Methodology

Soil Water Release Curve

The soil water release curve was determined using the KuPF apparatus (UGT, Germany; ICT International, Australia) between saturation and -80 kPa, supplemented with ‘dry end’ retention data determined by either WP4C dew point potentiometry (METER Group, Inc. USA), or pressure chamber data at -1500 kPa.  Saturated hydraulic conductivity was determined in situ by using a combination of the SATURO dual head infiltrometer (METER Group, Inc. USA) and single ring constant head infiltrometers in the A1 horizon, and Guelph permeameters in the B2 horizon.

The evaporative flux approach described by Schindler and Muller (2006), and Wendroth and Wypler (2008) has been routinely used to derive the SWC and hydraulic conductivity function for use with single and multiple pore domain models over the last decade (Köhne et al., 2002). The water retention characteristic is derived from paired consecutive soil water loss and mean matric potential data during free evaporation from a soil column (Schindler and Muller, 2006; Wendroth and Wypler, 2008).

Following profile description, three 250 cm³ intact cores were extracted from each soil horizon, in which two cores were analysed by the KuPf. If retention curves were found to vary, the third core was analysed, and the two closest matching retention curves reported. Pressure chamber analysis was conducted at the permanent wilting point (PWP) -1500 kPa using three replicate 20-30 g air dried < 2 mm soil subsamples. Volumetric soil moisture content was determined by multiplication with bulk density determined on the 250 cm³ cores at saturation. As an alternative approach, ‘dry-end’ retention datawas also determined by WP4C dew point hygrometer (Figure3) for around 20 soil horizons in whichtriplicate samples between -500 kPa and -5000 kPa were analysed by WP4C in the precise mode. Samples were sequentially dried under ambient conditions, then sealed for 1 hour to equilibrate before being re-analysed by WP4C. This was repeated up to 6 times until soils exceeded -5000 kPa.  Following the final WP4C reading, the gravimetric moisture content was determined by oven drying at 105  ̊C for 24 hours. Data was converted to a volumetric basis by multiplication with bulk density determined at saturation.

The soil water retention curve was fitted for the van Genuchten equation (van Genuchten, 1980)using Excel Solver software. The plant available water content (PAWC) was calculated as the water filled pore space between field capacity (FC) at -10 kPa and the permanent wilting point (PWP) at -1500 kPa (James, 1988; Marshall and Holmes, 1988; Brady and Weil, 2010). The readily available soil water content was determined between field capacity at -10 kPa and -50kPa. Drainable porosity was calculated as the pore space or loss in moisture between saturation at 0 kPa and field capacity at -10 kPa. 

Infiltration and Saturated Hydraulic Conductivity

Infiltration and saturated hydraulic conductivity at the soil surface (A1 or A11 horizon) was determined in triplicate by SATURO Dual Head Infiltrometer (Meter Group, Inc. USA) opperated for approximately 120 minutes. The Dual head Infiltrometer applies water to the soil surface a two pressure heads, repeated over three cycles, such that the effect of sorptivity and lateral flow can be excluded from the calculation of saturated hydraulic conductivity. Failure of the Dual Head Infiltrometers at several sites (especially the Victorian sites) resulted in measurement of surface soil infiltration and hydraulic conductivity by 200 mm diameter, single-ring, constant head infiltration in which the calculation of saturated hydraulic conductivity was solved according to Reynolds and Elrick (1990) assuming an alpha value of 0.12 cm¯¹.

Subsoil infiltration and hydraulic conductivity was determined by Guelph permeameter (Eijkelkamp, 2011) (Figure4) and purpose built thin tube constant head well permeameters, in which saturated hydraulic conductivity was solved by single head method and an assumed alpha value of 0.12 cm¯¹. Measurements were conducted in triplicate at approximately 600 mm depth (B2 horizon) in which a 100 -150 mm head was maintained in a 30 mm radius bore for at least 20 minutes. Saturated hydraulic conductivity was solved using the Guelph Permeameter software which is based on Reynolds and Elrick (1985); Reynolds and Elrick (1986); Elrick et al. (1989). At all sites, a 300 g disturbed sample was obtained from each soil horizon for analysis of chemical properties. Samples from Tasmania and Victoria were sent to CSBP laboratories for analysis, whilst samples from N.S.W., and South Australia were sent to DPI lab in Lismore, QLD.

Interpretation of measured values was based on (Hazelton and Murphy, 2007) in which threshold responses for some soil variables were re-interpreted based on knowledge of soil type, soil depth, or known soil conditions for apple production. The exchangeable aluminium toxicity threshold of 0.4 % was based on Voiculescu et al. (1989).

Soil Water Content

The amount of water that a soil can hold is usually described in terms of the saturation, field capacity and permanent wilting point. This is often represented as a bucket (Figure 5) in which the soil profile or bucket is said to be at saturated when the bucket is full. It is assumed that the bucket will drain by gravity over a period for up to two days to a moisture content known as field capacity. Further drying of the soil profile requires evaporation and transpiration. Eventually a point is reached when further loss of water causes the plant to wilt and die, this is known as the permanent wilting point (PWP). The amount of water, and thus pore space between saturation and the field capacity is known as the drainable porosity, a measure of the proportion of large macropores in the soil that are associated with drainage infiltration and supply of oxygen to roots. The amount of water, and thus pore space between the field capacity and the permanent wilting point is known as the plant available water content (PAWC). It is the total amount of water stored in the soil which is available for trees to use. However, the soil water within the PAWC range is not equally available to plants, the readily available water (RAW) between -10 and -50 kPa is most easily extracted by plants and is typically used for growth, whilst the more tightly held water between -50 and -1500 kPa is difficult for trees to extract and is only available to the plant after the RAW is extracted, and thus is mostly used by trees for survival. Whilst use of the soil water bucket concept is usually applied to the whole soil profile. The concept is equally useful on a soil horizon by horizon basis, in which the amount of moisture measured as a % is multiplied by the depth of each soil horizon to determine the amount of moisture in mm (same units as rainfall).