KM DeAngelis January 22, 20071 of 2Measurement of soil moisture content by gravimetric methodThe soil moisture content may be expressed by weight as the ratio of the mass of water present to the dry to the dryweight of the soil sample, or by volume as ratio of volume of water to the total volume of the soil sample. Todetermine any of these ratios for a particular soil sample, the water mass must be determined by drying the soil toconstant weight and measuring the soil sample mass after and before drying. The water mass (or weight) is thedifference between the weights of the wet and oven dry samples. The criterion for a dry soil sample is the soilsample that has been dried to constant weight in oven at temperature between 100 – 110 oC (105 oC is typical). Itseems that this temperature range has been based on water boiling temperature and does not consider the soilphysical and chemical characteristicsMaterials• Oven with 100 –110 0C temperature• A balance of precision of ±0.001 g.• Aluminum weigh tins• auger or tool to collect soil samplesProcedure1. Weigh an aluminum tin, and record this weight (tare”).2. Place a soil sample of about 10g in the tin and record this weight as (wet soil + tare).3. Place the sample in the oven 105oC, and dry for 24 hours or over night.4. Weigh the sample, and record this weight as weight of (dry soil + tare).5. Return the sample to the oven and dry for several hours, and determine the weight of (drysoil +tare).6. Repeat step 5 until there is no difference between any two consecutive measurements of theweight of (dry soil+tare).ComputationsThe moisture content in dry weight basis may be calculated using the following formula:(tare) - tare) dry soil of wttare) dry soil of (wt-tare) soil wetof wtd+++=((θIn some literature the moisture content is expressed in wet weight basis that is defined as theratio between water mass and the mass of wet soil (wθ). The conversion from dθ to wθ or visaversa can be carried out as follows:wt of water = wt of wet soil – wt of dry soildry soil of wt waterfo wtd=θ) dry soil of wt) dry soil of (wt-) soil wetof wtd((=θKM DeAngelis January 22, 20072 of 21((−=) dry soil of wt) soil wetof wtdθbut :) dry soil of wt) soil wetof wtwd((=θθthus:1−=wddθθθwhich result in the following formulas:1+=ddwθθθAnd:wwdθθθ−=1Water content in volumetric basis is expressed as: soilof Volume waterof Volumevd=θbut:density water waterof wt waterof Volume =And:density bulkdry soil of wt soilof Volume =Thus:desnity waterdensity bulkdry soil of wt waterof wtvd×=θwbdvddd×=θθReferenceBlack C.A. 1965. “Methods of Soil Analysis: Part I Physical and mineralogical properties”.American Society of Agronomy, Madison, Wisconsin,