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Question: The structure of the soil is disturbed when the FullStop is installed. How much does this affect the results?


Our experience shows that the dryness of the soil is much more important than changes to the soil structure following installation. This is because the speed a wetting front moves down into the soil depends strongly on the water content of the soil before irrigation. If the soil is dry before irrigation, the wetting front will move slowly because each layer of soil absorbs the water. A long irrigation is needed to get water down to the FullStop. If the soil is relatively wet before irrigation it cannot store much more water, so the wetting front penetrates quickly and deeply.

What about changes to the conductivity of the soil caused by disturbance? It is true that if soil above the FullStop is repacked to similar bulk density, disturbance changes the way soil pores are inter-connected, which impacts on the saturated conductivity. However, when water is applied at rates less than the infiltration capacity of the soil (as is typical for drip or sprinkler systems), the soil surface remains below saturation. In this case the large inter-connected soil pores usually do not fill with water, so the saturated conductivity of the soil is not the critical factor.

The dominant factor impacting the movement of the wetting front under sprinkler or drip irrigation is therefore the initial water content, not the saturated conductivity which would be affected by soil disturbance.

It is important to remember that all tools for measuring soil water content require soil disturbance during installation. In many cases the sensitivity of the instrument is greatest in the small zone of disturbed soil immediately adjacent to them. A large area of soil is disturbed during installation of a FullStop, but it is also sampling a large volume of soil. We consider the large sampling volume an advantage because research shows that a sample containing up to 20 elementary units (soil crumbs or peds) is best for measuring a soil property like water content.

FullStops are not designed for very deep installations (>60 cm) and they are not suited to installation in heavy clay subsoils. Roots need to grow back into the disturbed zone before results will be reliable. Care is needed when interpreting results from flood irrigation. In this case the saturated conductivity does impact on the rate of movement of the wetting front. We are currently working on new Wetting Front Detectors for furrow irrigation.

Finally there are a number of different sources of variability in a field that impact on soil water content apart from soil disturbance. These include variability of soil properties, difference in plant growth and, biggest of all, non-uniformity of irrigation.

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