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Assessing soil moisture probes

Updated: Apr 28, 2023

Despite the unseasonal weather, we really enjoyed our time at South Island Agricultural Field days, with many interesting conversations had. Our compliments go to the organisers for yet another great event.

A common topic around soil moisture monitoring was whether a quality vertical TDR (Time Domain Reflectometry) soil moisture probe (such as the Sentek Drill & Drop range) was as accurate and reliable as a neutron probe; alongside the cost-benefit of using a weekly neutron probe scheduling service versus purchasing a TDR probe that is either telemetered or that is downloaded via Bluetooth.

Neutron probes work by firing neutrons out into the soil and counting the number of ‘slow neutrons’ – those that have collided with the hydrogen atoms in water. This means they don’t require good soil to sensor contact so are ideally suited to providing accurate and reliable readings in stony soils. However, a neutron probe scheduling service typically only takes one measurement per week and only provide a single management trace. Infrequent readings can lead to more challenging irrigation decision-making on the shoulders of the season, particularly for pivot irrigation.

TDR probes work by calculating the travel time of a high frequency electromagnetic pulse through the soil. Accurate and reliable readings require careful installation, good soil to sensor contact is key. When installed properly TDR probes are proven to provide reliable and accurate data for 7 to 10 years. They also provide ‘real time’ data for each sensor down the profile alongside a management trace. This allows the irrigation depth applied (how far the irrigation has moved down the soil profile) to be monitored and refined to match or maximise plant rooting depth, in addition to the management trace.

When a Net Present Value analysis is undertaken based on current pricing (inclusive of capital and operating costs), the TDR Bluetooth probe is more cost-effective than a neutron probe by year 3, and the TDR telemetered probe is more cost-effective by the end of year 4. So, over a 7 to 10-year lifespan a quality TDR probe works out as the more cost-effective option. For cropping, if the probes are used in two crops per season and the equivalent neutron probe scenario is assessed against this, the time in which the TDR probe becomes more cost-effective is decreased, as there is less capital outlay.


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