A soil moisture probe weather station at Farming Forecaster site.
By Kimberley Beer, Murray Local Land Services senior land services officer farming systems
Soil moisture probes can be a useful tool to show how much water is in the soil profile, water infiltration patterns, the depth plant roots are active and where potential constraints might be present e.g. an acid layer.
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Having this information can help plan fertiliser applications, grain marketing, target soil investigations and plan grazing management.
However, to use probe data you need to understand how to interpret it, which isn’t always easy.
When looking at the graphs, the trick is to focus on the patterns of the sensor lines opposed to the actual figures, like the graph above.
Each line on the graph is a probe sensor at a different depth.
A soil moisture probe graph showing the distribution of water in the soil profile from the surface to 80cm deep.
The graph pictured has eight lines for an 80cm probe, with each sensor 10cm apart - starting just below the soil surface.
The key patterns on the graph to look out for are:
• Spikes - these indicate infiltration and subsoil recharge, from either rainfall or irrigation. The first circle above highlights a rainfall event that raised soil moisture down to 70cm.
• Rippling - the small ups and downs (ripples) in the horizontal lines show plant root activity. Dips show moisture withdrawals during the day, and rises are the small refills while plant activity reduces overnight.
• Draw down - while plants are actively growing, they withdraw moisture, gradually drying the profile. This is represented by the downward trend of the line. In the third circle, draw down can be seen in the first 40cm of the soil. The 50cm sensor line remains flat, meaning the plant roots aren’t accessing this moisture.
Looking at drawdown patterns over time, you can identify moisture availability and how deep roots are drawing from.
Kimberley Beer.
In wet conditions, roots access water from closer to the surface, using the path of least resistance, drawing from deeper as soil moisture depletes.
Although, it’s worth noting, if roots consistently halt at a specific depth year-on-year, there may be a physical or chemical barrier that’s limiting root growth and the size of the soil moisture ‘bucket’.
In this case, it may be worth further investigation and amelioration may be required.
An in pasture soil moisture probe and pasture cage.
