Well said Nilay and other colleagues , we dont have to air dry the soil samples for soil moisture determination . ..no point drying under air first followed by oven drying ....

I use to directly oven dry it. Do we have to air-dry it first?

No, You need NOT air dry the sample.

https://nature.berkeley.edu/soilmicro/methods/Soil%20moisture%20content.pdf

In soil moisture determination usually moist soil samples can be directly put into the oven for drying. However, the initial moisture content, size of the sample and total sample load (number of sample dried in a batch) may affect the drying time. Accordingly, air drying the samples before oven drying may yield better results. The following links may be explored for few relevant articles:

http://iopscience.iop.org/article/10.1088/1755-1315/43/1/012029/pdf

https://www.agronomy.org/files/publications/nse/pdfs/jnr005/005-01-0025.pdf

Previous discussions in this forum on similar topic may also be seen:

https://www.researchgate.net/post/How_long_should_a_soil_sample_needed_to_be_dried_to_obtain_soil_moisture_measurement

Well said Nilay and other colleagues , we dont have to air dry the soil samples for soil moisture determination . ..no point drying under air first followed by oven drying ....

Rightly said, in the thermo-gravimetric method, the measurement of water content in soil sample is based on the loss in the weight of moist soil after drying, so there is no need of air drying. Soil sample is commonly dry in an oven at 105 degree Celsius for 24 hours to a constant weight. Heat applied at this temperature is effective in evaporating water held in soil pores and on soil surface, however organic matter is not lost from the soil. The water content in the soil is expressed in percent which is the loss in the weight of moist soil, while oven drying, and expressed in the percentage of oven dry mass of soil. Now  soil moisture content can also be assessed indirectly by several improved techniques based on the determination of electrical resistance, slowing down the fast neutrons, gamma-rays attenuation, electrical capacitance and back-scattering coefficient as a function of water content. However, thermo-gravimetric approach is more flexible, as samples can be readily taken from any situation.

Dear Scientists,

In order to determine the moisture content at different tensions, we need to air dry the soil, then wet it and apply different tensions in pressure plate apparatus.  Other-wise hysteresis behavior of wet soil will result in lower moisture content even at same tension. Say for example wet soil has 10 % moisture at 15 tension, when moisture is added to it upto 30% it may reach 1.0 tension.  Suppose a soil with field capacity (0.33 tension) starts drying and reaches 1.0 tension. Then if you determine moisture content at this tension on oven dry basis it will be more than the wet soil moisture.  So before determining moisture content it is necessary to air dry the soil.

Note : To determine the moisture content in the field at any point of time, it is not required  to air dry. Directly the sample should be brought to the lab and oven dry it. Then work out % moisture. It is  quite obvious to determine field moisture.

Thanks

To measure soil moisture by thermo-gravimetric approach is more flexible, as samples can be readily taken from any situation. All analysis in the laboratory is related to air or oven dry basis, and therefore must be considering the actual soil moisture content ( Hesse, 1971).

Fascinating response from Dr Rajakumar , my compliments to him . But , if we are to measure the soil moisture content of a given soil thermo-gravimetrically , i feel , we dont have to air dry the soil samples. But ,  if we  are trying to develop soil moisture curve by taking gravimetric moisture  vis-a-vis soil moisture tension , i think , in that case, Dr Rajakumar is dead right ...