Protocols

KBS015: Gravimetric Soil Moisture

In use from sometime

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 nitrogen mineralization samples are being weighed. This reduces the amount of time spent on handling the soil samples and provides moisture availability for mineralization calculations.

Materials

  1. Labels

  2. small paper bags

  3. Grocery Bag

  4. Plastic spoons

  5. Balance

Procedure

1. 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 on scale (zero the balance).

3. Weigh approximately 40 g of sieved soil in tared paper bag. Record exact weight of soil in bag onto soil moisture spreadsheet. Place weighed bags into a grocery bag labeled with sampling date.

4. Place grocery bag into oven. Oven dry soil to a constant weight. Approximately 24 hours 105 degrees C or 48 hours 60 degrees C if C/N analysis is required.

5. 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.

7. Tare scale to zero. Place each individual bag containing soil on scale and record weight in lab book. Weigh empty paper bags and record as bag tare weight.

8. Dispose of soil unless needed for C/N analysis

9. Enter results into spreadsheet on computer.

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.

Abstract

Sampling Frequency: Monthly

Soil moisture analysis is completed monthly 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 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

Author: Tim Bergsma

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