It is generally done at 105 degree centigrade.

Dear Debra,

(1) Moisture content determination in soil by oven-drying method is based on removing soil moisture by oven-drying a soil sample until the weight remains constant. The moisture content (%) is calculated from the sample weight before and after drying.

(2) A thermostatically controlled oven preferably of the forced-draught type, capable of

maintaining a temperature between 105 °C and 110 °C is used for this purpose

(3) Soils containing gypsum lose water of crystallisation on heating. Therefore, a

moisture content determined by this method will be affected by approximately 0.1%

for each 1% of gypsum. If it is suspected that gypsum is present in the soil, dry the

moisture content samples at not more that 80 °C, for a longer period (till weight remains constant) and report the method used.

I attach herewith PDF copy of the manual describing the detailed method for determination soil moisture content.

Hope you find this input useful.

Dr Paul Sir and Dr Kundu Sir have ably addressed the issue.

On structural basis, soil water is classified into pore water, solvate water, adsorbed water and structural water. The capillary water and gravitational water may be considered as two types of pore water, and are capable of moving under hydrodynamic forces unless restricted by entrapped air molecules or when held at a higher force in capillary pores. Adsorbed water comprises i) hygroscopic and ii) film water. Adsorbed water is either supplied or due to the hygroscopic action of the mineral it keeps water in it. This adsorbed water cannot be removed by oven drying at 105 to 110 0C and may therefore be considered to be the part of the solid soil grain. That is the reason why, while determining water content the oven temperature is maintained around 105 to 110 0C so that the structure of the soil is not disturbed or distorted. In view of that if the soil is not having any organic portion temperature is maintained around 105 to 110 0C, and if the soil has got an organic portion then the temperature of the oven is required to maintain at 60 0C to determine the water content. In order to remove adsorbed water the soil is required to be heated more than 200 0C. Accordingly, a sandy soil is dried more quickly and efficiently than a clay soil since the water in a sandy soil is held by forces smaller than the latter. The following discussions related to the issue may also be seen:

https://www.researchgate.net/post/Can_oven_dried_soil_sample_at_70_degree_Celsius_for_72_hours_be_used_for_determining_soil_texture/1

https://www.researchgate.net/post/Whats_the_drying_temperature_when_drying_specimens_in_an_oven_to_obtain_the_dry_mass_for_water_content_calculation/1

it is done between the temperature range of 105 - 110 degree centigrade.

Thank you all for the great explanations.