Plant-available water capacity overview
More than 1000 Australian soils have been characterised for plant-available water capacity (PAWC). This information is available online and through SoilMapp, an iPad app available from the Apple Store. CSIRO research in collaboration with state agencies, consultants and growers continues to add soils of regional agricultural significance to the APSoil database.
The GRDC-funded project ‘Measuring and managing soil water in Australian agriculture’ is working with groups of consultants and growers across the country to characterise soils for PAWC and discuss how this information can be used to inform management decisions in farm businesses.
Understanding the soil’s capacity to store water can help in better predicting crop yields, particularly in years when rain does not follow traditional patterns.
CSIRO researcher Dr Kirsten Verburg says that understanding plant-available water capacity can help growers make more informed farm business decisions.
While some growers in southern Australia were surprised by higher-than-expected yields in the wake of a dry finish to the 2014 growing season, those who kept a close eye on their plant-available water (PAW) were able to more accurately predict yields, enabling better-informed management decisions.
CSIRO’s Canberra-based principal research scientist Dr Kirsten Verburg says when rain did not arrive in spring, growers may have underestimated the volume of water remaining in their soil following a wet start to the winter grains season.
The key terms Dr Verburg recommends growers become familiar with are plant-available water capacity (PAWC) and plant-available water (PAW).
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 or field capacity), and the lower limit where any remaining water cannot be absorbed by the plant (known as the crop lower limit 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,” Dr Verburg says.
“If, for instance, 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.”
PAW is the available soil moisture and indicates how full the ‘bucket’ is at any point in time.
Soil moisture terms
- Plant-available water capacity (PAWC) is the total amount of water that each soil type can store and release to different crops.
- The upper limit of PAWC is the drained upper limit (DUL) or field capacity, at which point any additional water drains, pools or runs off.
- The lower limit of PAWC is the crop lower limit (CLL) or wilting point where there may still be moisture in the soil, but it is contained in soil pores too small for plant roots to access.
- Soil moisture is the current moisture level (usually reported as a percentage of H2O) as measured by monitoring equipment or modelling, such as Yield Prophet®.
- Plant-available water (PAW) is the difference between the current soil moisture and CLL, and represents the volume of water stored in the soil that is plant-available at a point in time.
“In many areas of South Australia in 2014, the PAW was equal to the PAWC – the bucket was full – at the start of the season, and rains through to July kept it that way. This meant that where soils had a large PAWC, stored moisture was sufficient to get the crops through to harvest with good yields,” Dr Verburg says.
For example, Riverton in SA’s mid-north received only 50 millimetres of rain between August and October 2014 (Figure 1). However, good yields were still obtained because water stored in the soil compensated for the lack of spring rain.
“Knowing this, growers in this region who were on top of their PAW could see that regardless of rain from August onwards, there was still strong yield potential, which helped guide decisions on late fertiliser applications,” she says.
Models such as Yield Prophet® can incorporate PAWC data as well as other agronomic information and rainfall to simulate the growing season and help predict yields. Dr Verburg says that growers who used Yield Prophet® in the 2014 season would not have been surprised by the final yields.
“In regions where there was a lot of early rain and good soils, Yield Prophet® would have predicted good yield potential from the end of July regardless of any further rain.”
Figure 1 Riverton, South Australia, rainfall data for 2014 compared with the long-term average.
Dr Kirsten Verburg
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