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Analysis Of Soil Properties

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Understanding soil properties, and how they relate to soil quality and best management practices is important in tropical areas, particularly those areas with high climate variation. The aim of this study was to measure various soil properties at the surface and within a 1m profile depth at the Environmental Research Complex, JCU Campus, Smithfield. Various physical and chemical properties were measured at the soil surface along 4 transects with 16 points in each transect, and at depths (0-0.2m, 0.2-0.4m, 0.4-0.6m, 0.6-0.8m and 0.8-1m) at 8 profile locations within the grid. Soil properties measured include: electrical conductivity, topsoil water content, gamma radiation, pH, soil texture and soil colour. Our results found both spatial and temporal variation existed across the study site and that soil properties are interlinked. Generally, clay was the predominant soil texture, which influenced other properties like water content and electrical conductivity. Patterns between individual properties were also observed with water content, gamma radiation, elevation affecting electrical conductivity and soil texture in some areas of the study site. Our results suggest soil properties do vary spatially and temporally and their relationships can be complex in nature and connected.

Introduction

Soil is the unconsolidated material at the Earth’s surface. It is composed of organic and inorganic matter that functions as a natural medium for plant growth (Kuang et al.2012). Soils are the product of weathering and erosional processes and vary in structure and composition (Wilford 2012). Five factors affecting soil formation are parent rock, topography, climate, biological activity and time (Wilford 2012). These factors result in soils being heterogeneous and having spatial and temporal variation (Kuang et al. 2012). Soil and weathered material deposited above fresh bedrock is referred to as regolith (Wilford 2012). Parent material and weathering intensity will influence the nature and distribution of regolith (Wilford 2012).

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Soils are made up of physical, chemical and biological processes (Jung et al. 2005; Wilford 2012; Xu et al. 2012). Measuring various soil properties can give an indication of soil quality allowing for better land management (Jung et al. 2005). A study was undertaken to measure various soil properties at JCU campus, Smithfield. Physical soil properties measured included texture, colour and water content. Chemical properties included pH, electrical conductivity and gamma radiation. Soil texture is the proportions of clay, silt and sand in the soil and is influenced by particle size. Soil colour is affected by organic matter, water content, the minerals present and their oxidation states (Weil and Brady 2016). Soil colour gives an indication of soil condition and composition (Wilford 2012). Water content is the amount of water retained in the soil, it is important in determining plant available water and the permanent wilting point (Weil and Brady 2016). pH is a measure of the acidity or basicity of a soil. pH is measured on a scale from 1-14. 7 is neutral, <7 is acidic and >7 is basic (Weil and Brady 2016). Electrical conductivity is an indirect measurement of salt content and can also be an indirect indicator of other soil properties such as, cation exchange capacity (CEC) and both clay and water content (Gooley et al. 2014). Gamma radiation is a measurement of the natural decay of naturally occurring radioactive isotopes, Potassium (K), Uranium (U) and thorium (Th) (Kuang et al. 2012; Wilford 2012). The gamma rays emitted by these elements will be associated with the geochemistry and mineralogy of the parent rock, bedrock and weathered matter (Kuang et al. 2012).

Smithfield, Cairns is located in Tropical North Queensland, Australia (Nott 2003). Cairns has a wet/dry tropical monsoonal climate, with a noticeable wet season occurring between January and March (BOM 2018; Nott 2003). Cairns averages around 2000mm of rainfall annually, with 48mm falling in the week before the study was undertaken (BOM 2018). Cairns has a mean maximum and minimum temperature of 29.0oC and 20.8oC respectively (BOM 2018).

Cairns is surrounded by the high escarpments of the Macalister Ranges to the west and the Coral Sea to the East. (Nott 2003). The geology around cairns was influenced from the formation of the Hodgkinson Basin, 360 million years ago. Sediments that had runoff the land was put under pressure from major movements in the Earth’s crust (Government 2000). This pressure caused the sediments to be compressed, folded and lifted far above sea level (Government 2000). Between 310-260 million years ago, large pools of magma were pushed up near the earth’s surface. Continuous erosional processes exposed the granite rock, causing them to breakdown (Government 2000). The sediments were transported as colluvial deposits to the lowlands (Government 2000). Soils identified around the JCU campus, Smithfield are of the Mission series (Murtha et al. 1996). The Mission series are defined as well drained soils formed on alluvium or soils of metamorphic rock origin (Murtha et al. 1996). In general, the geology around Cairns is dominated by Quaternary sediments of metamorphic origin in the lowlands, with occasional intrusions of granite (Nott 2003).

A study of various soil properties across a cleared grassy paddock at the Environmental Research Complex, JCU Campus, Smithfield (JCU) was undertaken to determine if any variations and patterns exist across the landscape either spatially and/or temporally.

Method

The study area was located in a grassy paddock northwest of the Environmental Research Complex (Figure 1). There was a fenced off area running through the eastern section of the paddock. Within the fenced off area was 10 rows of cocoa trees under experiment with irrigation run up each row. The paddock was once forested before being cleared for sugar cane. After JCU acquired the land, regrowth started to grow before being cleared again and maintained as a grassy paddock (EA3007 2018). The study was undertaken on the 6th July 2018.

A grid of 4 x 16 points was set up consisting of 8 x 100m transects (Figure 2). Each transect was assigned a letter and each location in the transect was numbered; starting from the western side through to the eastern side (Figure 2). In addition, an extra 8 points were located within the grid and profile data to a depth of 1m from each point was collected (the same numbering structure as used for the transects was followed for the profiles) (Figure 2). Each group was assigned a transect with 8 points along each transect. At each point, various soil properties were measured including electrical conductivity (EC), gamma radiation (Y), and topsoil water content. EC was measured using an electromagnetic induction meter, MK2 EM38. It applies a magnetic field to the ground and measures conductivity horizontally (EMH) to a width 0-0.3m and vertically (EMV) to a depth 0.3-0.6m (Geonics Limited 2013). Gamma radiation was quantified using an Exploranium GR-320 gamma ray spectrometer. It measures gamma emissions of the radioactive isotopes Potassium(K), Uranium (U) and Thorium (Th) and total(K,U,Th) content of the soil surface (Gooley et al. 2014). Water content was determined using a Campbell Hydrosensor Probe with 120mm rods, which is a time domain reflectometry probe that measures pulses from the dielectric property of the soil (S.U et al. 2014).

At each profile location, samples were taken at every 20cm until a 1m depth was reached using a hand auger. At each 20cm interval, the soil was bagged and taken back to a nominated field location for analysis. For each 20cm interval; soil colour, texture, pH, and EC were recorded. Apparent EC (ECa) using the MK2 EM 38 was recorded at the soil surface (EA3007 2018). The soil colour was identified using the Munsell Soil Colour Chart. A naturally broken piece or a small amount of soil was sourced and wet with water (EA3007 2018). The colour was identified according to hue, value (darkness) and chroma (brightness) (EA3007 2018). Soil texture was categorised according to the behaviour of a moist bolus from the Australian Soil and Land Survey Field Handbook using a feel method (EA3007 2018). A handful of soil was wet to the sticky point. It was worked into a bolus, breaking up the particles of soil, until it was dry enough that it didn’t stick to your hand (EA3007 2018). The soil was worked into a ribbon and ribbon length along with coherence and texture (smoothness and sandiness) was used to identify the field texture (EA3007 2018).

EC and pH were measured using a 1:5 soil extract (EA3007 2018). For each soil sample a 50mL tube was filled up to the 30mL mark with distilled water (EA3007 2018). Soil was added to the tube until the suspension reached 32.5mL (EA3007 2018). The suspension was shaken for 2 minutes and poured into a wide mouthed container. Calibrated EC and pH electrodes were placed into the suspension. EC and pH were measured and recorded. Elevation of the study area was sourced from Google Earth.

Gamma Radiation (Y)

Total Y increased as elevation decreased (Figure 3 & 7). It also increased from the North to the South of the study area. The highest reading was in the south-eastern corner near the cocoa trees of 172cts/min. K contributed most to the pattern as it had the highest values and variation, ranging from 36 to 48 cts/min, Th had the lowest values and little variation ranging from 0-6 and U had values ranging from 10-18.

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