresses can be characterised in terms of two parameters: cohesiveness, the bonding of the soil particles which must be broken, and the angle of internal friction which is the resistance when soil is forced to slide over soil.
One type of strength that was measured in Narrabri is the shearing strength. This is induced by gradually increasing the lateral, or tangential stress until failure occurs. The instrument used is called a shear vane. The vane is driven into the soil to the desired depth, and then rotated. The torque required to shear the soil along the surface of a cylinder generated by the blades of the edges, is measured.
The shear vane could be used by farmers wishing to know when the field is dry or moist enough for ploughing as the process of tilling the soil is primarily one of shearing the soil and then following shearing, the soil is placed under compressive and tensile stresses. It would be necessary for the farmer to first calibrate his instrument against samples of the different soil types at different moisture contents so as to know what range of soil strengths corresponded with a crumbly consistence for the different soil types.
The results of plotting land management against wet and dry shear strengths, both show that the strength in the pasture sites was much higher than in the tilled sections. This correlation is what was expected, as the strength of a soil will decrease with tillage, due to breakdown of natural aggregates and pores. The pasture on the other hand would have well developed aggregates, as well as the roots of the grasses, which also add to the strengths of these sites.
A different technique for characterising soil strength in the field is by using a penetrometer. This instrument evaluates the soil resistance to the penetration of a narrow probe. The force required to cause penetration, per unit area of the of the probe tip is what is measured. The penetrometer could also have application to a farmer in determining timing of tillage operations in the same way as the shear vane. It would also have to be calibrated in a similar way to the shear vane. Other applications of the penetrometer could be in measuring if a soil crust is going to be restrictive to coleoptile emergence or if a plough pan is restrictive to root penetration. Or simply in measuring if the strength of the soil as a whole is going to impede root or shoot elongation below the soil