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During the earlier days of March 2004, ten Advanced Soil Physics students from Sydney University arived in Narrabri, to begin four days of intense study and practical aplication, on the concepts of soil physics. During these four days,various experiments and research development applications, were used to test soil hydraulics, water movement and content, and site conditions, as a means to compliment the theoretical component of the course. The Narrabri office of the Australian Cotton Research Centre, provided the site, approximately 25km from Narrabri and consisting of a pasture and a cultivated sub-site (below left, green pasture, brown cultivated). Each site was transected with five study pegs (below left), as loci for experiment replicates. Because of the conversely used and managed sites and the array of experiments conducted, comparitive analsyses and land use management conclusions can be deduced from the excursion, as evident throughout this summary. |
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| General Discritpion and Agricultural Significance: General descriptions of the area locate the site within a moderate summer predominant rainfall (650mm) district and hence, in a generalised semi-arid climatic region. The site lies within the Namoi Plains district, bounded by the undulating and geomorphically influencing, Mt Kaputar and Warrumbungles. Futhermore, soils wtihin the region are alluvial dominated and are dark cracking clays, classified as Vertosols (under the Australian Soil Classification System), evident by the pit description within the study site being a Black Vertosol (above right). Characteristics of Black Vertosols, subsequently result in high moisture contents, medium to high bulk densities (mean cultivated: 1.2 g/cm^2, mean pasture: 1.0 g/cm^2) and slower infiltration rates; due to smaller pore size. Hence, Vertosols are very important soils for irrigating high water-demanding crops and sustaining, high yeilding crop and animal production. Furthermore, they are environmentally important in regards to minimising deep drainage and groundwater recharge. In addition to, ground water contamination and the secondary effects of irrigation and dryland cropping within high rainfall regions, like Narrabri.
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Due to the diversity in procedures and their individual limitations and suitability to the site, a select few experimental results are summarised and farther on discussed. Soil strength was most accurately measured by the dynamic field penetrometer field survey (measuring shear and compressive stress), that showed the changes in strength accross the entire site (center below). From this, it is surmised that soil strength is greater within the pasture soils, with the cultivated soils, exhibiting only limited resistence to penetration, which was less than the more structured and root bound pasture soils. |
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| In addition to this, the shear vane peg studies suplemented these results and provided statistical analysis of the determined strength diferences between pegs, concluding that there is a <0.0001 F probability of no statitical difference between landuse sites. Moisture studies determining volumetric water content were performed accross the feild using EM38 and TDR instrumentation. Moisture content was greatest on average within the cultivated soil (below left, predicted water content greater than 30cm/cm represented by blue area; below right EM38 results blue is dry and with low salt). However, capacitance probe analysis which measured a continuous relative moisture content of the soil at depth and at the surface, concluded that moisutre content reduced throughout the day, with moisture greatest in the morning. Water content was larger due to the influence of adhesive forces within the clayey soil in addition, to the cultivated soils not having plant roots to extract available water from soil pores, unlike the pasture soils that had lower mositure contents. Fluctuations throughout the day are due to a culmination of factors; including the thermal capacity of the soil, the diurnal fluctuations of the soil and the surface exposure to radiation increasing evapotranspiration. Cultivated soils are bare and exposed and hence, during the hottest hours of the day, water loss will be greater (below centre, partial pressure of water over time), corresponding to moisture content declines in the afternoon. |
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| Aditionally,experiments were conducted that tested the saturated hydraulic conductivity and hence, infiltration rate of the soil surface and at depth. Many methods were used, however due to their limitations and a lack of continuity within the data, the ponded disk and six-lady sub soil experiments were subsequently percieved as the best experimental means, to measure specific site hydraulic conductivity and sub soil water characteristics. Infitration determined through saturated hydraulic conductivity evluation concluded that the pasture soils ahve a higher infiltration rate (average K0 = 1.665 cm/min) and therefore, a greater ability to transmit throught the soil profile where as, the cultivated soils had an average rate, significantly smaller (average K0 = 0.0565 cm/min). However, these results are highly effected by the minimal number of replications; only two and the presence of roots, macropores and cracks that distort infiltration results and hence, undermine these results. Subsoil experiments explored the wetting and drying cycles of the soil and quantify subsoil infiltration. These 'six-lady' experiments (with or witout solute) combined principles of both the tension disk permeameter and the TDR and concluded, that the ability of the soil to hold water decreased with increasing depth and that surface layers, dried more quickly as the wetting front moved down the soil profile.
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There is evidence of a difference in soil properties between the cultivated and pasture sites, within the Narrabri study area. The physical difference between the sites are illustrated such that landuse is having a intrinsic influence on soil structure, strength, infiltration and soil-water behaviour. Hence, management decsisions and the overall implications of this study, are influenced by the expected land use and objective of production. Furthermore, it must be acknowldeged that each of these conclusions is only as effective as the stated limitations, with each experimental method being highly limited and effected by the same physical parameters that they are trying to quantitate. |
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