SoilMoisture Measurement Classic Reprint

Excerpt from Soil-Moisture Measurement

As has been noted, the gravimetric method is the only commonly used direct means of measuring soil moisture. Simply, it involves taking a soil sample, weighing it, oven-drying it, reweighing, and expressing the original moisture content in percent of oven-dry weight of soil. The first step, securing the sample, is the most troublesome, the degree of difficulty depending on the condition of the soil.

Sampling conditions are ideal when soil is just moist enough for easy ingress and egress of the sampling instrument, and where stones, roots, and organic matter are not a problem. Such conditions are seldom encountered.

With free water in the soil, moisture can never be sampled accurately: water will drip off as the sample is removed from the ground or compaction may squeeze it out. Rapid sampling is essential to prevent undue losses. Stickiness, often a problem in wet soils, can be alleviated by keeping instruments clean.

Where a wet layer overlies a dry one, samples taken from the drier levels may be contaminated. After the upper soil is wetted by a summer rain, for instance, removal of a sample may permit water to run down the hole toward drier soil. The same problem is en countered when samples are taken below a perched water table. Wet instruments add to contamination and should be dried between samplings.

When the soil is dry and hard, the principal problem in fine textured soils is to get the sampling instrument in and out again. In coarse-textured soils particularly, samples may slide out of the in strument as it is withdrawn from the soil. Fortunately, after the soil dries in summer, its moisture content may fluctuate so little that frequent sampling may be unnecessary.

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