Lessons learnt from using soil moisture probes

Background

Soil capacitance probes have been in use since the early 90’s when the probes technology and the interpretive software was developed to enable the visualisation of soil moisture at various depth in a soil profile. Initially probes found use in permanent plantations and high value water sensitive crops. In more recent times as the cost of the probe hardware and installation has become more affordable there has been a steady uptake in the use of probes in annual crops. I also believe growers and advisers are also more acutely aware of the impact soil moisture management can have on overall production.

Where to start?

I have yet to see a farm that has all the same soil type and every paddock is the same. So for me the starting point with a probe position is putting the probe in an area of the paddock that represents the most uniform type of soil or from where the average of the paddock production comes from. 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. The type of irrigation and system layout are also taken into account.

Poor site selection will result in the collection of unrepresentative soil water changes which can lead to poor irrigation scheduling decisions.

In my opinion it is important to remember that in the initial years of using probes the aim is to build a good back ground picture of what is occurring to soil moisture and to fine tune irrigation timings to maximize water use and crop production. Initially having a probe installed isn’t going to dramatically change how you irrigate and manage the crop.

It is important to develop a good relationship with the business who is supplying and servicing the probes in your area. Ask your agronomist who they recommend. In most instances the probes are installed after the crop has emerged so the probe position captures a good representation of the established plant stand. With this timing in mind the more you can let the installer know where your program is up to the simpler it will be for them to schedule the installation, which helps to ensure data collection starts on time. Once the crop water use has finished it is a good to again communicate with your supplier regarding removal. Once harvest is underway the small things like removing a probe can be forgotten with disastrous results.

When positioning the probe in the paddock give thought to any tractor passes that may occur in crop, spraying, spreading etc. By having the probe positioned at the end of a spray boom makes it easier to navigate it in the paddock

Clearly mark the site and inform anyone who may be operating machinery in or around the area of the probe’s location.

As the data is collected and the graph starts to build you can see where the changes are occurring. This builds into the step pattern that shows the change from reading to reading. By comparing one reading to the next you are able to obtain a figure which represents the change. Although this figure may not read directly in MM I have found that it is usually close enough to correlate. It is this figure I will compare with evapotranspiration data to estimate where the water levels will be in the future, thereby giving a good indication of when to start irrigating or to help decide if there is sufficient moisture to finish the crop.

After an irrigation or a large rainfall event the full capacity of the soil profile can be established, this sets the useful upper limit of the field water holding capacity. Reviewing the rate at which the moisture level draws down allows a lower level to be set, a refill point, it is at this point at that time of plant growth you don’t want the moisture level to fall below the set limit. The refill point will change as the crop root development increases.

Getting the refill points at the correct level can take more than one season but the refill point setting is a key advantage of the probe as it is driven by plant root growth.

Rooting depth sets the available moisture deficit. The deeper the roots the greater the available deficit.

The crop water use will be influenced by temperature, humidity, wind speed, solar radiation and rainfall. These factors need to be considered when reviewing the data. As the temperature increases the crop needs more water, during overcast days crops will slow their growth resulting in a lower water use.

What can go wrong?

• Loss of data from chewed cables Foxes will chew cables if they come across them. Bury and sleeve the cables as required.
• Probes damaged or run over by farm equipment. Clearly mark probe sites and think about machinery width.
• Telemetry problems can occur with probes located in tall crops such as maize. Talk to your supplier 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.

End of season

When the season is over it is important to have a look at the probe data and review what it can tell you about the crop.

• What depth was moisture being removed from?
• How long did it take for the roots to grow to various depths?
• Did the applied irrigations always recharge the profile to the same extent or did the recharge amount change over time?
• How significant were rainfall events on wetting the profile?
• How was the timing of irrigation? Could a slow down 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?

Reviewing these questions will help you develop better irrigation strategies for future crops

Conclusion

The data provided by the probe and graphed by the software combined with the season evapotranspiration data will give a site / crop specific measure on changes in the soil moisture over the growing season. When an understanding of were the crop is drawing moisture from, usable limits can be developed to provide an operational range of ideal soil moisture level through the stages of crop growth.

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