I've been getting some calls as of late concerning the correct soil moisture for growers to have prior to fumigation with chloropicrin, now that fumigation with methyl bromide is no longer an option. Anyway, I checked with a colleague at Trical, who stated that the moisture requirement has definitely not changed, methyl bromide or chloropicrin, and a requirement of 50% or greater available water content in the soil has been in place for a long time.
One of the labels for chloropicrin attached below with specific instructions on soil moisture, see pages 11 & 12.
http://www.cdpr.ca.gov/docs/emon/fumigants/labels/triclor_trical.pdf
Soil moisture prep for fumigation is the same as it always was, end of story.
Also found a pretty nifty pictorial guide put out by the NRCS on estimating soil moisture, including the critical 50% or greater available water content, for different soil types.
Link here:
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_051845.pdf