How wet is your soil: Tools for working out plant available water capacity

Author: Toni Somes | Date: 19 Apr 2016

Knowing your plant available water capacity is critical for variety selection, time of sowing and yield predictions.

Photo: Image copyright/Evan Collis



Intuitive judgement, based on seasons of experience, has traditionally guided grain growers’ decision making, but new tools offering accurate data about elements like a soil’s capacity to store water are now adding to their planning ability.

At a recent Grains Research and Development Corporation (GRDC) grower update at North Star CSIRO researcher Brett Cocks explained how new technology to gauge plant available water capacity (PAWC) was complementing old ways of making crop management decisions.

“A key determinant of potential yield in dryland cropping is the amount of water available to the crop, either from rainfall or stored water,” Mr Cocks said.

“In the northern region the contribution of stored soil water to crop productivity has long been recognised. The amount of stored soil water influences decisions to crop or wait, to sow earlier or later and the level of inputs like nitrogen fertiliser.

“If growers understand how rainfall is captured and stored for future crop production, know their soil characteristics and moisture levels at key times like planting, as well as what tools to use for monitoring it can assist in the decision making.

“By combining their existing knowledge with technology they can get more accurate real-time soil water information, which can help manage risk, make better informed decisions and assess yield consequences.”

Now as part of a GRDC-supported project Mr Cocks and his team have a practical toolbox of methods growers can use to help characterise their soils for plant available water capacity (PAWC).

He said the key to using these tools was understanding different soil types and their water storage ability. Or in simplified terms consider each soil type a ‘bucket’ with each only capable of holding a certain volume of liquid. Knowing the water-holding capacity of a particular soil allows rainfall capture to be described in terms of its availability to crops.

 “It is critical growers understand and have access to this information if the value of stored soil water in grain paddocks is to be used in planning decisions,” Mr Cocks said.

“For growers to make more informed decisions and use decision-support tools such as Yield Prophet®, they need knowledge of the soil’s PAWC.”

What the terms mean…

Plant available water capacity (PAWC) is the soil’s total water-holding capacity, or the ‘bucket’. It is the difference between the upper limit where the soil can hold no more water (known as the drained upper limit – DUL - or field capacity), and the lower limit where any remaining water cannot be absorbed by the plant (known as the crop lower limit – CLL - or wilting point).

PAWC depends on soil texture and crop type and is independent of seasonal conditions.

“The reason it is important to understand PAWC is that it will affect how the crop responds to stored moisture and rain,” Mr Cocks said.

“For example if a soil has a large PAWC, then the crop will perform well for quite a while after significant rain. However, if the PAWC is small, the crop will need more frequent rain because the soil can’t store as much moisture.”

Plant-available water (PAW) is the available soil moisture and indicates how full the ‘bucket’ is at any one time. It is calculated by the difference between the current soil moisture and CLL.

Soil moisture is the current moisture level (usually reported as a percentage of H2O) as measured by monitoring equipment or modelling.

While bulk density (BD) is the weight or density of soil. Soil with a high bulk density tend to restrict plant growth. Bulk density also increases with compaction and tends to increase with depth.

So how and why do we characterise soils?

Characterisation is about defining a soil’s ability to hold water for the use of a particular crop and should not be confused with soil monitoring, which is about measuring the quantity of water in the soil ‘bucket’ at a particular point in time. 

Soil characterisation determines the PAWC of the soil at a particular site in the landscape. Generally, the site is representative of a broader area and considered a similar ‘soil type’.

Characterising soils allows growers to develop a better understanding of the size of the soil ‘bucket’ where the water resources to grow a crop are stored.

In determining a soil characterisations, a soil’s physical characteristics, along with chemical information need to be collected to determine what sub-soil constraints might impact on the soil’s ability to store water or the plant’s ability to extract water from the soil.

Factors influencing bucket size?

PAWC or size of the soil bucket is determined by the soil type – the proportions of sand, silt and clay, the soil depth and the crop which is being grown.

The amount of water a plant can extract from the bucket varies. Differences in crop root growth and the presence or absence of physical and chemical barriers all affect how much water can be extracted from the soil bucket. Texture is a major determinant of PAWC, but constraints like stoniness, toxicity (from aluminium or boron), salinity and sodicity (e.g. extremely dense subsoil) can also limit the PAWC.

When it comes to soil texture clay particles are small, but collectively have a larger surface area than sand particles occupying the same volume. Water is held on the surface of soil particles, which means clay soils have the ability to hold more water than sand. However, because the spaces between the soil particles tend to be smaller in clay than sand, plant roots have more difficulty accessing space and as a result the more tightly held water, so this too impacts on PAWC.

It is also common to have different PAWCs for different crops grown on a particular soil type. CCL may differ between crops, because of variances in root density, root depth, crop demand and crop growth. Tolerances for subsoil constraints can also vary between crop types.

Measuring PAWC in the paddock

Paddock-based soil characterisation is relatively simple and centres on establishing the DUL , CCL and bulk density (BD)  on a site, which is representative of the landscape.

To determine DUL: Water is applied to the site (using drip irrigation and under plastic to prevent evaporation) until it is considered to be fully wet to the depth of rooting. This may take a number of months in the heavier clay soils, but can be as short as a few days on sands. After drainage has finished, soil moisture and bulk density measurements are taken at drained upper limit (DUL).

To work out CCL: An estimate of the lower limit of crop extraction (CLL) for a particular crop grown is then done on a site with similar soil to the DUL test area. At around crop flowering, a rain-exclusion tent is erected over a healthy area of the crop (either on, or adjacent to the DUL site) which is then allowed to grow through to maturity.

Monitoring of soil moisture is then done to determine CLL. It is recommended CLL be measured for the range of local crops (ideally over a number of seasons) to determine differences in ability to extract water and therefore differences in PAWC.

“Soil variability may mean there is more than one PAWC in a paddock, which can make selecting a representative site difficult,” Mr Cocks said.

“High soil variability can also cause the DUL and CCL measurements to effectively be on different soils, even though they may only be 2-3cm apart. So it is critical to measure DUL and CCL on the same soil type.”

Other options for measuring PAWC in your paddock

Another simple way for growers to measure PAWC, according to CSIRO farming systems researcher Neil Dalgleish is by using the wettest and driest soil profile. This method requires a grower or agronomist to make judgement calls on when a soil profile is fully wet to depth, and when it is dry after the growing of a particular crop.

The measurement of soil water content at these two points will give a reasonable estimate of the upper and lower limits of extraction and will allow a calculation of PAWC which will suffice for most uses.

This method of determining PAWC is particularly appropriate for electronic soil water monitoring systems e.g. capacitance probes such as the EnviroSMART, where soil moisture is monitored through time and the wet and dry points are able to be easily identified. The catch with this type of device is that data is generally outputted in millivolts or similar which is a measure that many would have difficulty in relating to water availability. The Soil Water Express tool has been developed to assist in this process allowing the user to create estimates of DUL and CLL, based on the device’s electronic output, in units of meaningful measurement i.e. mm of available water.

Using online tools to measure on-farm PAWC

Plant-available water capacity (PAWC) for many soil types can now be found online. To help growers and agronomists take advantage of the current data the national soils database APSoil was created and features the PAWC of more than 1000 soils across Australia.

The database software is free and can be downloaded here.

This information can also be accessed via Google Earth (from the APSoil website) and using SoilMapp, an iPad application available from the app store.

The yield forecasting tool Yield Prophet® also draws on this database. [Yield Prophet® is a simulation model incorporating PAWC data as well as information on pre-season soil moisture, mineral nitrogen and other agronomic information and rainfall to simulate the growing season and help predict yields.]

Most of the PAWC data included in the APSoil database has been obtained through paddock trials.

In the Google Earth application, characterisation locations can be viewed by clicking on the shovel icons or by selecting the most appropriate test sites near your farm. For each site the soil type and PAWC for different crop types are provided.

In SoilMapp the APSoil sites are represented by green dots. Tapping on the map allows users to discover nearby APSoil sites and soil characteristics. Users can also find physical and chemical soil analysis data. Knowledge of soil properties, like texture and particle size and subsoil constraints can help when using the APSoil database as part of the PAWC calculations.

Tips for making an APSoil selection

Soil PAWC can vary significantly, so how do growers choose the most appropriate soil characterisation, if they aren’t in a position to do a local PAWC test? Here is a guide:

  • Be wary of just selecting the nearest APSoil location, as soil and landscape position can vary.
  • Compare soil descriptions – texture, colour, soil classification and chemical analysis. Some APSoil characterisations also include particle size.
  • Dig a hole using an auger, a corer, a backhoe trench, roadside bank or cutting. Note the surface features (like cracking or hard setting), subsoil issues (salinity, sodicity etc), rooting depth. This can assist with APSoil selection as well as adapt soil profile to local conditions (eg if depth of texture change or rooting is different).
  • Check for nearby soil survey characteristics (using tools like SoilMapp, Espade, Queensland Globe or local soil reports) to help describe soils.
  • Draw on soil-landscape mapping (where available) to find APSoil sites in similar landscape positions.
  • Native vegetation is also often a useful indicator of soil type and is often included in land resource information.

Knowledge makes for better management

When growers know their soils PAWC they can use the data, along with other tools like PAW to use use modelling tools like Yield Prophet® or APSIM (available online here) to help make management decisions about variety, time of sowing and canopy management.

“Knowing the PAWC can assist growers make and assess the risk of crop production decisions,” Mr Cocks said.

“If a grower knows the soil water level at critical seasonal decision points, like planting and fertilising they can use the information to determine aspects like crop type and input levels. For example, in wetter years they can increase inputs, in drier times they can reduce inputs and potentially they can optimise production and hopefully profits.

“So in a sense understanding the characteristics of soil and its water storage limitations is another tool in the toolbox to assist growers manage risk and allow them to make realistic yield predictions and crop management decisions.”

Contact

Brett Cocks
07 4688 1580,
brett.cocks@csiro.au

For more information, go to:

  • APSoil database: includes link to Google Earth file
  • SoilMapp (soil maps, soil characterisation, archive and APSoil sites): Apple IPad app available from App store; documentation 
  • GRDC PAWC booklet 

NSW

  • ESpade (soil-landscape and land systems mapping and reports, reports on soil characterisation sites from various surveys)

Queensland


GRDC Project Code CSP00170

Region North