Measuring and predicting plant available water capacity (PAWC) to drive decision-making and crop resourcing: ways to estimate PAWC in the data limited Surat, Qld area

Take home messages

• Plant available water (PAW) is a key determinant of potential yield in dryland agriculture. Obtaining a measurement or estimate of PAW can, therefore, inform crop management decisions relating to time of sowing, crop type or the level of fertiliser inputs
• Estimating PAW, whether through use of a soil water monitoring device or a push probe, requires knowledge of the plant available water capacity (PAWC) of a soil
• PAWC estimations using the standard field and laboratory protocol are often beyond the scope of growers and advisors, but more than 1000 PAWC characterisations linked to geographic position are publicly available 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
• Similarity of soil properties are key when extrapolating from these PAWC characterisation sites to a location of interest – the nearest characterisation is not necessarily the appropriate one.

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 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 crop or wait (for the next cropping 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 1000 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 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® first simulates soil water and nitrogen dynamics as well as crop growth with the weather conditions experienced to date. It 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 georeferenced 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 on PAWC. It discusses how soil mapping and other soil and land resources can help in matching the grower’s soils to existing PAWC sites. We illustrate this with an example from an area around Surat, Qld in the Northern cropping region - an area where data resources for the approach are presently limited.

Plant Available Water Capacity (PAWC)

To characterise a soil’s PAWC, or ‘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 water content.

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. 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 (PAW)

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 in order 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 in the 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’ by Burk and Dalgliesh (2013) (see Resources Section). Briefly, to determine the DUL an area of approximately 4 m x 4m 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 (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

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 APSIM Initiative. 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 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 in Burk and Dalgliesh (2013), 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 held more tightly 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 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, sodicity, 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 (a) Google Earth and (b) 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 4a,b). 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 (Dalgliesh and Foale, 1998).

Figure 4. Select soil PAWC characterisations from the Roma-Murrilla-Tara-Chinchilla region

(a) APSoil 1024 located east of Warkon. The different CLL for oats, lucerne and lablab illustrate crop rooting (depth and duration) effects.

(b) Grey Vertosol (Kupunn) near Condamine (APSoil 105). In this soil the crop effects on CLL appear to be much smaller, although it is not known if the characterisations were done in the same year.

(c, d) Grey Vertosols near Wallumbilla (APSoil 843 and APSoil 100). APSoil 843 is downslope from APSoil 100. Subsoil salinity in this soil affected the ability of roots to extract water from about 70 cm depth (chloride exceeding 600 ppm), whereas in APSoil 100 subsoil salinity only reached these levels below 120 cm depth. This illustrates that within paddock variation can be significant but may be explained by slope/landscape position.

Soils and PAWC characterisations in the Surat area

Soil properties like texture and subsoil constraints that are important drivers for PAWC relate to a soil’s position in the landscape, its parent material and how the soil formed, or how the soil material got there. Soil landscape information can hence be used to help estimate PAWC or find a similar soil in the APSoil database.

One of the important sources of soil information available for part of the wider Surat area is the “Roma District Land Management Field Manual” (Macnish, 1987). The report was compiled from a land conservation and farming perspective, and covers the soils including potential and limitations. Accompanying the survey to generate the report is a Land Resource Area (LRA) map suitable for use at a scale of 1:250,000. This map distinguishes 12 LRA units that each represent a unique combination of soils, vegetation, landform and geology. Contained within these LRA units are 20 soil types. These soil types are not mapped, but their descriptions in the documentation include common landscape position and the description of the LRAs lists the dominant and minor soils within each unit.

The area west of Surat not covered in Roma LRA mapping falls under LRA mapping described in “Resource Information, in Understanding and Managing Soils in the Murilla, Tara and Chinchilla Shires” (Maher, 1996b). This identifies 10 LRA units mapped at 1:250,000 scale that contain 31 unmapped soil types. These Murilla, Tara and Chinchilla soil types are described in similar ways to the Roma reporting format in the Field Manual (Maher, 1996a). There is no LRA mapping >60 km south of the Roma mapping.

Parts of the two LRA maps are shown in Figure 5 with the APSoil sites indicated by the shovel symbols. Although mapped at the same scale it is notable that the mapping units do not fully adjoin at the boundaries. This is likely to reflect different mapping concepts/legends or correlations applied by each of the survey teams and suggests the need for caution when extrapolating across each map.

Figure 5. Land Resource Mapping (LRA) near Surat with APSoil sites. Roma LRA units are mapped yellow and Marilla, Tara and Chinchilla LRA in pink.

The Roma LRA contains 13 APSoils near Surat (Figure 5): six occupy the Brigalow Uplands LRA (# 1284, # 851, # 843, # 64, # 100, # 91). The unit predominantly comprises cracking and non-cracking grey, brown and red clays (Macnish, 1987). There are minor examples of texture contrast soils. Five APSoil sites (# 1282, # 1281, # 845, # 853, # 63) occupy the Open Downs LRA that predominantly comprises cracking clay soils with gilgai. One APSoil (852) occupies the Merivale LRA (predominantly very shallow, texture contrast and sandy soils), and finally, one (1283) is in the Coogoon LRA, which again is a variable unit containing red or sodic texture contrast soils, very shallow soils, and clays.

In the Murilla, Tara and Chinchilla LRA mapping (Maher, 1996b), 6 APSoil sites (105, 93, 75, 216, 217, 218) occupy the Brigalow Plains LRA comprising moderately deep to deep clays. Two APSoil sites occupy the Rolling Downs LRA, which comprises cracking and non-cracking clays. Finally, one APSoil site is located in the Clay alluvial plains LRA, comprising black cracking clays.

It is likely that LRAs Open Downs (Roma) and Rolling Downs (MTC) correlate, as do reasonably well the Brigalow Uplands (Roma) and Brigalow Plains (MTC) LRAs. Other LRAs appear to be less easy to correlate.

Variability of soils within the LRAs is reflected in APSoil records, as shown in Figure 6 for the six APSoil PAWC characterisations within the Brigalow Uplands LRA from the Roma mapping (the sixth had estimated CLL). Table 1 shows the soil description using Australian Soil Classification (Isbell and CSIRO, 2016). The key observation is that, even within the same LRA, soils vary in terms of soil type and PAWC. The latter ranging from 91-193 mm. Even within soil classes the PAWC values vary. Clay-rich Vertosols are generally expected to have large PAWC values. However, the presence of subsoil constraints like salinity can dramatically reduce the PAWC. The extent of the reduction in PAWC depends on the depth at which salinity impacts on the crop roots.

Table 1. APSoil records from sites in the Brigalow Uplands LRA from Roma LRA mapping (Vertosol – cracking clay soil; Sodosol = texture contrast soil with a lighter texture surface soil over a sodic, clay subsoil)

Figure 6. APSoil records from the Brigalow Uplands LRA unit (highlighted green) from Roma LRA mapping

The take-home message from the evidence presented in the preceding discussion and from Figure 6 is that soils and their PAWCs can vary, often considerably, within the mapped LRA units. This means that identification of LRA unit on its own is unlikely to be enough to reliably match a soil to an APSoil PAWC. Identification of the soil type within the LRA and level of any subsoil constraints will be required to estimate PAWC. However, the descriptions in the reports accompanying the LRA maps (i.e. in Macnish, 1987; Maher, 1996a; Maher, 1996b) are quite technical.

Work is currently underway to develop and evaluate digital soil maps. These are maps that predict soil properties on a 90 mx90 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.

Guidance for choosing an APSoil characterisation and modifying PAWC in data-limited circumstances

• The nearest APSoil characterisation may not be the most appropriate as its soil, parent material and landscape position could be quite different.  APSoil characterisations in proximity to the site of interest would often be considered first as possible candidates, but caution is needed!
• Dig a hole (soil auger, soil core, backhoe trench, roadside bank or cutting); note surface features (cracking, hard setting), subsoil issues (salinity, sodicity, etc), rooting depth. Crop roots can be a reliable indicator of the depth to a constraining layer because roots will not pass through the hostile layer. This can be done by carefully looking through the soil profile from top to bottom for fine roots using a hand lens like a magnifying glass, or even by eye. Care should be taken, however, as dark, more ‘woody’ roots may indicate pre-cleared native roots that may have greater tolerance to constraints than modern crops. Recent crop roots will probably look very light brown and very fine, and where these terminate with 1 m is likely to indicate an invisible constraint (e.g. salinity or other toxicity).
• Compare the soil in question with descriptions of the APSoil sites (texture, colour, soil classification, chemical analysis). More recently collected APSoil characterisations include chemical analysis and particle size. Use APSoil displayed through Google Earth™ or the SoilMapp app and use a search pattern radiating away from the site. Users should seek APSoils that seem similar in landscape setting and satellite image surface colour. Use soil textural information presented with APSoil (see examples in Figure 6) to match the soils.
• Once the soil of interest and APSoil candidate have been matched, use the APSoil PAWC size as the starting point. Adjust the rooting depth and PAWC according to local observations.

Conclusion

Estimates of Plant Available Water Capacity (PAWC) can assist dryland farmers to reduce uncertainty associated with cropping decisions. This is because understanding the amount of water that a specific soil can supply to a specific crop assists in decisions around what to grow, when (or if) to seed, and what level of inputs (fertiliser, weed control) should be used in season.

Field measurement of PAWC is preferred, but time consuming. As an alternative the APSoil database contains >1000 PAWC characterisations nationally, which growers need to match to their soils to use effectively. Matching requires experience and is greatly assisted by access to good local soil data and information (mapping and reports). However, some places like the Surat area are data-limited which makes matching harder, although not impossible.

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 is still research in progress.

Resources

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 is freely available here (includes link to Google Earth file)

GRDC PAWC booklet

Soil Matters book

SoilMapp: for Apple iPad devices, for link to Apple App Store

References

Burk, L., Dalgliesh, N. (2013) Estimating plant available water capacity. Grains Research and Development Corporation.

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

Isbell, R.F., CSIRO, (2016) The Australian Soil Classification. Australian Soil and Land Survey Handbook Series (Vol 4). 2nd ed. CSIRO Publishing, Melbourne.

Macnish, S.E. (Ed.), (1987). Land Management Field Manual Roma District. Training Series QE87001. Queensland Department of Primary Industries, Brisbane, 182 pp.

Maher, J.M., (1996a) Field Manual, in Understanding and Managing Soils in the Marilla, Tara and Chinchilla Shires, Department of Primary Industries, Brisbane, Queensland.

Maher, J.M., (1996b) Resource Information, in Understanding and Managing Soils in the Marilla, Tara and Chinchilla Shires, Department of Primary Industries, Brisbane, Queensland.

Contact details

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

® Registered trademark