KBS015: Gravimetric Soil Moisture
The amount of water or moisture in a given mass of soil is highly variable and is important to measure in field studies because it affects microbial activity, nutrient movement, and plant growth. To measure soil moisture content by the gravimetric method, a subsample of a fresh, sieved composite sample or a fresh soil core is weighed, oven dried until there is no further mass loss, and then reweighed. The moisture content is expressed as the moisture percentage of the soil dry weight.
Sampling frequency: Depends on experiment and research objective. Soil moisture is routinely measured as part of our LTER core soil sampling for inorganic N, N mineralization, and greenhouse gas fluxes.
Sample preparation: For sieved composite soil samples, such as those for soil inorganic N, take subsamples for both soil moisture and inorganic N at the same time to reduce time spent on handling. For soil cores taken with greenhouse gas flux sampling and stored in soil moisture tins, analyze the entire sample as collected from the field..
• Soil moisture tins and lids; pre-weighed and numbered (since 2004*)
• Plastic spoons
- Record sampling date, experiment, treatment, replicate, variate, etc. on the soil moisture spreadsheet.
- Weigh out soils:
A. For sieved composite soil samples, place soil moisture tin on the balance and tare the weight by zeroing the scale. Use a plastic spoon to subsample the composite sample and place 40-50 g of soil into the tared soil moisture tin. Record tin number and the exact fresh weight of soil (to nearest 0.01 g) in the tin on the soil moisture spreadsheet. Weights can be recorded directly from the scale to the spreadsheet using linking software. Cover tin with matching numbered lid.
B. For soil core samples stored in moisture tins, moisture content is determined on the entire sample in the tin. Record tin number and the exact total weight of soil and tin, including the matching numbered lid, on the soil moisture spreadsheet. Weights can be recorded directly from the scale to the spreadsheet using linking software. Subtract the tare weight of the moisture tin and lid (weights are in a linked spreadsheet) from this value to determine the fresh weight of soil.
- Remove lids and place tins into the oven. Oven dry soil to a constant weight: at least 24 hours at 105 degrees C for soil moisture only or 48 hours at 60 degrees C if C/N analysis is required.
- When drying is complete, recap tins with numbered lids as they are removed from the oven. Allow to cool for 15 minutes.
- Tare scale to zero. Place each tin containing soil, including the lid, on the balance and record total weight of tin + soil in spreadsheet. There is no need to weigh the tin+lid separately because these weights are already in the spreadsheet.
- Dispose of soil unless needed for C/N analysis.
*Prior to 2004, small paper bags (#1, #2 or #3) were used instead of soil moisture tins. Bags were labeled with sampling information and samples processed using the above procedure, with the following exceptions. The dried sample weight was determined by subtracting an estimate of the bag tare weight, determined as the average weight of 10 empty bags that were treated the same as bags containing soil samples. Also, individual sample bags were stored in larger grocery bags while in the oven to reduce handling time and maintain order.
Subtract the tare weight of the designated soil moisture tin and lid (weights are in database) from the oven-dried sample weight (soil + tin + lid) to determine the dry weight of soil.
Determine % soil moisture on a dry-weight basis using the equation:
% soil moisture (dw) = 100 * ((fresh weight – dry weight)/dry weight)