Volume-Accuracy Relationship in Soil Moisture Sampling

The number of soil samples required to achieve a given level of accuracy of the mean value is directly proportional to the variance of the samples. Analysis of a set of soil samples of different sample volumes showed that there is a relationship between the variance of the samples and the sample volume. Generally, a larger sample volume results in a lower variance because the larger sample includes more of the overall spatial variability than a smaller sample. Samples of eight different sizes were collected; sample volumes ranged from about 7 to 825 cm³ (1/2 to 50 in³). Analysis of variance techniques were used to determine the relationships between both bias and sample volume and precision and sample volume. This analysis showed that when sampling to a depth of 10 cm (4 in), the minimum desirable sampling volume is about 50 cm³ (3 in³); smaller volumes will result in lower precision of the mean value. There does not seem to be any direct relationship between bias and sample volume. The minimum desirable sampling volume is a function of the homogeneity of the area being sampled and the elapsed time since the last significant rainfall.