A SOIL moisture probe might have been a temporarily, unnecessary tool in parts of Queensland and NSW recently given rain events, but knowing the moisture levels in paddocks is critical ahead of winter planting and irrigation planning.
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However, growers and agronomists were generally becoming more adept at using ‘soil capacitance probes’ and interpretive software to assess paddock moisture levels, agronomist Adrian Hayes said.
Yet site selection is critical when it comes to positioning probes to assist with irrigation decisions.
The Coleambally-based adviser, who has presented information at GRDC updates, said it was important growers located probes in an area of the paddock that represented the most uniform soil type or produced consistently average yields.
“Topography survey maps, yield maps, prior season’s NDV vegetation maps can all be used to help define a representative area of where to install a probe. But the irrigation type and system layout also need to be considered,” Mr Hayes said.
He said poor site selection would equate to poor data collection and in turn poor irrigation scheduling decisions.
“Growers and advisers should aim to build a thorough picture of soil moisture, so they can fine tune irrigation timings to ensure maximum water use efficiency and crop production,” Mr Hayes said.
“Initially installing a probe isn’t going to dramatically change irrigation management, but longer term it can assist with decision making.”
He said probes were usually installed after the crop had emerged so the probe position captured a good representation of the established plant stand.
“When positioning the probe in the paddock consider tractor passes that may occur in crop during spraying and spreading. Having the probe positioned at the end of a spray boom makes it easier to navigate.”
He advised growers to clearly mark probe sites so it was apparent to contractors or others operating machinery in the paddock.
“As data is collected and the graph starts to build you can see where the changes are occurring. This builds a pattern that shows the change in soil moisture levels. By comparing one reading to the next you are can obtain a figure which represents the change.
“It is this figure growers need to compare with evapotranspiration data to estimate where the water levels will be in the future, giving a good indication of when to start irrigating or whether there is sufficient moisture to finish the crop.”
Mr Hayes said after an irrigation or a large rainfall event the full capacity of the soil profile could be established, setting the upper limit of a paddock’s water holding capacity.
“Reviewing the rate at which the moisture level draws down allows a lower level to be set, or a refill point. You don’t want the moisture level to fall below this limit. But be aware the refill point will change as the crop root development increases,” he explained.
“Getting the refill points at the correct level can take more than one season, but setting the refill point is a key advantage of the probe as it is driven by plant root growth.”
He advised growers to keep in mind crop water use will be influenced by temperature, humidity, wind speed, solar radiation and rainfall and these factors need to be considered when reviewing probe data.
What can go wrong
- Loss of data from damaged cables. Foxes will chew cables. Bury and sleeve the cables as required.
- Probes damaged or run over by farm equipment. Clearly mark probe sites and consider machinery width.
- Telemetry problems can occur with probes located in tall crops such as maize. Inquire about extended antenna or mounting the telemetry unit out of the crop.
- Water moving down the tube resulting in the profile being refilled from the bottom to the top. This can be a problem in cracking clays, however your installer should be aware of this and will use an appropriate installation method.
- Bad site selection. Review the criteria that was used and modify the selection process.
Review data at the end of the season
- What depth was moisture being removed from?
- How long did it take for the roots to grow to various depths?Did irrigations always recharge the profile to the same extent or did the recharge amount change over time?
- How significant were rainfall events?
- How was the timing of irrigation? Could a slowdown of crop growth be attributed to water availability?
- Could the refill points be adjusted?
- After an irrigation, how long did it take for the crop to start using water at peak usage? Is it possible through irrigation timing to reduce this sitting period?
Mr Hayes said considering these points would assist growers develop more effective irrigation strategies in future crops.
