Hello Dulce:

The answer depends. of course, an several soil properties, sieved or non-sieved,  and whether you have multiple replicates to work with.  A general approach might be to dry the 7 g sample for 3 or 4 days, remove and obtain a dry weight, and then freeze dry for another 24 hours to see if the weight is still changing. Repeat until you achieve constant weight.

-B

Hello David,

Thanks a lot for you answer, I was worried because we have soil ejection, we freeze dry for 24 hours, and I thought that was too long, but if you suggest the same amount of time I'm confident that the time was not the problem.

It seems that the problem was other (we initially try with 3g of sample, and the tubes were too small, so that was probably the issue), hopefully we don't have problems next time.

Thanks again.