Soil characterisation in Australian apple orchards

By calibrating new data with soil moisture data, growers can know when, how much and for how long to irrigate to supply enough water to meet tree demand, writes Marcus Hardie and Nigel Swarts from the Tasmanian Institute of Agriculture (UTAS).

Despite the economic importance of the Australian apple industry little is known about the condition, characteristics, and properties of Australian apple growing soils. This study, as part of PIPS II Nitrogen fertigation project (AP14023), aimed to determine the physical and chemical attributes of typical apple growing soils in four regions. Improved soil information is needed to support growers to better understand and manage their soil, nutrient, and water resources and assist with the development of perennial tree crop decision support tools. This study measured and reported on the condition of 31 soils including 5 sites in Tasmania, 9 sites in South Australia, 8 sites in Victoria, and 9 sites in New South Wales.

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Figure 1: Field sampling and analysis. Note use of dingo with 600 mm post hole auger

At each site the soil profile was exposed to around 70 cm depth using a Dingo powered 600 mm auger. The soil profile was described for horizon layers, texture, colour and structure. For each soil horizon, soil chemistry including pH, cations, and nutrients were determined. The water holding capacity of the soil was determined from intact cores brought back from each site at the soil laboratory at UTAS (Figure 1).

Water is held within the pores and voids of a soil in which the size of the pore or void determines what that water can be used for. Large macropores pores larger than about 30 um are visible to the eye, these pores are responsible for infiltration, drainage and oxygen supply but do not tend to hold water long enough for plants to use. The amount of water held between 30um to 0.2 um pores or (between field capacity and wilting point) is the plant available water content (PAWC). The water held here is described as readily available water which supports plant growth and fruit development. However, water held between the readily available water content and the permanent wilting point is less available to plants and mostly contributes to plant survival rather than growth (Figure 2).

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Figure 2: Soil water bucket concept (find out more)

 

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Figure 3: Yellow dermosol Victoria with soil horizons identified in yellow

 For each site we reported the drainable porosity, readily available water content and plant available water content for each soil layer, and the soil profile as a whole. For example, we describe the characteristics of a Yellow Dermosol in Victoria (Figure 3).

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Figure 4: Total soil water, Yellow Dermosol, Victoria

Figure 4 shows that the Yellow dermosol holds a total of 401 mm soil moisture. However, of this total moisture the amount of water which is actually available to the trees (PAWC Green + Orange bars) is only 167 mm, the moisture used for rapid plant growth (readily available water- Green bar) is relatively small at only 51 mm, while 134 mm of moisture is too tightly held by the soil and is not available to the tree (red bars). Comparison between the three soil horizons (Figure 5) shows that most of the readily available soil moisture is in the topsoil A1 horizon and the second B (or B22) horizon, whilst around 22 mm of the water in the A1 horizon is not available to trees, around 89 mm of water in the B22 horizon was not available for tree use.

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Figure 5: Soil water in each soil horizon layer of a yellow dermosol in Victoria

These numbers together with the soil chemistry indicates the topsoil (A1 horizon) appears to be a bit compact with a bulk density of 1.37 g/cm3 however all other measures indicate it to be a very well-structured soil. The saturated hydraulic conductivity at 500 mm/hr is excellent, and the drainable porosity at 7.9 % is good and close to the desired value of 10% which is rarely achievable in a clay loam. The structure of the B horizons on the other hand are dense and have low hydraulic conductivity, limited drainage capacity and are highly likely to become waterlogged in winter, as indicated by the pale-yellow colours and presence of mottling throughout the B horizons.

So how can this information be used by growers?

This data will underpin SINATA, a major output of the fertigation project, which will incorporate tree size, age and crop load with modelled environmental and climate data to provide growers with the ability to strategically manage their irrigation and nitrogen resources.

The soil data will be available on the APAL website, in the SINATA decision support tool or by contacting Nigel Swarts at nigel.swarts@utas.edu.au for more information.

 

Acknowledgement

This project has been funded by Horticulture Innovation Australia Limited using the Apple and Pear levy and funds from the Australian Government.

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About the Author:

APAL is an industry representative body and not-for-profit membership organisation that supports Australia’s commercial apple and pear growers.