Author - David Hunt, GIA Smart Farming Project Officer
Can soil moisture sensors be used in containerised nursery production? The short answer is yes, they can, but there are some things that you need to consider before buying them.
Irrigating to the plants needs has always been the focus of plant production managers, and there are various methods used to find that balance between ensuring the plants have sufficient moisture and nutrients while using resources efficiently. This has become easier now with the advancement of digital or smart farming sensors, and modern communications technologies.
Soil moisture or growing media moisture sensors are now cheaper and easier to install, and with new communications technologies incorporating cloud-based interpretation services, these sensors can provide real time remote monitoring of root zone moisture content for irrigation automation. Some can also monitor the temperature and electrical conductivity of the root zone. These sensors have proven to be beneficial at the Smart Production Nursery as part of the Smart Farming Partnerships project funded under the National Landcare program, Greenlife Industry Australia (GIA) and Hort Innovation, with estimates of a 30% reduction in irrigation runtimes reducing the cost of production. However, there are some aspects of incorporating moisture monitoring sensors into production systems that need to be considered.
Why use soil/media moisture sensors?
Irrigation scheduling is usually a pre-determined schedule developed from experience and the visual appearance of the plants, which is only re-assessed after the plants show signs of stress. The amount of water used by the plants fluctuate daily with the quantity of solar radiation, temperature, wind, and humidity of the growing environment. Fixed irrigation schedules can either over-irrigate or under-irrigate the plants. Over-irrigating wastes water, energy, labour, reduces growing media aeration, and leaches nutrients from the containers, which can cause algal blooms in drains or dams. Under-irrigation can stress the crop, reduce yield, reduce crop uniformity, and adversely affect plant quality.
Using soil moisture sensors to monitor the root zone moisture content in real-time allows for quicker adjustments of the irrigation schedule to match the actual plant water use for the daily conditions. This will not only reduce the volume of water used but will reduce pump run times reducing energy use and cost, and reduce nutrient leaching keeping nutrients within the root zone and available to the plants.
What should you consider before purchasing/installing moisture sensors?
Does the irrigation system meet industry best practice?
Although soil moisture sensors can be a valuable tool, they are only as good as the efficiency of the irrigation system. A soil moisture sensor will not improve an inefficient irrigation system with an uneven application or inappropriate application rate. These sensors will only help to manage an inefficient irrigation system more efficiently. To achieve the best results from installing moisture sensors and to reduce production costs, the irrigation system needs to meet industry best practice.
That is, the pumping system and mains pipe must be capable of supplying the correct flow rates and operating pressure for multiple zone irrigations. The sprinkler height and spacing need to meet the manufacturers specifications for the sprinkler being used. The uniformity of application or coefficient of uniformity (CU) must be greater than 85% meaning the water is applied evenly over the irrigation zone so at least 85% of the plants get the same amount. The mean application rate (MAR) should meet industry standards of 10mm/hr to 25 mm/hr and match the infiltration rate of the growing media, and the scheduling coefficient (SC) should be less than 1.5. If the irrigation system does not meet the industry best practice, then upgrading the irrigation system should be considered before installing moisture sensors.
How can you choose a soil moisture sensor?
There is a wide range of commercially available soil moisture sensors that can be used for irrigation scheduling. Choosing the right type of sensor will depend on the irrigation system, growing media, the container size, and whether the sensors are to be used to monitor root zone moisture content or automate the irrigation system.
Monitoring root zone moisture content as a check mechanism or tool for refining the irrigation schedule is cheaper. The sensors may only need to be connected to the nursery’s WiFi network, and they usually come with a basic visualisation program or phone app to present the data. Automating irrigation with soil moisture sensors is more expensive, requires an efficient irrigation system, a stable communications network or WiFi, accurate sensors, a more sophisticated irrigation controller, and possibly a subscription to a cloud-based interpretation service. Automation will require a staff member to have an understanding of how the sensor system works in case there are problems, and will need to be familiar with the irrigation trigger points, such as, wilt point, irrigation start and irrigation stop points.
No matter how you intend to use these sensors, the most important deciding factors are the growing media type and container size. Many soil moisture sensors have been designed for inground use in mineral soils, which work very well when calibrated to the soil type. Unfortunately, not all sensors work as well in organic growing mediums with a high air-filled porosity. The different physical and chemical properties of organic growing media and their large air spaces tend affect the signal of some soil moisture sensors.
Two common types of moisture sensors available are –
Capacitance sensors, these tend to be cheaper but only monitor a small volume around the sensor and need to be buried within the root zone. They can be susceptible to changes in temperature and salinity, and require a specific calibration for each organic growing media blend. However, these would be suitable for landscape tree producers with long-term crops growing in the ground.
Time Domain Reflectance (TDR) sensors – are not affected by temperature and salinity, transmit an electromagnetic signal through the growing media over a larger volume, do not need a specific calibration for organic growing media, and are easy to install. These sensors can be inserted through the side of a plastic container and are small enough that one sensor could be used in small containers, and two or more could be positioned at different depths in large container. Some are available with interchangeable prongs of varying length for different container sizes. Unfortunately, there are no soil moisture sensors smaller enough for seedling trays, a weight scale or weighing system is more appropriate in that situation.
An alternative to installing a moisture sensor network throughout the nursery is to use a handheld insertion moisture probe that can be carried and inserted into any container for an instant moisture measurement. This type of moisture sensor is cheaper than installing a network of sensors, and is easy to use. However, measurements will need to be taken manually and downloaded to a phone app or PC software for graphing and interpretation. This type of moisture sensor cannot be used to automate irrigation but is a good starting option for those interested in monitoring container moisture content.
In most cases there will be a soil moisture sensor available that can help to identify plant water use and improve irrigation scheduling. However, for any sensor to correctly represent the larger growing area or crop, the irrigation system should meet industry best practice for efficiency and uniformity of application. The sensors need to be appropriate for how they will be used, and a staff member must be familiar with how the sensor data relates to the irrigation schedule before the full benefits can be realised.
The summary of criteria to consider when choosing soil moisture sensors is:
- Size of sensor - the sensor needs to fit into growing containers.
- Type of sensor – Appropriate for organic growing media, be robust and reliable, and not affected by environmental conditions.
- Cost - financially viable with no or limited ongoing costs.
- Availability - Be readily available on the local market, with after-sales technical support if required.
- Simplicity/ease of use - Must be able to be installed and maintained by nursery staff, does not require specialized personnel or equipment for calibration, and is easily transferred from one container to another.
- Interface/connection – comes with a visualization program for data interpretation, or can be connected to existing irrigation controllers for integration.
References and Further Reading
Hunt, D.S., 2008. Increasing Adoption of Irrigation and Water Recycling Technologies in Australian Nurseries: FINAL Report for Project PA 57134 http://era.daf.qld.gov.au/id/eprint/2502/1/PA57134_Increasing_Adoption_of_Irrigation_and_Water_Recycling_Technologies_in_Australian_Nurseries.pdf
Rolfe, C., Yiasumi, W., & Keskula, E., 2000. Managing water in plant nurseries. 2nd Ed. ISBN: 0734711808. NSW Agriculture. (3rd Edition available)
Rolfe, C., 2006., ‘Scheduling irrigation to maximise efficiency’ Greenlife Industry Australia, Nursery Paper August 2006 Issue no.8, viewed 23 August 2021. https://www.greenlifeindustry.com.au/static/uploads/files/ngia-np-2006-08-august-wfpuyrjtefqu.pdf
Video – Best Management Practice Irrigation Layouts. https://nurseryproductionfms.com.au/download/best-management-practice-irrigation-layouts/