Soil Moisture content in The field
Volumetric Soil Moisture Content refers to the volume of water in a given volume of soil as discussed on previous pages. This type of Soil moisture Content is measured in m3.m-3 (so how much of a cubic metre is water out of the entire cubic metre of soil sample). Alternately, the Volumetric Soil Moisture content can be referred to as a % of volume (which is much easier).
Effect of Soil Type
Soil moisture content is very dependent on soil type. A saturated coarse, sandy soil can hold far less water than a saturated heavy silty clay. Sand has large particles which take up a lot of physical space. Also, as sand particles do not bind water, a lot of water will drain out of the sand due to gravity before field capacity is reached. For these two reasons, sand has a much lower maximum and minimum water content than a clay soil does.
What this means for you is that if you are monitoring a number of sites across your property, the soil moisture content values should not be compared between paddocks with different types of soil. Reading 20% moisture in one paddock and 20% in another paddock does not mean that the plants will be equally happy.
In sand, plants in 20% moisture will be very happy as sand readily releases its moisture and the most sand can hold is around 30%.
However, in clay, a plant in 20% moisture will be extremely stressed. Clay soils often have a maximum moisture reading of 50% or more, so 20% is very dry. The clay particles also bind water to themselves and at low moisture contents like 20%, the clay will not give the water up for the roots to use.
It is therefore important, for each site need to be considered individually!
When you start out, you have to learn what the various content values mean for your soil. What is the maximum reading you are getting after heavy rain? As the soil dries out, compare the values you are getting to how the soil looks when you dig around a bit and to how water stressed the plants are. What is the minimum reading you are getting in very dry soils at the end of summer when you've not irrigated for awhile? All of this information will give you a good start on working out at which soil moisture % you need to start irrigating and when you should stop.
Which sensors measure Soil Moisture Content?
Measures soil moisture using Capacitance (Frequency Domain Reflectometry):
- EnviroPro Probe
Measures soil moisture using TDR (Time Domain Reflectrometry) and TDR-like technology:
- Profile Probe
- WET Sensor