Using Wetting Front Detectors (WFD) as a Cheap and Easy Way to Manage Water and Nutrients in Macadamias – Dr. Theunis Smit

When it comes to managing water and nutrients in macadamias, we are often left with two options. We either have to spend a fortune on high-end equipment to help us manage our resources or we simply neglect to monitor water and nutrient use. Fortunately, some basic and inexpensive tools are available to help growers manage these critical components of the production system. Wetting front detectors (WFD) were developed in response to low adoption of existing irrigation tools. Prof. Richard Stirzaker describes a WFD as a switch, which alerts the irrigator that a front of a given strength has passed a given depth in the soil. The WFD comprises a specially shaped funnel, a filter and a mechanical float mechanism (Figure 1).

FIGURE 1 FullStop Wetting Front Detector (WFD). The funnel part is buried in the soil with the black tube protruding above the soil surface. When a wetting front reaches the detector, a red indicator pops up. Detectors are usually placed in pairs, about one third and two thirds down the active root zone.

When rain falls or the soil is irrigated, water moves downwards through the rootzone.  The infiltrating water converges inside the funnel and the soil at the base becomes so wet that water seeps out of it, passes through a filter and is collected in a reservoir. This water activates a float, which in turn operates an indicator flag above the soil surface.  There are no wires, no electronics and no batteries.

According to Prof. Stirzaker, If the soil is dry before irrigation, the wetting front will not penetrate deeply because the dry soil absorbs most of the water. A long irrigation would be needed to activate a detector. However, if the soil is relatively wet before irrigation, it cannot store much more water, so the wetting front penetrates deeply (Stirzaker 2003, Stirzaker and Hutchinson 2005).

He further notes that knowing how deep a wetting front moves into the soil is critical for irrigation management. If a crop is given frequent but light sprinklings of water, the wetting front will not go deep and the WFD will not be activated. Much of the water will evaporate from the soil surface. If too much water is applied at one time, the wetting front will go deep into the soil, perhaps below the rooting depth of the crop, wasting water, nutrients and energy.

Furthermore, the WFD retains a sample of water which can be extracted via a tube using a syringe. This can be analysed for its salt or nitrate concentration using simple tools such as nitrate test strips and an Electrical conductivity (EC) sensor. Monitoring EC or nitrate levels can tell you more about irrigation management than measuring water content itself. For example, nitrate levels will drop sharply if over-irrigation occurs (Stirzaker and Wilkie 2002). Depending on the quality of the irrigation water, EC levels will gradually rise during periods of under-irrigation (Stirzaker et al 2004, Stirzaker and Thomson 2004).

Of particular interest to most growers will be watching the videos of Prof. Stirzaker explaining these and other useful irrigation and nutrient management tools in his own garden:

Installing a wetting front detector – https://youtu.be/pFdLjPepVL0

The wetting front explained – https://youtu.be/ptBlPBK_Zxs

Detecting the wetting front – https://youtu.be/gYk07ZGZbqo

Nutrient leaching explained – https://youtu.be/46ff1FILQhw

These tools and many other, reasonably priced and practical tools, are explained in great detail on the Virtual Irrigation Academy (VIA) website (https://via.farm/). VIA is a global community that aids farmers and communities to learn how manage water and nutrients to grow more food.

Why are these tools so useful in macadamia production systems?

Macadamias have shallow root systems and are particularly sensitive to overwatering. The sensitivity to overwatering is not only linked to the increased risk of root rot (Phytophthora), but also to the fact that nutrients are leached passed the rootzone. Using simple tools such as a WFD, macadamia growers would be able to monitor that both water and nutrients are not going past the rootzone, which will not only help them use resources more efficiently but could potentially increase both yield and quality. Furthermore, these tools are reasonably priced, practical and user friendly and could therefore be used by an array of macadamia grower.

For more information regarding these and other tools, please feel free to contact the folks on the VIA platform, or to contact myself (theunis@darwinhort.com).

References

Stirzaker RJ and Hutchinson PA (2005). Irrigation controlled by a Wetting Front Detector: field evaluation under sprinkler irrigation. Australian Journal of Soil Research (in press)

Stirzaker R, Stevens J, Annandale J, Maeko T, Steyn J,Mpandeli S, Maurobane W, Nkgapele J & Jovanovic N(2004). Building Capacity in Irrigation Management with Wetting Front Detectors. Report to the Water Research Commission No. TT 230/04.

Stirzaker R and Thompson T (2004). FullStop at Angas Bremer: A report on the 2002-3 data to the Angas Bremer Water Management Committee Stirzaker RJ (2003).

When to turn the water off: scheduling micro-irrigation with a wetting front detector. Irrigation Science 22, 177-185.

Stirzaker RJ and Wilkie J (2002). Four lessons from a wetting front detector. Irrigation Australia 2002 conference, 21-23 May, Sydney.

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