22nd December 2021

Soil Moisture measurement

Not only a useful measurement in Agriculture but also widely used in Meteorology: the measurement of Volumetric Water Content (VWC). A little explanation…

Soil moisture measurements are known to be useful in Agriculture. It helps greatly to determine the irrigation schedule for crop. It is also known to be a helpful measurement when it comes to defining efficient use of fertilizers.

As water is one of the carriers of the fertilizer through the soil and it also can positively or negatively influence the effect of the fertilizer, it is clear that it is important to know how much water the soil contains. Soil moisture is of course greatly influenced by precipitation. Depending on the type of soil, precipitation will drain down easily or will be stopped in lower layers that are impermeable.
A part of the water will however be bound to soil particles (hydroscopic effect). Plants do of course need water to grow, so it is important to know that the moisture of the soil, carrying the nutrients, is sufficient for that. This makes the use of soil moisture measurements in agriculture quite obvious.

Maybe less obvious, is that soil moisture is also important in local weather forecasting and local weather prediction. When the soil on the surface has a high water content, it is likely that when the temperature get higher during the day, there will be evaporation. Higher water content of the soil clearly means more evaporation and in fact this higher evaporation will limit the air temperature to rise.

So in fact: when having high soil moisture values, the air temperature will rise less.
More evaporation will bring more water in the air; a logical consequence will be that the dewpoint temperature will rise. Dewpoint is the temperature where water vapor will condensate: hot air can hold more water than cold air. When the temperature goes down (drops below Dewpoint), the cooler air is saturated with water and the vapor will condense.
This energy of the condensation process will have a warming effect on the air and will thus limit the air temperature to keep on falling.

High evaporation, meaning high water content of the air, will also influence the probability that precipitation may occur. When hot air rises, combined with e.g. a low pressure front, it is likely that the higher the water content of the air, the higher the chances are it will rain.

Measurement of soil water content, or soil moisture, is mostly expressed in %. The percentage of water in the soil, relative to the volume of the soil. A measurement value of 15% means that 1m3 of soil contains 15% water: so in this calculation example it means that 1m3 of soil contains 15 liters of water.

In a soil moisture sensor that measures VWC (Volumetric Water Content) often a combination of measurements is used: soil moisture, combined with temperature and EC is a very common combination.
It is clear that the content of ‘soil’ will not be the same everywhere, so this may cause differences in readings from one place to another. On the other hand, highly accurate measurement results are not always a necessity: often it is sufficient to know the relative changes over time for a certain area.
To obtain accurate measurement values, it is advised to use a ‘soil specific’ calibration curve.