The adage ‘you can’t manage what you don’t measure’ rings true in assessing the merits of using plant-available water capacity (PAWC) and soil moisture measurement to make on-farm decisions.

CSIRO principal research scientist Kirsten Verburg has led a range of GRDC projects into measuring and predicting PAWC to help growers with decision-making. AgByte director Leighton Wilksch is an industry leader in soil moisture monitoring.

Dr Verburg and Mr Wilksch agree that combining PAWC information with soil moisture monitoring can give growers an edge in decision-making, providing valuable information to help with decisions, including what and when to plant, the likely response to mid-season fertiliser applications, and yield predictions.

To help growers improve their understanding of PAWC and soil moisture, GRDC is in the process of publishing the Water Smart Farming manual. This manual, featuring contributions from industry experts, will also include a range of other topics on farm water use including decisions growers can make before and during the growing season to improve water use efficiency and making decisions using uncertain weather forecasts.

Understanding PAWC

PAWC is the maximum amount of water a soil can hold and release to a crop. Knowing the PAWC can help with management decisions, particularly when accompanied by data on current soil water content.

While on-site measurements of PAWC are the most accurate way to get a full picture of a soil’s water-holding capacity, Dr Verburg explains growers can start with a simpler approach.

“More than 1100 Australian soils have already been characterised for PAWC and growers can access this information online through APSoil,” she says.

“The essential challenge when using this pre-existing data is how to select the right reference soil that is similar to a growers’ own soil, since PAWC can change dramatically between soils.”

There are also other soil resources and digital soil maps that can help estimate PAWC, and I’d suggest growers keep an eye out for the Water Smart Farming manual, which includes links to soil resources and a process growers can use to help analyse their soils and estimate their PAWC.”

Measuring soil moisture

Mr Wilksch says capacitance probes are the best solution for most growers, with or without online telemetry systems to automatically capture the data.

“When selecting a capacitance probe system, the user must decide what level of data collection and storage is required,” he says.

“The most low-cost simple system won’t have network connectivity and requires the user to plug into the logger and download the data on a laptop or via Bluetooth on to their phone, which is more effort but will save on the initial cost.”

The next decision is where to place the probe.

“There are various elements to consider to get maximum value from the site,” Mr Wilksch says.

“Growers should consider representative soils, access, factors like run-off from tracks or tree roots that would skew the results, and the risk of the unit being struck by machine operations.

“Where a farm has a range of soil types, I would recommend starting with a ‘better’ soil type, as these types will typically give the best return on investment for nutrition applications.”

With a soil moisture system installed and PAWC understood, growers can monitor their soil water ‘bucket’ in real time, always knowing how much water is available to the crop.

Soil moisture probes lead to confident decision-making

Since 2017, Matt Nihill and his cousin Danny – who farm on 1100 hectares at Elmore in North Central Victoria – have progressively invested in soil moisture and weather monitoring equipment across their farms to assist them with management decisions.

“We started with soil moisture monitoring because we wanted greater control of some of the difficult decisions that determine profitability in dryland cropping,” Matt says.

“Agriculture Victoria has a soil moisture probe installed not far away from our farm, but it is in a loamy soil, which is a very different soil type.

“While I found the information from the probe interesting, I knew it wasn’t going to be very relevant to our heavier soil and red sodic subsoil.”

Starting with two 120-centimetre capacitance probes and telemetry systems in 2017, the Nihills expanded their systems to include paired probes across different soil types with 160cm probes to observe infiltration to a deeper profile.

Understanding rainfall infiltration

With a range of soil and crop types, the data has been useful in understanding how deep rainfall events infiltrate into the subsoil and how this deep soil moisture is being used during grain fill.

“Since we installed them, the soil moisture probes and weather stations have influenced a lot of our decisions,” Matt says.

“For example, in 2018 we fallowed a heavy soil paddock for the first time in many years, based on low soil moisture, which gave us a great weed and disease break and in 2019 that paddock yielded 1.5 tonnes per hectare more than the rest of the farm.

“In the same 2018 season, soil moisture data convinced us that cutting wheat for hay would be more profitable than leaving it for grain. On the flip side, in 2022, the data gave us confidence to put in faba beans and canola instead of hay as break crops.

“In 2021 we applied a higher rate of late urea on our wheat, based on the moisture, and as a result we achieved higher grades than most local growers.”

A key benefit has been monitoring for frost events.

“One season we had a late October frost, which we initially didn’t think was too severe, but the canopy sensor showed it was cold enough that damage was possible,” Matt says.

Victorian grower Matt Nihill says moisture probes and weather stations influence many of his decisions. Photo: Supplied by Matt Nihill

Other benefits have included monitoring canopy humidity to help with canola Sclerotinia fungicide decisions, choosing when to start and stop harvest for fire risk and more-timely decisions on when to bale hay.

The Nihills have learned a lot about their soils and their capacity to support a crop through monitoring their soil moisture.

“We were surprised to learn how different the rooting depth can be in different soils,” Matt says.

“We changed a couple of probes from 120 millimetres to up to 180mm when we saw that our self-mulching soils would draw well beyond 120mm.

“In our sodic subsoils, I was pleasantly surprised to learn the roots could overcome difficult subsoil constraints and reach the 120mm sensor on a couple of occasions.

“In part, this finding encouraged us to try long-season wheat and canola, to grow deep roots that can better cope with either waterlogging or dry conditions later in the season.”

Along with soil moisture probes, the Nihills have used a range of tools to assess spatial variability, including regular satellite normalised difference vegetation index (NDVI), soil type maps generated from soil grid sampling and subsoil maps.

“I have no doubt that having access to all this information has benefited the farm and helped me be a better agronomist,” Matt says.

“It’s that little extra confidence in decision-making that it brings to the table – confidence to chase yield potential when the going is good, and confidence to pull back or invest more in hay when it’s dry.”

More information: Kirsten Verburg, 02 6246 5954, [email protected]; Leighton Wilksch, 0408 428 714, [email protected]; Matt Nihill, [email protected]

Useful resources: APSoil