How to correct the soil weight for soil moisture at air dry (AD) conditions to oven dry (OD) soil moisture
The objective is to outline how to accurately weigh soil, to learn how to correct the soil weight for soil moisture at air dry (AD) conditions to oven dry (OD) soil moisture, and to determine a correction factor to account for the air dry moisture content in subsequent samples weighed for analysis.
- Weigh soil accurately
- Calculate soil moisture
- Determine soil air dry (ADsoil) to oven dry (ODsoil) correction factor (AD2OD correction factor)
Air dry correction factor
Soil properties generally are expressed on an oven-dry soil (ODsoil) weight basis. The calculation of the air-dry to oven-dry, or AD2OD, ratio is used to adjust all sample weights to an oven-dry basis. To calculate the sample weight that is equivalent to the oven-dry soil weight the AD2OD ratio is applied.
When a soil is sampled it is spread out and dried to air dry moisture content. The exception to this is when drying the soil would affect a biological assay or investigation of biological organisms that are so common in soils. When biological organisms are to be assayed the soil is often not dried and is sometimes placed in cold storage.
Even when a soil sample feels dry after air drying, it contains some water. This water is held too tightly to be useful to plants, but in order to quantitatively report on soil characteristics this tightly held water must be accounted for. All soil weight data are typically reported, or corrected, to an ODsoil weight basis.
The ODsoil moisture status is used because it is repeatable and reproducible. Why? When the soil is air dried the soil moisture is in equilibrium with the relative humidity of the air. The relative humidity of air changes from day to day. Soil in equilibrium with the humidity on a humid day weighs more than soil on a dry day due to the water associated with the soil.
Therefore, a baseline moisture status is needed. The agreed upon established water content is that content achieved after drying in an oven at 105¬∫C until the weight no longer changes. It is inconvenient to dry every sample prior to every analytical procedure. In fact in some cases heating the sample influences the results of subsequent analysis. Because of this it is common to measure the fraction of water that is associated with the soil between air dry and oven dry on a separate sample. In this way a convenient correction factor can be applied to samples that are under similar conditions which really means the same day. Every time a soil is weighed for an analysis, another sample is weighed and the AD2OD correction factor is determined for that moment that the sample was weighed. The drying of the sample takes days so the calculations cannot be completed in one day.
To determine the amount of water in your air-dry soil (ADsoil) sample, you will weigh a soil sample, dry it in an oven to a constant weight and after cooling it, weigh it again. The second weight is the oven-dry soil (ODsoil) weight of the sample and the difference between the ADsoil wt. and ODsoil wt. is the weight of water in the ADsoil sample. The moisture content is expressed as a ratio of the air-dry to the oven-dry weight (ADsoil/ODsoil), but the correction factor is less than one so (ODsoil/ADsoil). Weigh the weighing tin separately and subtract its weight from the soil plus tin weights.
Traditionally, the most frequently used definition for a dry soil is the soil mass after it has come to a constant weight at a temperature of 100 to 110¬∞C (American Society for Testing and Materials, 1993). Many laboratory ovens are not capable of maintaining this prescribed temperature range. Temperatures that are > 50¬∞C may promote oxidation or decomposition of some forms of organic matter. Samples may not reach a constant weight with overnight drying.
Do not add moist samples to an oven with drying samples unless the drying samples have been in the oven for at least 12 to 16 h. Soil samples may adsorb significant amounts of moisture from the atmosphere after cooling. Prompt weighing, i.e., less than 30 min after samples have cooled, helps to eliminate this problem. Also it is advisable to cool samples in a desiccator to avoid atmospheric moisture from re association with the soil surfaces. This is more prominent in fractions with great surface area such as clays. During the weighing or drying processes, the non-uniform weight of weighing vessels, sample contamination, or sample loss may lead to erroneous results.
Use heat resistant gloves to remove weighing containers from a hot oven. No other significant hazard is associated with this procedure. Follow standard laboratory procedures.
- Electronic balance, ¬±0.01-g sensitivity
- Oven, thermostatically controlled, 105 ¬± 5¬∞C
- Thermometer, 0 to 200¬∞C
- Aluminum foil dish, 57-mm diameter x 15-mm deep, with lifting tab; or a drying tin
Air Dry Soil Moisture Procedures
- Record the weight of the dish or tin. Record the label number, or name, of each weighing tin. This is important. Do not simply tare the weight of the dish or tin.
- Add 10 to 20 g of air-dry soil to each moisture dish and record the weight in the laboratory notebook.
- Weigh the dish plus the sample and record the weight to the nearest 0.01 g. and record the weight in the laboratory notebook.
- Place the sample dish in a drying oven set at 105 ¬± 5¬∞C. Allow the sample to remain in the oven overnight (12 to 16 h). Do not add moist samples to an oven with drying samples unless the drying samples have been in the oven for at least 12 to 16 h.
- Remove the sample dish and allow to cool before reweighing. Record the oven-dry weight to the nearest 0.01 g in the laboratory notebook.. Do not allow the sample dish to remain at room temperature for >30 min before reweighing.
A suggested reporting data table consists of the results from:
- Calculate the weight of the air dry soil (ADsoil) in the tin. Sample calculation: ADsoil wt. = (tin wt. & ADsoil wt.) – tin wt.
- Calculate the weight of the oven dry soil (ODsoil) in the tin after removing from oven. Sample calculation: ODsoil g = (tin g & ODsoil g) – tin g
- Calculate a “correction factor” to apply to any sample to represent the ODsoil weight from any sample given the samples ADsoil weight. Sample calculation: AD2OD correction factor = ODsoil/ADsoil
- American Society for Testing and Materials. 1993. Annual book of ASTM standards. Construction. Section 4. Soil and rock; dimension stone; geosynthesis. Vol. 04.08. ASTM, Philadelphia, PA.