Analytical Methods:Soil Moisture
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Contents |
[edit] Soil moisture
| Scale | Field or basic laboratory technique |
| Questions | Moisture content is usually recorded to support other analyses |
| Samples and storage | Bulk samples are usually fine unless the sample is being used for another technique such as bulk density. Samples should be sealed and analysed as soon as possible. |
| Time and Cost | Very quick and low cost technique. Many soil analyses require air or oven-dried samples and it is very quick to record the loss in weight on drying. |
| General comments | Individual soil moisture measurements provide little information it is spatial and temporal trends that are usually of most interest. |
[edit] Questions
Soil moisture is highly variable hence individual measurements provide very little information. It is spatial and temporal trends in soil moisture that are usually of most interest either as an indication of changing preservation conditions across a site or over time or to support the interpretation of other analyses. For examples relative differences in soil moisture content across a site may help to explain element concentration patterns.
Example case studies where soil moisture content has provided useful information to support archaeological interpretations include:
[edit] Analysis
Soil moisture status is usually very easily assessed in the field as either soil wetness or soil drainage status.
| Drainage class | Criteria |
|---|---|
| Poorly drained | Soil remains wet to a shallow depth, and excess free water is evident in the profile for long periods of time. |
| Moderately drained | Water drains somewhat slowly due to impervious nature of substrate, low slope, and/or shallow water table. |
| Well drained | Water drains easily but not rapidly. Free water is uncommon. |
| Excessively drained | Water is removed from the surface very rapidly. Free water is very rare or very deep. |
| Dry | Soil feels completely dry and when wetted will show a distinct colour change (unless entirely quartz sand). |
| Moist | Soil feels damp though no water is visible on grain or aggregate surfaces. |
| Wet | A film of water is visible on grains and aggregate surfaces but there is no free water. |
| Saturated | The sample contains easily visible free water, working or squeezing the soil liberates the water. |
Where exact measurements are required to support geophysical survey or other analyses these can be made in the field using a soil moisture probe such as a theta probe. The probe is inserted into the soil and the reading recorded; because soil moisture is so variable it is advisable to take multiple measurements at each sampling point.
If a moisture probe isn’t available soil moisture can be determined by:
- weighing fresh soil samples
- drying the samples at 105oC until a steady weight is reached.
- reweighing the dried samples.
% Soil Moisture = ((Fresh weight – Dry weight) / Dry weight} x 100
If an oven isn’t available soils can be air-dried but this may not drive off all the moisture, particularly in clay rich or organic soils.
[edit] Data and interpretation
Soil moisture values can not be interpreted meaningfully in isolation because of the high temporal variability. Spatial patterns of moisture can be mapped and compared with other soil properties across the same area, or soil moisture values can be plotted against another variable, such as an element concentration to identify linear or non-linear relationships and to perform regression analysis.
[edit] Related techniques
