The survey site is on the plains of the Namoi Valley near Narrabri in northern New South Wales. The climate is warm to hot with summer-dominant rainfall averaging 650 mm annually. The soil of the area has been formed in situ from alluvial deposits and colluvial deposits from nearby mountains. The dominant soil-forming process is believed to be episodic washing of sediments. The local soils are generally 'cracking clays', a soil type suitable for cotton, a common crop of the area. The soil on the site is an epicalcareous, epipedal, black Vertosol, which is strongly structured throughout the largely medium clay profile.

The site consisted of two areas: a cultivated field with no vegetation cover and an adjacent area of pasture. A number of properties of the soil in each area were studied and compared. Those properties and the instruments used are:

hydraulic conductivity - in the topsoil with a double ring infiltrometer, a single ring infiltrometer and a ponded disc infiltrometer and in the subsoil with an Amoozemeter and borehole liner

water holding capacity - TDR and EM units and capacitance probes

strength - dynamic penetrometer

thermal conductivity - thermocouple probes

evaporation - thermometers and humidity instruments at two heights above the surface

A summary of the values (averaged over all relevant instuments) for these properties in each area of the site is in Table 1.

Table 1    Summary of soil property values

Hydraulic coductivity of the topsoil was variable because of the cracks characteristic of Vertosols. The topsoil pasture values are overestimated because of these cracks, while the cultivated soil values are underestimated owing to slaking. The subsoil values are low due to the high clay content and are higher in the pasture area because of the root channels formed by the pasture plants. These experiments were not run for long enough given the high clay content of the soil to obtain accurate values.

Volumetric water content is lower in the pasture soil because of evapotranspiration of the pasture plants, which dry out the profile. Lower water content of the pasture soil is also shown by higher sorptivity values. EM measurements over the site also showed this pattern. Measurements from the capacitance probes showed little consistency. TDR instruments measure electrical conductivity effects of soil and so are sensitive to both water content and bulk soil solution conductivity, the individual sizes of which cannot be determined by these instruments.

The shear strength values show that soil strength is higher in the pasture soil than the cultivated soil. Bulk density values support this, demonstrating loss of structure and greater compaction in the cultivated soil. This loss of structure is due to tillage.

The thermal conductivity pattern of soil is demonstrated by the graphs in Figure 1. The graph on the left begins in mid-morning and shows the sinusoidal temperature pattern, here plotted on a daily basis. The amplitude of the temperature cycle decreases with depth and the maximum (and minimum) points move forward in time with increasing depth. Mulching clearly reduces the surface temperature amplitude by trapping air, which has a lower thermal conductivity, while black plastic absorbs radiation and increases soil temperature.

Figure 1    Temperature at different depths over time (left) and effect of mulches (right)

Figure 2 shows the sinusoidal variation in air temperature and the inverse relationship with relative humidity. These data can be used to calculate the Bowen ratio, which allows actual evapotranspiration rates to be calculated for a partcular land use and ultimate estimation of the water balance.

Figure 2    Daily variation in air temperature and relative humidity

The above information is important for effective management of soil resources in the agricultural and environmental contexts. Hydraulic conductivity and water content information can guide irrigation application rates and scheduling. Soil strength measurements provide an indication of soil structure, which is important for optimal seed germination and crop growth, as well as an indirect indication of soil organic matter content. Heat measurements provide information relating to the water balance, which is the basis of quantifying land use effects on ground water. All these sources of information have a role in better informing land managers so that soil and water resources can be used more sustainably.