A Soil Physics excursion was conducted in early March 2004 at the Australian Cotton Cooperative Research Centre, Narrabri.
The study area consisted of two different land uses, pasture and cultivated soil. Numerous methods for measuring soil physical properties were used to quantify the Black Vertosol soil in the study area and investigate any possible differences between the pasture and cultivated sites. Soil properties were tested using different equipment at ten specific peg sites (5 in pasture and 5 in cultivation), as well as at random through out the test site.

The experiments that were conducted can be grouped into four main categories based on the characteristics being tested. 1. Infiltration 2. Moisture content 3. Soil strength experiments 4. Soil thermal properties

Infiltration experiments

Table 1. Summary of Infiltration experiments, showing replicate tests for each.

Generally, the hydraulic conductivity values (K) are higher for the pasture sites than the cultivated sites, which can be contributed to the existence of macro pores (present as cracks) in the pasture along with a dryer profile. The K values obtained in the tension disk experiment, which excludes infiltration through macropores (cracks), leading to a reversal of results as seen in other experiments with a higher K value in the cultivated than the pasture. The cultivated sites had smaller K values also as the soil slaked and crusted a less permeable layer in the profile impeading infiltration.

Generally, vertosols have slow to moderate hydraulic conductivities due to the abundance of clay and high resistance to water movement. As the resistance increases, the K values decrease.

Water content experiments

The water content drops from the cultivated site to the pasture site as a result of the pasture transpiring the water out of the profile. Water contents were found directly by a Time Domain Reflectrometry (TDR) instrument, and indirectly using a Capacitance Probe which measured the dielectric of the surrounding soil which could be used to equate the water content. Bulk density measurements which were taken also support that water contents in the cultivated site were higher than those in the pasture site. The bulk density of the cultivated sites was greater than those in the pasture site, which would be a result of cultivation destroying the soil structure. Generally, any form of cultivation decreases the topsoil structure and increases the volumetric water content. Water content was also monitored during infiltration from a Ponded Disk Permeameter, at depths of around 3cm, 6cm, 9cm, 11cm, and 15cm, using wave guides connected to a TDR. During the experiment a salt solution was used to replace the constant head, which showed up as a reduction in water content. This showed that water content measurement can be effected by solutes.

Soil Strength experiments

Soil strength is the resistance of soil to deformation. This was tested by a Shear Vane which tests the shear stress of the soil, and a Cone Penetrometer which tests the compression and shear stress of the soil. The soil strength of the pasture site was much higher than that in the cultivated site. This can be attributed to the cultivated site having less structure, and also the wetness of the soil has a direct effect on soil strength, reducing it with greater water content. A compacted layer can occur in places of frequent trafficing, a clay pan can also form from cultivation. These layers would be seen by an increased force strength at depth, however these were not found in teh cultivated site at all by the Cone Penetrometer to a depth of 15cm.

Soil thermal properties experiments

The soil temperature was measured using thermocouples placed at different depths. It was found that there was a distinct reduction in the amplitude of temperature variation when mulch is applied to a soil surface. Using mulch can therefore reduce soil temperature fluctuations. Therefore the pasture site would almost certainly have less temperature fluctuations due to surface cover. Evaporation rates would be lower and thus soil water would be conserved and available for plant growth. The cultivated site would experience major fluctuations in temperature producing unfavourable conditions for microorganisms and root growth.

Implications
According to Bouma et al. 1999, “Identical soils from a soil-genesis (taxonomic) point of view may have significantly different MSC (Moisture supply capacity) values because of different organic matter contents and soil structures formed by different types of land-use and management.” For example, the original soil (genoform) has been altered over time to produce two different phenoforms due to different management practices on the Narrabri site. It is apparent that there are clear differences between the cultivated and pasture sites, especially when looking at volumetric water content, bulk density and shear stress which had significant differences between the sites. These differences highlight the varying soil physical and hydrological characteristics under contrasting management as observed by Bouma et al 1999.

Characteristics of black vertosols, subsequently result in high moisture contents, high bulk densities (1.6 cm/cm) and slower infiltration rates, due to smaller pore size and hence, make such soils very important for irrigation and high water demanding, crop production. Furthermore, they are environmentally important in regards to deep drainage, groundwater recharge and contamination, and secondary effects of irrigation and dryland cropping in high rainfall regions.

Reference
J. Bouma and P. Droogers. Comparing different methods for estimating the soil moisture supply capacity of a soil series subjected to different types of management, Geoderma, Volume 92, Issues 3-4, 1 October 1999, Pages 185-197

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