Soil Moisture Measurement

A physical measurement of soil moisture content can determine the amount of water present within a soil, being expressed as either (1) the mass of water present, as a ratio of dry soil weight (called the gravitational water content), or 92) the percentage of water present within a defined volume (called the volumetric water content).

1. Gravitational water content: This is simply a mass of soil being weighed, for example, 50g, it is then oven dried (at 105°C) to remove water present, and then reweighed. This is recorded and then the soil is continued to be oven dried until there is no loss of weight, meaning that all the water has been removed from the soil. The final weight is then recorded, for example 37g. This would mean that 13g of water has been removed, and the water content is expressed as a ratio of the initial soil mass.

The equation can be: GWC% (of dry soil) = Masswater / Massdrysoil or 13 / 50 = 26%, or about ¼ if expressed as a fraction.

2. Volumetric water content: This is the volume of water present as a ratio of the total volume of soil. The mass of water is 1g per cm3, so effectively 1 unit of mass equals 1 unit of volume for water. The volume of soil used for this example is a block of soil measuring 4cm x 5cm x 5cm, or 100cm3. The soil is dried just like in the first example, and the starting mass of the soil block is 200g; whilst the dried mass is 150g, meaning that the loss of water is 50g.

The equation can be: VWC% (of dry soil) = Vwater / Vsoil or 50 / 150 = 33%, or about 1/3 if expressed as a fraction.

With the volume of the soil being known as well as the dried soil mass, the bulk density of the soil can also be determined, and this is really helpful in assessing soil compaction which will help in making better informed maintenance and management decisions.

Bulk density = Dried soil mass / total volume of soil core, or 150g / 100cm3 = 1.5g/cm3.