Measuring the Percentage of Moisture in Soil Samples

Why is % moisture important?

The weight of a moist soil consists of the weight of the dry particles of soil plus the weight of the water within the soil. As the water content of a soil varies, the wet weight will also vary, but the weight of the dry soil particles remains constant. That is, there is only one fixed weight of a portion of a dry soil (“dry weight”), but essentially an infinite number of possible “wet weights”, as the water content of the soil changes.

Why do laboratories report soil results on a “dry weight” basis?

The dry weight of a portion of soil is constant. The wet weight of the same portion of soil can vary as the moisture content varies. For this reason, laboratories report analytical results for soil samples on a dry weight basis.

As an example, if the soil concentration of a parameter of interest is expressed on a dry weight basis, it will be constant, regardless of whether the soil sample was collected shortly after a rain event, or during a period of drought. Conversely, if the concentration of the same parameter is expressed on a wet weight basis, e.g. “as sampled”, it will vary depending on the amount of water in the sample (% moisture), i.e., it will be lower on a sample that is wetter.

Dry weight measurements for non-volatile parameters

From an analytical perspective, the key to constant, reproducible and defensible soil results is reporting data on a dry weight basis using accurate moisture measurements on the sample being analysed. One approach to reporting laboratory data on a dry weight basis is to simply dry a soil sample in an oven to evaporate any water within the soil, and then analyze the dried sample.

A significant advantage to this approach is that the dried soil can be crushed, ground and well mixed to achieve a very homogenous blend of dry soil particles, compared to a wet or “as received” sample, which is a heterogeneous mixture of particles and moisture. This allows the laboratory to achieve more consistent replicate analyses since the impact of heterogeneity is not as much of a factor as it would be for tests that analyze soils on an “as sampled” wet basis.

This approach only applies to parameters that are non-volatile and not affected by introducing heat. As an example, metals are, for the most part, non-volatile, and the heat of a drying oven is far below their boiling points, so they remain in the sample upon drying.

Dry weight measurements for volatile and heat-sensitive parameters

For all analyses, it is important to ensure that steps in sample processing do not affect the integrity or change the concentration of the parameter(s) being measured. Hence drying a sample as described above becomes problematic for volatile parameters and those that are heat-sensitive, as the drying step, whether it is performed at room temperature (air drying) or in an oven set slightly above 100 °C, may drive off volatile parameters or chemically/physically alter them.

In this case, the soil must be analyzed “as received” (i.e., wet) to maintain the integrity of the parameter(s) of interest. A separate aliquot of the homogenized wet soil is analysed to determine the % moisture, and the wet weight is converted to a dry weight for calculating the parameter concentration. In other words, the contribution of water to the weight of a wet sample aliquot can be subtracted out, giving a final result that would be equivalent to a result that had been obtained had the sample been analyzed dry, but avoiding parameter loss due to a drying process.

It is notable that the analysis of heterogeneous mixtures decreases the precision of the analyses. Therefore, it is important to maximize the accuracy and precision of the % moisture determination so that it does not contribute to the measurement uncertainty introduced by sample heterogeneity.

The value of accurate % moisture determinations

For regulatory compliance and as an industry-accepted best practice, soil results are reported on a dry weight basis. This ensures consistency and comparability of the data, regardless of when the samples were collected, differing amounts of water in different aliquots of the same sample or who analysed the sample. Hence, it is important to accurately and reliably measure % moisture on a separate aliquot of each sample being analysed that cannot be air or oven dried.