Unravelling the mysteries of soil water repellence

Western Australian PhD students unravel the mysteries of soil water repellence


Water Resources
Enoch Wong, PhD student at UWA researching factors that affect expressions of soil water repellence. PHOTO Evan Collis

Enoch Wong, PhD student at UWA researching factors that affect expressions of soil water repellence. PHOTO Evan Collis

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Students on quest to better understand soil water repellency.

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Three Western Australian PhD students are on a quest to better understand soil water repellency in an attempt to provide growers with greater understanding and practical solutions to improve crop germination and yield results

Soil water repellence is one of the most significant factors affecting crop germination and early vigour.

With an estimated three million hectares across Western Australia’s broadacre regions considered at high risk of water repellence, this ever-increasing problem requires urgent and soil-type-specific solutions. A wide-ranging GRDC-invested research program into water-repellent soils is developing strategies to help growers tackle this challenge. As part of the program, three PhD students are working closely with industry experts to develop a greater understanding of what factors influence and increase water repellence in the soil.

Research may provide valuable repellence forecasting tool

Access to forecasts that can predict a host of variables – such as temperature, rain, wind speeds and humidity – are critical business tools for grain growers.

But what if the next available tool for a grain business is a forecast for the water repellence levels of the soil, particularly at the time of sowing?

Enoch Wong, a PhD student studying at the University of Western Australia with investment from both GRDC and CSIRO, hopes his three years of research into factors affecting soil water repellence may provide a better understanding of soil water repellence at certain times of the year.

When we move from late summer to autumn the temperature starts to drop and the humidity increases – meaning the soil water content also increases slightly. My research is indicating that it is at this point that the water repellence in the soil increases significantly. - Enoch Wong

Soil water repellence is dynamic, with its severity varying through the seasons. Mr Wong is researching factors that affect the expression and severity of soil water repellence, particularly at seeding time. He says this may form the basis for forecasting the seasonal severity of soil water repellence.

Specifically, his research is aimed at understanding the relationship of soil water repellence to soil water content and external air temperature. His experiments in a controlled laboratory environment are showing some interesting results. To date, he has identified what he describes as a “break point” of soil water content and its relationship to soil water repellence. When soil water content (SWC) increases beyond 0.6 per cent in a non-wetting sand, he has found water repellence also dramatically increases.

Conversely, once the soil water content increases to “critical soil water content”, usually at percentages greater than the break point, soil water repellence decreases rapidly.

All experiments were carried out in temperature-controlled rooms at three different temperatures (4˚C, 20˚C and 40˚C) and different soil water contents were achieved in desiccators with different saturated salt solutions.

With the change of seasons, Mr Wong says, particularly from summer through to autumn, soil water content can be solely reliant on the external humidity levels when there is an absence of rain.

“When we move from late summer to autumn the temperature starts to drop and the humidity increases – meaning the soil water content also increases slightly,” he says. “My research is indicating that it is at this point that the water repellence in the soil increases significantly.”

This may mean that dry seeding at this point in time, before significant rain, could be coinciding with the highest level of soil water repellence.

Using a molarity of ethanol droplet (MED) test to measure the severity of repellence, Mr Wong observed an increase of MED in soils incubated at 40˚C under increasing humidity from approximately MED 1.8 (0.6 per cent SWC) to 4.6 (1.4 per cent SWC).

Higher MED values indicate more severe soil water repellence, with anything higher than MED 3 being severely water repellent.

His research could provide better guidance at seeding time regarding the need to apply banded wetting agents or invest in other soil water repellence management strategies.

It might mean sowing should be delayed until critical soil water contents have been reached to achieve better crop germination and establishment in certain soils.

Future research could investigate the impact of temperature and humidity on the expression of water repellence in the field, taking into account the time of day, soil surface temperature and other practical factors.

Mr Wong hopes to complete his studies in 2019.

  • More information: Phil Ward via phil.ward@csiro.au

Organic Matter Research Considers Clay Effects

Nicholas Daniel, PhD student at Murdoch University researching the molecular chemistry of soil water repellence. PHOTO: Evan Collis

Nicholas Daniel, PhD student at Murdoch University researching the molecular chemistry of soil water repellence. PHOTO: Evan Collis

Understanding soil water repellence at a microscopic level, and how certain soil types have higher expression of repellence in the presence of specific organic matter, is the next phase in research looking to find solutions to the problem.

PhD student Nicholas Daniel, who is based at Murdoch University, has designed a series of experiments aimed at understanding why certain soil types express water repellence more than others.

While there is a general understanding that soil organic matter causes soil water repellence, Mr Daniel’s research investigates what is happening between the organic and mineral components in the soil at the molecular level.

His studies are looking at four key classes of organic molecules identified in both agricultural and bushland soils in WA as being associated with soil water repellence. These include saturated fats, saturated fatty acids, alcohols and sterols.

The work combines laboratory experiments and computer simulations to compare the degree of repellence induced by these components in both sandy and clay soil types, and to discover the chemical mechanisms involved.

“At the moment, we know that clay is used as an ameliorant for water-repellent sandy soils because the surface area of a grain of clay is much larger than a grain of sand, so is more difficult to coat with the repellent organic molecules,” Mr Daniel says.

However, he believes this explanation is incomplete, not taking into consideration the complex chemical interactions occurring in the soil at a molecular level.

While claying has been shown to be effective in the field at reducing repellence, he says a lack of research makes it difficult to propose a more thorough explanation with confidence.

Now into the third year of the research, he has identified that sandy soils respond differently to organic molecules than the clay soils.

“My early simulations looked at the interactions between saturated fatty acid and alcohol molecules on sand and clay surfaces, their structure and how they layer on the surface,” he says.

“But what we found with these early models was that they were too simple, and we couldn’t discern any difference between the interactions to clay and sand which would account for the significant difference observed in water repellence of the two minerals.”

He says the complexity of the models was then increased to include water/soil charge effects, as well as charged metals (including calcium, sodium, magnesium and potassium).

“With these more complex models we are now starting to see a significant difference between what is happening with the organic molecules on the sand surface compared to that on the clay,” he says.

“On the sand models, these organic molecules arrange themselves so they lay parallel to the surface, effectively acting as a shield, inhibiting water from interacting with the sand surface,” he says.

“Conversely, on the clay surface, the organic molecules arrange themselves vertically, or perpendicular, to the surface, which allows water to still interact with the clay surfaces – suggesting much lower degrees of water repellence.”

Mr Daniel’s research, while theoretical and very detailed, could provide paddock-specific solutions in the future for growers to target and improve their water-repellent soils.

“If we can deepen our understanding of how organic molecules interact with the mineral components of soil, at a molecular level, then it may be possible to manipulate these interactions to reduce repellence and then start applying this understanding to the development of practical solutions,” he says.

He is hoping the outcome of his research will allow soil water repellence solutions to be better targeted at the molecules causing the issue and soil-type differences, therefore more effectively combating the issue.

“For example, before a grower spends money on a generic banded wetting agent, they might take a soil sample first and send it for a round of testing designed to identify the specific organic material and associated interactions with the mineral surface, which could then lead to the design of a wetter formula specific to that one paddock or soil type,” he says.

  • More information: Nicholas Daniel via tzeek.1@gmail.com

Insights Into Repellence And Nutrient Uptake

Simon Yeap, PhD student researching interactions between soil water repellence and crop nutrition. PHOTO: Evan Collis

Simon Yeap, PhD student researching interactions between soil water repellence and crop nutrition. PHOTO: Evan Collis

Water-repellent soils are challenging enough, but does water repellency have a dual impact by reducing a plant’s ability to take up nutrients, as well as limiting water availability?

Researcher Simon Yeap, who is completing his GRDC-invested PhD studies at Murdoch University, is attempting to understand the relationship between soil water repellence and nutrient availability and uptake.

Despite Mr Yeap’s observation that repellent soils can act as a barrier for roots to access nutrients, his on-farm experiments at Meckering, Kojonup, Badgingarra and Moora have so far been inconclusive.

Mr Yeap is also conducting glasshouse experiments to ratify his results, with these trials so far demonstrating a greater link between water-repellent soils and nutrient uptake. “We know that water-repellent soils can limit crop nutrition in a number of ways,” he says.

“Because of the reduction in volume of soil that wets up, the chemical availability of nutrients is limited, and therefore the amount of nutrients diffused towards the root zone changes.

“Dry zones in the soil might act as a physical barrier to root growth as well, and preferential flow can accelerate nutrient loss away from the root zones.”

Surprisingly, in most of the experiments, while crop emergence was affected, this did not have an impact on overall yield results and nutrient uptake.

“In a field experiment at Badgingarra, soil compaction appeared to have a greater impact on wheat growth and total nutrient uptake when compared with water repellence,” he says. This highlights that managing multiple constraints in combination may be necessary to improve overall nutrient availability.

Also, high spatial and temporal variability of water repellence expression in the field can make these observations difficult. But Mr Yeap suggests the nutrients may become available later in the season once the soil has become wet.

While this remains an untested theory, he believes more research into this hypothesis could be of benefit to growers. “This observation has been an interesting outcome of this research, and it may be that water repellence early in the season has the benefit of a greater amount of nutrients left in the soil for uptake at critical times later in the crop growth cycle.

“However, this is only of benefit if the plant can overcome the early soil water repellence challenges and can germinate with adequate water and nutrient supply for strong early growth.”

Through his research, he has also seen the benefits of water-repellent topsoils in regard to harvesting water into the furrows.

With water-repellent topsoil, the water is repelled from the top of the ridge and sides of the furrow and directed straight into the seedbed, having an almost irrigation-type effect on the seed, encouraging better germination and early crop growth.

What this means for growers, he says, is that any attempts to alleviate water repellence in topsoils, particularly using wetters, should be directed only to the base of the furrow to encourage this water-harvesting strategy, provided the furrow can be treated to wet up consistently.

“This observation is suggesting that furrow geometry is very important in directing the water to the seedbed, and also that a blanket application of a banded wetting agent may not be the most economical way of encouraging early crop germination and growth,” he says.

Mr Yeap’s research began in 2016 and will be completed at the end of the 2018 season.

  • More information: Simon Yeap 31249848@student.murdoch.edu.au

GRDC Research Code: DAW00244

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