Key points
- Subsoil constraints affect productivity across 60% of Australia’s 20 million hectares of cropping soils. Investigations to ameliorate these constraints have led to mixed results and there are questions as to why different treatments have either worked or not.
- Conventional techniques for investigating treatment effects use single-point-in-time destructive sampling and often do not link changes in soil condition to a plant response in an environment. Advances in logging and sensing technologies now allow continuous, non-destructive monitoring of multiple soil, plant and weather properties.
- In 2023 and 2024, sensors at three field sites measured crop water use and determined soil water retention curves at several depths in the soil. In 2024, canopy temperature of the crops was also measured at one of these sites. This showed how plant water status responded to the amount of water available in the soil.
- Continuous monitoring of soil water content at two sites showed how different soil amelioration strategies improved plant available water capacity. Pairing soil water content with soil water potential measurements allowed in-situ soil water retention curves to be determined at each monitored depth. This showed where in the soil profile the treatments changed pore size distribution, and by how much.
- Measuring canopy temperature alongside soil water content and soil water potential is expected to improve the modelling and prediction of plant responses to subsoil amelioration across climates with different evaporative conditions.
The challenge
Plant available water capacity (PAWC) is the amount of water a soil can store in the root zone between the wettest limit for crop growth (the drained upper limit, DUL) and the driest limit for crop water extraction (the crop lower limit, CLL). Since the early 2000s, much effort has gone into removing subsoil constraints to increase PAWC so that roots can access more stored water. However, yield gains have been inconsistent, partly because the effect of treatments on PAWC has been hard to measure.
Measuring water content at the DUL and CLL gives the PAWC, but it does not show how hard it is for plants to extract water from the soil between these two limits. This information is needed to fully identify how soil constraints impact plant water supply. When water is harder to extract, plants take up and transpire less water than they need. As a result, crop growth does not reach its potential, even while the soil still holds water it can use.
A soil water retention curve (SWRC) describes how hard it is for plants to “suck” water from a soil (i.e. the soil water potential) at any given water content. In wet soil, water sits in large pores, and a plant needs little energy to draw it into its roots. As the soil dries, the remaining water is held in progressively smaller pores, at greater suction, and the plant must work harder to extract it. The SWRC is measured in two main ways:
- In the laboratory, using soil cores taken from different depths in the soil at a single point in time (which disturbs the soil and affects the results).
- Estimated from field measurements of DUL and CLL using calibrated soil moisture sensors.
Pairing water potential sensors and water content sensors allows the SWRC to be measured in the field, continuously, and under a growing crop. This produces a SWRC that shows how hard it is for plants to extract water from the soil as it dries. Field conditions capture the combined effects of soil, plant and weather on soil water dynamics. This method may therefore be a better way to identify how soil amelioration affects plant water use across the full range of plant available water and over time, as well as the effect on crop growth and yield.
Our research
This project tested a field method to characterise how easily plants can access soil water (the SWRC) and how much water the soil can store for plants (the PAWC). The method was tested at three sites, each with a history of management aimed at overcoming soil constraints.
