Learning Centre

Soil Moisture Tension in the Field

Measurement Units
You will hear both kPa and negative 'something' kPa bandied around in reference to Soil Moisture Tension.

KiloPascals (kPa) are units of pressure measurements. Suction is a negative pressure which is also referred to as a tension.

Soil Moisture Tension is a measure of suction, and the correct way to refer to it is minus or negative X kPa. However, it is quite common for the minus to be dropped.

It can be a little counter intuitive when thinking of wet soil vs dry soil with soil moisture tension as smaller numbers that are closer to zero don't mean less water. In fact these low numbers indicate more water, wetter soils. They show how much suction is needed to extract water from the soil. So small numbers, mean it's easy for the plant roots to get a drink. As a soil dries out the kPa values becomes larger (and more negative) and the hard it is to extract water.

Effect of Soil Type
One great thing about measuring in Soil Moisture Tension is that soil type is largely irrelevant. -25kPa in clay is the same as -25kPa in sand. Plants in these conditions in either of these soils are working exactly the same to extra moisture from the soil.

Irrigation Scheduling
Permanent crops, such as tree crops and vines are relatively forgiving to irrigation practices - they have extensive root zones, which can access large volumes of stored water; and episodes of water stress may damage the current year's crop or reduce the following season's fruit set but rarely lead to loss of the plants.

The picture for seasonal crops is much more critical - a single episode of water stress can lead to the immediate and complete loss of the plants.

Whilst tree crops may be happy extracting water to -60 kPa, most of the annuals can only extract to levels of -20 or -30 kPa.

A bit of research on the internet or a chat to your agronomists can tell you which kPa level you should be stressing your crop type to, before irrigation is required. You can also do this by ground truthing, digging in the soil and watching crop water stress responses.

Which sensors measure Soil Moisture Tension?
1 Gypsum blocks
2 Tensiometers

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