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Soil moisture monitoring part 2

At their meeting the Stewart PIT also discussed initial data from the 5 Observant capacitance probes installed on the Stewart farm. Throughout the project the PIT farmers have had numerous discussions on the value of moisture probes and how to use them most effectively.

Soil moisture probe take home messages

  • Look for the decline in daily water use and ground truth what the cause is. Is it moisture stress or another factor?

  • Set your own refill and full point management lines. Probe software might give you indicative positions but you have to tailor it to your farm, your soil, your crop and your water infrastructure. If it takes a few days to order or apply water give yourself a bigger buffer between timing of irrigation and the point of water stress, in case things go wrong.

  • Check it regularly! Read the soil moisture probe graph every couple of days and more frequently if you are nearing irrigation or there is a big change in weather-either rain or heat wave.

  • Look at both your summed or average graph and your individual sensor data graph and understand how they fit together and at what depth water is being used from at different times.

  • Make notes in your software if you can. This will help you improve your irrigation practice over time and identify how different factors e.g. rain and heat, have impact on soil moisture use.

How do you use your capacitance probes?

here are many different software options for different commercial moisture probe offerings. Understanding how the software is set up and what information or parameters are fed into the graphs you are looking at is critical to interpreting soil moisture conditions and actioning irrigation accordingly.

The most important question when considering information from soil moisture graphs and other sources is "when will the pasture or crop start to become too limited by soil moisture?" This is the refill point for your particular soil and pasture type. If irrigation is not applied prior or at this point then a yield reduction will occur, depending on the stage of crop development.

There are many ways to work out where your refill point is. One of the simplest ways is to look for a change in water use in your soil moisture graph. When a crop or pasture is becoming water stressed, water use will decline as it works harder to access moisture. The slowing of the rate of uptake of soil moisture will flatten off, showing a shallower slope, compared to peak uptake of soil moisture, or the "steeper" part of the graph.

Graph 1: change in water use over time

Note: other factors such as heat stress, water logging and soil compacting will also cause a decline in the daily water use of a crop that is not related to water stress, so it really important to ground truth your decision making with other methods. Refer to our previous article on moisture probes for more information.

Once you have monitored changing conditions on the soil moisture graph and ground truthed conditions physically in the field, you can adjust the refill and full point management lines on your software to help you plan future irrigations.

It is really important to revisit and revise your refill and full point management lines because they can change in different stages of crop or pasture development over the season.

Should I calibrate my moisture probe?

You can use your moisture probe "uncalibrated". As mentioned above, look for the trends in change in water use and ground truth with your existing irrigation scheduling methods. Calibration becomes important when you are using the probe to understand the exact magnitude of plant water use or the soil moisture deficit. Calibration is not a straight forward process and is usually only done in research projects. In an uncalibrated probe, the absolute figures will not necessarily be an accurate representation of mm of soil water use, or % of soil moisture content. But the relative changes to soil water use will be. This is why it is easiest to use soil moisture use trends provided by soil moisture graphs rather than absolute numbers to schedule irrigation.

Summed graph vs individual sensor data graph

Most moisture probe software can provide you with a summed graph of soil moisture, which is either the average or the summed value of soil moisture at each sensor depth. On a 100cm probe there is typically 10 sensors, one at each 10cm interval. You should also be able to look at a graph that shows individual soil moisture data at each sensor. This graph shows how soil moisture is being used at different depths under a perennial pasture and lucerne.

As you can see from the below example from the Humphris farm, the majority of water use by pasture is from the top 50 cm. This is typical of where the majority of roots will be in a perennial pasture.

Know how your probe software operates. Some software will take an average of all the sensor depths and some will add them to form an average soil moisture graph or total soil water graph. Accordingly, some software gives you options to change the weightings of each sensor and/or turn them on and off. By understanding how this works for your gear, and looking at soil moisture use at each depth you will have a clearer picture of where soil moisture is being extracted. If you do turn off lower sensors or adjust them, it is important to regularly look at the lower depths so you can detect any water movement.

Graph 2: soil moisture use at each sensor depth. There is significant use at 10-30cm with gradual decreasing use with depth.

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