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How many sensors do you need?

Continued rain at regular intervals along with colder than average temperatures has resulted in very little irrigation being required for much of Canterbury since late January. This is shown in the adjacent graphs using weather station data from Lincoln, Winchmore, and Timaru airport.

Given the NIWA Climate outlook to June is forecasting near average to average temperatures and average rainfall, it’s quite tempting to start winterising irrigators. However, it’s best to wait a couple more weeks (Easter), keeping a close eye on your moisture probe trace in case the climate predictions turn out to be incorrect.

Primary Insight’s recent irrigation workshops in Canterbury and the Bay of Plenty resulted in much discussion around how many soil moisture probes are needed to meet good practice, and how to minimise the costs of soil moisture monitoring?

The answer to how many sensors is farm specific. It requires consideration of the soil type’s present, the different crops grown, and most importantly a good understanding of how the farms irrigation system operates, including any limitations. During the workshop we work through how to practically do this, using one of the attendee’s farms as an example. For a dairy farm the process typically results in 2 to 4 sensors being required; for an arable farm it can be double this, which can make monitoring soil moisture monitoring using soil moisture probes an expensive exercise for these farms.

However, an advantage arable farms have over dairy is crops are not grazed. This allows for a greater range of soil moisture probe options to be used, including Bluetooth probes. These probes download their data to a smart phone when you get within 10 to 20 metres, the smart phone then uploads the data to the website so you can view and make decisions from the trace. Bluetooth probes are half the cost of telemetered probes as no telemetry hardware is required; their running costs are also less as there’s no telemetry cost.

Another way to reduce costs is to develop a technology plan for the farm; identifying the range of current and future technologies and how to rationalise running costs over time, particularly from a telemetry viewpoint. I’ve recently been on a farm which had four separate telemetry systems in operation!

If you’d like to learn more about whether Bluetooth probes are an option for your farm, how to work out how many sensors you need, or you want to brush up on your irrigation knowledge, Primary Insight’s next FREE Irrigation workshop will be held 1-4pm 12 April at the Hinds Community Centre. Attendees will get a certificate of attendance which can be used to meet your Farm Environment Plan requirements. Registrations for this and future workshops are available through our website.


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