KBS015: Gravimetric Soil Moisture

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Abstract

Soil moisture analysis is completed as part of the nitrogen mineralization potential measurements. A 40 g fresh weight subsample taken from the composite soil sample of each plot is weighed and placed into a paper bag (or tin) and then oven dried at 60 degrees C for 48h. After drying the samples are reweighed.

Moisture is expressed on a % dry weight basis by the formula:

%soil moisture (dw) = 100 * ((Fresh weight – dry weight)/dry weight)

and on a fresh weight basis by the formula:

%soil moisture (fw) = 100 * ((1- dry weight)/fresh weight)

Protocol

Overview

Soil moisture content affects microbial activity, nutrient movement, and plant growth. Soil moisture content is needed to calculate other soil biological parameters (e.g. N mineralization) on a common (dry weight of soil) basis. The gravimetric method calculates soil moisture by the difference between the fresh weight and dry weight of a given soil sample.

General Procedure

Soil moisture samples are taken from the sieved soil samples at the same time inorganic N samples are being weighed. This reduces the amount of time spent on handling the soil samples and provides moisture availability for mineralization calculations.

Materials

  • Labels
  • small paper bags or unique numbered tins (since 2004)
  • Grocery Bag
  • Plastic spoons
  • Balance

Procedure

  1. If using bags, prepare labels and label small paper bags (1#, 2# or 3#) with appropriate information about the sample (e.g. project, date, plot,variate).
  2. Tare paper bag or tin on scale (zero the balance).
  3. Weigh approximately 40 g of sieved soil into tared paper bag or tin. Record exact weight of soil in bag onto soil moisture spreadsheet. Record tin number when using tins instead of bags. Place weighed bags into a grocery bag labeled with sampling date.
  4. Place grocery bag or tins into oven. Be sure to remove tin lids. Oven dry soil to a constant weight. Approximately 24 hours at 105 degrees C or 48 hours at 60 degrees C, if C/N analysis is required.
  5. If using bags, weigh 10 empty paper bags and place them in the oven with soil.
  6. When drying is complete, remove bags from oven and allow to cool for 15 minutes. Be sure to recap tins as they are removed from the oven.
  7. Tare scale to zero. Place each individual bag or tin containing soil on scale and record weight in spreadsheet. This can be done using directly using software linking the balance and computer or manually entered into a spreadsheet.
  8. Weigh empty paper bags and record as bag tare weight.
  9. Dispose of soil unless needed for C/N analysis.

Calculations

% soil moisture = ((wet soil wt) – (dry soil wt – bag wt.) / (dry soil wt. – bag wt.)) * 100

Approximate time required

  1. Label preparation and bag labeling will take 0.75 person hours.
  2. Soil weighing will take 1 person hours.
  3. Oven drying will take 24 to 48 hours, 0 person hours.
  4. Dry Weight recording will take .75 person hours.

The MCSE:Baseline Soil Sampling Protocol describes the sampling frequency at the KBS LTER site.

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