Anna summary

Summary

Author: Anna Houston

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In March 2004 the 4th year advanced soil physics group from Sydney Uni conducted soil physical experiments at the Australian Cotton Research Center The study was conducted on a plot of land containing cultivated and pastured land. The aim of the study was to compare and contrast landuse impacts on soil physical properties.

The study site was conducted on a black vertosol. Initially 10 sites were used to observe the differences, five sites were located on each land where intensive studies where conducted. A field survey was conducted by randomised points using the easily portable equipment

It should be noted that vertosols are quite problematic when measuring the hydraulic properties, this is due to the shrink swell nature of the soil, cracks may be present that accelerate the conductivity and alter the bulk density results. The clayey soil may cause smearing on the borehole when studying the sub-surface conductivity.

The landuse in the area has changed significantly in the last 40 years, continuous pasture was the dominant enterprise, however the irrigated cotton and cultivation industries soon surpassed the pasture due to the vertic properties of the soils. This change in the landuse has caused some effects on the physical properties of the soil. The most apparent properties to change are the volumetric water water, bulk density and shear strength of the soil, each of these parameters have a significant difference between landuse. The cultivated land contains higher volumetric water content, a lower shear strength and increased bulk density compared to the pasture site. The higher water content is attributed to the fallowed field, the water is not being used by plants. However the bulk density of the field is higher due to the disturbance and decreased structure of the soil by machinery. The shear strength is related to the volumetric water content, as the presence of water allow objects to compress and shear the soil with ease. The strength of the soil will decrease with tillage as it breaks the natural aggregates and therefore the pore size decrease. At the pasture site the organic matter and roots aid in a well structured soil and adding to the strength.

The saturated hydraluic conductivity in the pasture site was faster possibly due to the cracks that were evident in the soil, this was indicated by the high sorptivity rates in the pasture. The cultivated sites contained lower Ksat rates in all methods applied, this is possibly due to the dispersion and slaking effects of the clay, impeding infiltration. Infiltration rates were slow to medium in cultivated area, from moderate to rapid in the pasture site. The wetting front sensor indicated infiltration was fast for the first 6 cm however decreased rapidly from 6 - 12 cm, possibly due to increased clay content as no plough pan was detected by the dynamic penetrometer. During the wetting front experiment solutes were applied that gave an error result of a higher water content. This error is important for farmers in the area who are irrigating with saline water, there soil water content will be misleading therefore the correct amount will not be water supplied to the crop. The use of equipment to determine infiltration rates, shear strength or water content can initially be expensive however will be cost effective as water use is minimized.

Mean hydraulic conductivity of apparatus used in the field

	Sub surface Ksat		Surface Ksat			Unsaturated K
Landuse	Amoozemeter (cm/hr)	Lined Borehole (cm/hr)	Double Ring (cm/hr)	Single Ring (cm/hr)	Ponded Disk (cm/hr)	Tension Disk (cm/hr)
Cultivated	0.13	0.005	0.25	1.2	3.39	10.89
Pasture	0.41	N/A	25.5	6.4	69.96	0.45

The 40 years of the cotton and cultivated enterprises in the area have changed some physical parameters of the soil as mentioned above. These changes do have implications for agricultural management, since the soil structure in the cultivated area has declined, rotations would be highly recommended. This will add organic matter to increase water holding capacity and porosity for infiltration. Stubble retention would be highly recommended as this will give a mulch effect, decreasing evaporation from the soil, increase infiltration and organic matter content. The decrease in evaporation was shown in the mulched temperature waves experiment, whereby the soil temperature decreased during the day and insulated the soil at night keeping it warmer. The addition of crop rotation and stubble retention will also minimize run-off, (however in this area run-off is not a major problem), increase infiltration and decrease evaporation since ponding will be minimised.

These management techniques will also aid environmental outcomes as less water is wasted to evaporation and deep drainage, allowing more water to stay in the waterways. Deep drainage can be minimised by calculating the required volume of water to the desired depth to allow optimum yield. One of the major concerns for the environment in this area is the use chemicals, (along with water use), with increased deep drainage the chemicals will enter the watertable. This also needs to be carefully monitored as addition of solutes will alter the water content reading. Overall soil health and efficient water use is vital for a productive future on and off the farm.