When and how much to irrigate? Answers to these questions give the soil moisture sensors of the leading Australian manufacturer – Sentek. They show the soil water content every 10 cm depth – e.g. at 10, 20, 30, … down to 120 cm. The water content is displayed in litres (mm) per square meter. This way you have information how much water you have available in the soil at any given moment.
Sentek uses as a “sensor” the root of the plant. On the following chart you can see how the soil water content drops steeply during the day and gradually during the night. The reason for that is that during the day the plant “pumps” water from the ground and evaporates it through the leaves. This does not happen during the night. That is why the chart looks like stairs whenever the soil moisture is within the optimal range.
When the soil dries so much that plants cannot extract water from it any more, the chart goes straight and levels out. This is exactly the moment when another irrigation is necessary. When the soil is saturated, plants cannot use water from it in an optimal way and the chart is flat.
This way you can see when to irrigate – when soil moisture drops below the lower limit, under which plants get stressed. In the case on the next chart, this means one irrigation every 6-7 days.
How much to irrigate?
Sentek sensors measure soil water content at every 10 cm depth. This way you see how much water is necessary to provide optimal soil moisture down to the depth of the roots at the current moment. On the following chart you can see soil water content from 0 to 60 cm depth. In the middle of July (where the left arrow points), water is used from a depth of 20 cm (i.e. the roots reach down to 20 cm). This means it is necessary to irrigate that much, so that the water reached this exact depth. More irrigation would mean a waste of water, because roots will not be able to reach the greater depth and use the water from there.
Towards the end of August however, roots use water from 40 cm depth, because plants have already developed. That is why in August you would need to irrigate more, so that the water goes down to 40 cm in the soil.
This way you see exactly how much to irrigate at any given moment.