Take home messages

• Information regarding the plant available water (PAW) at a point in time, particularly at planting, can be useful in a range of crop management decisions. Estimating PAW, whether through use of a soil water monitoring device or a push probe, requires knowledge of the plant available water capacity (PAWC) and/or the crop lower limit (CLL)
• A wide variety of soils in the northern region have been characterised for PAWC and details are available to growers in the APSoil database, which can be viewed in Google Earth and in the ‘SoilMapp’ application for iPad
• Knowledge of physical and chemical soil properties like texture or particle size distribution and (sub) soil constraints helps interpret the size and shape of the PAWC profiles of different soils. It can also assist in choosing a similar soil from the APSoil database
• Knowing something about how soils are distributed across the landscape - helped by understanding how the soils have been formed - boosts the ability to find a suitable soil type in APSoil or from other publicly available local soil and land resources (mapping, resource guides and field manuals, etc.), or allows users to reasonably predict the PAWC at a site
• Once the match between the soil of interest and the soil type is made, the PAWC can be adjusted using soil, land and chemical information, and if necessary, should constraints to crop roots be noted in the soil profile.

Plant available water and crop management decisions

A key determinant of potential yield in dryland agriculture is the amount of water available to the crop, either from rainfall or stored soil water. In the GRDC northern region the contribution of stored soil water to crop productivity for both winter and summer cropping has long been recognized. The amount of stored soil water influences decisions to plant or wait (for the next opportunity or long fallow), to sow earlier or later (and associated variety choice) and the input level of resources such as nitrogen fertiliser.

The amount of stored soil water available to a crop - plant available water (PAW) – is affected by pre-season and in-season rainfall, infiltration, evaporation and transpiration. It also strongly depends on a soil’s plant available water capacity (PAWC), which is the total amount of water a soil can store and release to different crops. The PAWC, or ‘bucket size’, depends on the soil’s physical and chemical characteristics as well as the crop being grown.

Over the past 20 years, CSIRO in collaboration with state agencies, catchment management organisations, consultants and farmers has characterised more than 1100 sites around Australia for PAWC. The data are publicly available in the APSoil database, including via a Google Earth file and in the ‘SoilMapp’ application for iPad (see Resources section).

Many farmers and advisers, especially in southern Australia, are using the PAWC data in conjunction with Yield Prophet® to assist with crop management decisions. Yield Prophet® is a tool that interprets the predictions of the APSIM cropping systems model. It uses the information on PAWC along with on pre-season soil moisture and mineral nitrogen, agronomic inputs and local climate data to forecast, at any time during the growing season, the possible yield outcomes. Yield Prophet® simulates soil water and nitrogen dynamics as well as crop growth with the weather conditions experienced to date and then uses long term historical weather records to simulate what would have happened from this date onwards in each year of the climate record. The resulting range of expected yield outcomes can be compared with the expected outcomes of alternative varieties, time of sowing, topdressing, etc. to inform management decisions.

Others use the PAWC data more informally in conjunction with assessments of soil water (soil core, soil water monitoring device or depth of wet soil with a push probe) to estimate the amount of plant available water. Local rules of thumb are then used to inform the management decisions.

The APSoil database provides geo-referenced data (i.e. located on a map), but the PAWC characterisations are for points in the landscape. To use this information to predict PAWC for the soil in a paddock of interest, the challenge is to find a similar soil in the APSoil database. Similarities between soils are related to parent material and the conditions under which the soil formed, or the material was deposited. This is often related to landscape position. Information on soil-landscape associations, therefore, provide an avenue to assist with PAWC prediction. The soil-landscape information is captured by the soil surveys undertaken by state government departments and other research organisations and is increasingly becoming available online.

This paper describes the concepts behind PAWC and outlines where to find existing information. It discusses how soil-landscape associations can be used to inform extrapolation from existing PAWC sites and assist with predictions. This is illustrated by current work from the Central Darling Downs in which relationships between landscapes, soil and PAWC are being explored to inform better PAWC predictions for growers to inform management decisions.

Plant available water capacity (PAWC)

To characterise a soil’s PAWC, or ‘soil water bucket size’, we need to determine (Figure 1a):

• drained upper limit (DUL) or field capacity – the amount of water a soil can hold against gravity;
• crop lower limit (CLL) – the amount of water remaining after a particular crop has extracted all the water available to it from the soil; and
• bulk density (BD) – the density of the soil, which is required to convert measurements of gravimetric water content to volumetric.

In addition, soil chemical data are obtained to provide an indication whether subsoil constraints (e.g. salinity, sodicity, boron and aluminium) may affect a soil’s ability to store water, or the plant’s ability to extract water from the soil.

Figure 1. (a) The plant available water capacity (PAWC) is the total amount of water that each soil type can store and release to different crops and is defined by its drained upper limit (DUL) and its crop specific crop lower limit (CLL); (b) plant available water (PAW) represents the volume of water stored within the soil available to the plant at a point in time. It is defined by the difference between the current volumetric soil water content and the CLL.

Plant available water is the difference between the CLL and the volumetric soil water content (mm water/mm of soil) (Figure 1b). The latter can be assessed by soil coring (gravimetric moisture which is converted into a volumetric water content using the bulk density of the soil) or the use of soil water monitoring devices (requiring calibration to quantitatively report soil water content).

An approximate estimate of PAW can be obtained from knowledge of the PAWC (mm of available water/cm of soil depth down the profile) and the depth of wet soil (push probe or based on a feel of wet and dry limits using an uncalibrated soil water monitoring device).

Knowledge of PAW can inform management decisions and many growers in the GRDC Northern Region have, formally or informally, adopted this. Several papers at recent GRDC Updates have illustrated the impact of PAW at sowing on crop yield in the context of management decisions.

Field measurement of PAWC

Field measurement of DUL, CLL and BD are described in detail in the GRDC PAWC Booklet ‘Estimating plant available water capacity’ (see Resources section). Briefly, to determine the DUL an area of approximately 4 m x 4 m is slowly wet up using drip tubing that has been laid out in spiral (see Figure 2a). The area is covered with plastic to prevent evaporation and after the slow wetting up it is allowed to drain (see GRDC PAWC booklet for indicative rates of wetting up and drainage times). The soil is then sampled for soil moisture and bulk density.

The CLL is measured either opportunistically at the end of a very dry season or in an area protected by a rainout shelter between anthesis/flowering and time of sampling at harvest (Figure 2b). This method assumes the crop will have explored all available soil water to the maximum extent and it accounts for any subsoil constraints that affect the plant’s ability to extract water from the soil.

Figure 2. (a) Wetting up for DUL determination and (b) rainout shelter used for CLL determination (source: CSIRO).

Where to find existing information on PAWC

Characterisations of PAWC for more than 1100 soils across Australia have been collated in the APSoil database and are freely available to farmers, advisors and researchers. The database software and data can be downloaded from the database. The characterisations can also be accessed via Google Earth (KML file from APSoil website) and in SoilMapp, an application for the iPad available from the App store. The industry yield forecasting tool Yield Prophet® also draws on this database.

In Google Earth the APSoil characterisation sites are marked by a shovel symbol (see Figure 3a), with information about the PAWC profile appearing in a pop-up box if one clicks on the site. The pop-up box also provides links to download the data in APSoil database or spreadsheet format.

In SoilMapp the APSoil sites are represented by green dots (see Figure 3b). Tapping on the map results in a pop-up that allows one to ‘discover’ nearby APSoil sites (tap green arrow) or other soil (survey) characterisations. The discovery screen then shows the PAWC characterisation as well as any other soil physical or chemical analysis data and available descriptive information.

Most of the PAWC data included in the APSoil database has been obtained through the field methodology outlined above, although for some soils estimates have been used for DUL or CLL. Some generic, estimated profiles are also available. While field measured profiles are mostly geo-referenced to the site of measurement (+/- accuracy of GPS unit), generic soils are identified with the nearest, or regional town.

Factors that influence PAWC

An important determinant of the PAWC is the soil’s texture. The particle size distribution of sand, silt and clay determines how much water and how tightly it is held. Clay particles are small (<2 microns in size), but collectively have a larger surface area than sand particles occupying the same volume. This is important because water is held on the surface of soil particles which results in clay soils having the ability to hold more water than a sand. Because the spaces between the soil particles tend to be smaller in clays than in sands, plant roots have more difficulty accessing the space and the water is thus more tightly held in clay soils. This affects the amount of water a soil can hold against drainage (DUL) as well as how much of the water can be extracted by the crop (CLL).

The effect of texture on PAWC can be seen by comparing some of the APSoil characterisations from the GRDC northern region, as illustrated below (Figure 4). The soil’s structure and its chemistry and mineralogy affect PAWC as well. For example, subsoil sodicity may impede internal drainage and subsoil constraints such as salinity, toxicity from aluminium or boron and extremely high-density subsoil may limit root exploration, sometimes reducing the PAWC bucket significantly.

Figure 3. Access to geo-referenced soil PAWC characterisations of the APSoil database via (top) Google Earth and (bottom) SoilMapp (APSoil discovery screens as inserts).

The CLL may differ for different crops due to differences in root density, root depth, crop demand and duration of crop growth (Figure 4b, c). Some APSoil characterisations only determined the CLL for a single crop. The CLL for deeper rooting crops are often considered the same, but care needs to be taken with such rules of thumb as different tolerances for subsoil constraints can cause variation between crops. A detailed explanation of the factors influencing PAWC is included in the Soil Matters – Monitoring soil water and nutrients in dryland farming book by Dalgliesh and Foale (1998).

Figure 4. Selected soil PAWC characterisations from the central Darling Downs

(a,b) Variation within the broad level recent alluvial plains (LRA unit 1a) stems from the mixed basaltic and sandstone origin of the alluvium and from particle size of the sediments in response to position within the plains. The Condamine soil is a deep, coarse structured black cracking clay adjacent to the Condamine river. It has coarse sand and gravel throughout and moderate to high subsoil salinity, both of which limit PAWC to about 150-200 mm, as seen in the example of APSoil 8 from near Yandilla, Qld (a). The Anchorfield black cracking clay has a finer structure which is reflected in its larger PAWC (usually >250 mm) as illustrated by APSoil 6 from near Brookstead (b).

(c) The Waco soil is more common in the broad level older alluvial plains of basaltic alluvium. The high clay content of these soils along with the smectite clay minerals contributed by the basaltic origin are responsible for the severe cracking and self-mulching nature of these soils and their large PAWC (APSoil site 16 near Jimbour, Qld).

(d) The undulating to steep, low hills and rises of Walloon sandstone of the Brigalow Uplands (LRA unit 6b) are characterised by grey-brown cracking clays with brown sands over brown clays. The Diamondy soil of APSoil site 88 near Jinghi, Qld is a texture contrast soil with a hardsetting surface and impermeable subsoil. The lower PAWC (50-100 mm) is in response to the limited depth, lower water holding capacity of the sandy surface layer and sodicity and salinity at depth.

Linking central Darling Downs soil-landscapes to PAWC

When it is not feasible to measure PAWC locally, one can try to estimate it by extrapolating from existing characterisations. This is not an easy task. The nearest PAWC characterisation may not be the most appropriate as its soil properties could be quite different. The presence or level of subsoil constraints may vary too. The challenge is therefore, to find a PAWC characterisation for a soil with similar properties.

The soil properties that affect PAWC (texture, stones and gravel, chemical constraints) change within the landscape as a function of parent material and how the soil formed, or how soil material got there. These aspects are reflected in soil-landscape models that underpin soil survey maps. In the Central Darling Downs, soil and land mapping spanning the 1960s to 1990s have been drawn from, along with new survey, to create land resource area (LRA) mapping and associated documentation, including an overview of the soils and landscapes from a land and farming capability/limitations perspective (Harris et al., 1999b), the landscape and soils within them (“Field Manual”, Harris et al., 1999a) and soil chemistry (Biggs et al., 1999) (see Resources section). Combined, the references describe the local soils and how they fit within the landscape.

The current project is testing whether we can draw on this information to predict PAWC or find a similar soil in the APSoil database. Below we illustrate for two transects how the soil properties change and are likely to affect PAWC.

The central Darling Downs is on southeast Queensland’s western flanks of the Great Divide and covers an area of approximately 2M ha of the upper Condamine river catchment (Figure 5). The regional geology features basalt along the Great Divide that mostly overlays sandstones. The area forms a broad valley and the soil characteristics are governed by source materials (sediments, basalt, sometimes granites) and the sequence of weathering, erosion and redistribution, i.e. the history of soil development. The alluvial soils in the broad valley floor are generally very fertile clays, as are the soils on the upper basalts (clays, and loams).

The central Darling Downs area contains 73 APSoil characterisations shown in Figure 5. These include the earliest APSoil described, and because techniques were still evolving, many lack the physical and chemical data that is standard in the current protocol described in the PAWC Booklet .

Generally speaking the soils are suited to dryland cropping by virtue of the rainfall and soil types, e.g. deep clay or loamy soils often resulting in very high PAWCs (>250 mm).

Early in the study a conceptual understanding of soil processes was drawn together in discussion with local experts (Qld. government, agricultural advisers, growers) and through a desktop evaluation of data including terrain, soil mapping, geology, APSoil profiles, etc.. Following, a soil survey was devised based on transects along ridge-to-valley bottom soil sequences (toposequences), typically 15-20 km long. Four soil survey sites were positioned along each transect, with one site at an APSoil site to ‘calibrate’ our PAWC understanding. The remaining three sites spread to cover soil-landscape variations (Figure 5).

Soil samples were analysed for physical and chemical properties, and site and profile characteristics recorded (e.g. depth to bedrock, coarse fragments, rooting depth).

Figure 5. Survey transects and sampling points (orange dots) overlaid on LRA mapping

Using soil landscape information to help estimate PAWC

The current project is testing whether we can draw on the information that is available to growers in the Central Darling Downs information (e.g. from APSoil, SoilMapp, LRA mapping, etc.) to predict PAWC or find a similar soil in the APSoil database. The steps below indicate how this may be done. We illustrate the steps using one/two soil sampling transects undertaken in our project. The initial steps reflect those in the LRA Field Manual.

Step 1: Find the LRA for your site of interest.

1. A map of LRA is available through the Queensland Globe. In this online mapping tool, select ‘Add Layers’, then choose ‘Land resource area mapping’ under ‘Geoscientific information’ and zoom into the area of interest.
2. Compare the LRA unit number with its description in Table 2.1 of the Field Manual and the diagrams of the relationships between different LRAs in Figure 2.2 of the Field Manual to confirm this LRA provides a good description of the landscape position of the location of interest.

Step 2: Consider the main differences between soils that are found within this LRA. Table 2.1 of the Field Manual lists the major soil types and indicative positions within the LRA are given in Figure 2.3 of the Field Manual.

Step 3: Considering the position in the landscape (e.g. higher or lower on hill slope, location on levee or in depression) as well as soil observations such as depth to bedrock, stones and gravel, rooting depth, pH, salinity, sodicity, identify the likely soil type and compare with the description of this soil type in the back of the Field Manual. Rooting depth is an important observation because it indicates the limits of root exploration in the soil caused by limiting constraints, regardless of the type or combination of constraints.

Step 4: Most APSoils in the area have been classified using the soil type classification in the Field Manual. In addition to this source of reference, Table 1 provides a means to identify likely APSoil profiles for several the soil types in the area. Work is in progress to extend this list. Where a soil type does not have a matching APSoil, use the PAWC range from the Field Manual as a starting point for selection of possible APSoils.

Step 5: Use observations on rooting depth that will reflect any subsoil constraints to adjust the PAWC. In this preliminary approach pending refinement, the PAWC should be adjusted by using the rooting depth observations if available, or failing this, a reasoned estimate of root depth in the soil. The reasoned approach may draw on past cropping performance of the soil, or observations made in similar soils. The adjustment takes into consideration the crop root architecture, which is likely to diminish in density in response to the type of response; a chemical constraint (e.g. pH, salinity) is likely to progressively strengthen with depth and cause roots to peter out, so the adjustment to the “bucket” should be non-linear with depth. For example, for a saline-constrained soil where roots are observed to 100 cm, and the Field Manual indicates a PAWC of 200 mm for 150 cm (maximum depth) non-constrained profile, the adjusted PAWC would not be a 30% (100 versus 150 cm) but rather 50% of the full potential, i.e. 100 mm PAWC. For a depth-constrained profile where the roots meet an abrupt stop (e.g. at bedrock), the adjustment should subtract the proportion of ‘missing’ subsoil depth given indicatively in the Field Manual for the soil. For soil containing stones or gravels, yet the profile depth matches the Field Manual, the PAWC adjustment should account for the proportion of stones and gravels.

The above provides a first estimate of PAWC. It is likely to require further refinement over time as yield predictions based on the PAWC are compared with achieved yields.

The project has undertaken sampling along five transects in the Central Darling Downs to evaluate the above approach. Below we present two of the transects to illustrate the steps above. The transect figures show the sites projected on the survey slope, the LRA boundaries, whether salinity or sodicity were measured in the soil, course (>2 mm) fragments, indicative PAWC range for the LRA (from Field Manual), and the APSoil PAWC (wheat) if co-located with the site. In line with Step 3, the soil analyses, and site and landscape observations enabled each site along the transect to be matched to a soil type from the Field Manual and the PAWC adjusted if the data dictated. One site in each transect was located as close to an existing APSoil site to reference the PAWC.

Transect 4 (Figure 6) is 20 km long and in the southern part of the Central Darling Downs. It traverses LRA 8a “Poplar Box Walloons” formed on fine grained Walloon sandstone and has a gently sloping relief featuring undulating rises and low hills. The major soils are self-mulching, black cracking clays. This LRA gently transitions to LRA 2b “Older Alluvial Plains” on older, broad level plains of mixed basaltic and sandstone alluvium. Major soils for LRA 2b are grey cracking clays. The transect finishes at the base of the hillslope with LRA 1a, which is formed on recent, broad alluvial plains, again of mixed basaltic and sandstone origins. Common soils include black and grey cracking clays or texture

Table 1. Selected soils from the Central Darling Downs and possible APSoil profile candidates

¹ from local soil classification in the Field Manual; ² as a main soil type; 3 from Field Manual

contrast bleached sands or loams over brown or black clays. The four sites in the transect are 4a (LRA 8a), 4b and 4c (LRA 2b) and 4d (LRA 1a).

The soils associated with each of the LRAs that were crossed are presented in the table in the Appendix, which also shows the site match to soil type. Site 4a is an Elphinstone soil on slopes (2-6%) of gently undulating plains and rises and consists of deep (100-150 cm) self-mulching black cracking clay that grades to brownish and yellowish and is formed on fine grained sandstone. At depth it can be highly sodic and saline and the PAWC is moderate (100-150 mm). Site 4b matches the Waco soil on gently sloping to flat alluvial plains. This soil comprises deep to very deep (100-180 cm), fine, self-mulching, dark cracking clay becoming browner at depth and formed on basaltic alluvium.  It may be sodic and saline at depth. The unconstrained PAWC is very high (>250 mm). Lower in the landscape, site 4c is a Mywybilla soil on gently sloping to flat alluvial plains. The PAWC may be very high (>250 mm). This site is near APSoil # 4, which has a PAWC recorded as 281 mm and consistent with the Field Manual. Finally, 4d matches the Waco soil (gently sloping to flat alluvial plains), and with a very high PAWC (>250 mm). Roots were seen over the full length of all four profiles, meaning that none were constrained and so likely to reflect the PAWCs allocated to each soil in the Field Manual.

Transect 11 is 12 km long (Figure 7) and in the northern part of Central Darling Downs on the south-facing range. It firstly traverses LRA 7a, “Basaltic Uplands”, which comprises undulating rises and rolling low hills. The major soils of the LRA include grey-brown and brown clays or clay loams. Lower in the landscape it cuts across LRA 2a of “Older Alluvial Plains” and occupies broad level plains of basaltic alluvium. The major soils include black, self-mulching cracking clays. The transect then moves into LRA 1a, “Recent Alluvial Plains”. The LRA is also featured in Transect 4 and is part of the same valley system, so, again, is formed on recent, broad alluvial plains of mixed basaltic and sandstone origins. Common soils include black and grey cracking clays or texture contrast bleached sands or loams over brown or black clays. The four sites in the transect are 11a (LRA 7a), 11b (LRA 2a), 11c and 11d in LRA 1a.

Figure 6. Transect 4 sites positioned on terrain with accompanying LRA boundaries, coarse fragments, profile and root depth, saline and sodic conditions, APSoil PAWC and clay %

Figure 7. Transect 11 sites positioned on terrain with accompanying LRA boundaries, coarse fragments, profile and root depth, saline and sodic conditions, APSoil PAWC and clay %

The soils associated with the LRAs covered by transect 11 are presented in the Appendix. Site 11a is a Burton soil on long gentle slopes and broad flat basalt ridges and features moderately deep to deep (75-150 cm) non-cracking red-brown to red clay. The PAWC is moderate (100-150 mm), and given the observed soil depth was 90 cm, the PAWC is likely to be ~110 mm after adjustment. Site 11b is a Waco soil on gently sloping to flat alluvial plains. This soil is deep to very deep (100-180 cm), fine, self-mulching, dark cracking clay becoming browner at depth and formed on basaltic alluvium, and may be sodic and saline at depth. PAWC from the Field Manual is very high (>250 mm) and consistent with the APSoil PAWC near the site (# 11; 332 mm). Roots were seen all the way through the soil. Sites 11c and 11d match the Anchorfield soil on gently sloping to flat alluvial plains. This is a self-mulching, dark clay becoming grey, yellow or brown at depth. The soil may be sodic and moderately to highly saline at depth and the unconstrained PAWC can be very high (>250 mm). Roots were noted throughout both soil profiles so the PAWC at the site was not constrained.

Conclusions

Plant Available Water Capacity (PAWC) is an important soil parameter that assists dryland growers in particular to plan and manage their crops. PAWC data for over 1100 Australian soils are freely available to growers through the APSoil database, Google Earth or SoilMapp for iPad. However, not every soil is covered so that most growers need to use the resources available to match their soil to APSoil. These resources include soil and land mapping and reports that can lead the farmer to make a strong match. This paper illustrates one such approach being tested in the Central Darling Downs that applies a knowledge of the soils in the landscape, how they were formed and their distributions. Once the match between the soil of interest and the PAWC from APSoil or other resources has been made, the PAWC for the soil of interest can be adjusted based on the depth of roots observed in the soil profile, which is an indicator of depth to PAWC constraining factors, if any.

Work is currently underway to develop and evaluate digital soil maps. These are maps that predict soil properties on a 90 m x 90 m grid (Soil and Landscape Grid of Australia). This provides the opportunity to map within LRA unit variability, but this is still research in progress.

Resources

Queensland soils and others

LRA (and other) mapping available through the Queensland Globe: (Select ‘Add Layers’, then choose ‘Land resource area mapping’ under ‘Geoscientific information’ and zoom into the area of interest)

LRA manuals

APSoil, PAWC characterisation protocols

The APSoil database (includes link to Google Earth file)

GRDC PAWC booklet

Soil Matters book

SoilMapp: for Apple iPad devices, see www.csiro.au/soilmapp and links to Apple App Store

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers throughout the Central Darling Downs - especially those allowing entry on their land the generous spend time with us in discussion. Our thanks also go for the good discussions we have had with Drs Andrew Biggs and Mark Silburn from the Queensland Government Department of Natural Resources and Mines.

References

Biggs, A.J.W., Coutts, A.J., Harris, P.S. (Eds.), (1999) Central Darling Downs Land Management Manual - Soil Chemical Data Book, DNRQ990102. Government of Queensland, Coorparoo, Qld, 107 pp.

Dalgliesh, N. and Foale, M.A., (1998) Soil Matters: Monitoring Soil Water and Nutrients in Dryland Farming. CSIRO Tropical Agriculture, Agricultural Production Systems Research Unit, Toowoomba, Qld.

Harris, P.S., Biggs, A.J.W., Coutts, A.J. (Eds.), (1999a) Central Darling Downs Land Management Manual - Field Manual, DNRQ990102. Government of Queensland, Coorparoo, Qld, 191 pp.

Harris, P.S., Biggs, A.J.W., Stone, B.J., Crane, L.N., Douglas (Eds.), (1999b) Central Darling Downs Land Management Manual - Resource Information Book, DNRQ990102. Government of Queensland, Coorparoo, Qld, 321 pp.

Contact details

Mark Thomas
CSIRO Agriculture and Food
Waite Road, Urrbrae, SA 5064
Ph: 08 8303 8471
Email: mark.thomas@csiro.au