Normally from plant nutrition point of view we need determination of plant nutrients at field moisture content. So we carry out incubation studies at field capacity of soil or 60 percent of water holding capacity. If we analyze directly the field moist samples we have to determine the moisture content in the sample. While sampling and processing one has to maintain the same moisture content. Also as per extraction procedure one has extract  nutrients at certain soil  extractant ratio.If it is dry soil we can easily add appropriate amount of reagent to obtain right ratio .Air drying may not alter the nutrient content in soil especially cationic nutrients. While drying biological activity may get suppressed and when sample is wetted it may again increase. So drying may affect the nutrient mineralization especially incase of N,P,S.Also when soil solution ratio is wide  there is solubilization and exchange of nutrients between exchange phase and soil solution.Dilution may bring more nutrients into solution phase.

Normally from plant nutrition point of view we need determination of plant nutrients at field moisture content. So we carry out incubation studies at field capacity of soil or 60 percent of water holding capacity. If we analyze directly the field moist samples we have to determine the moisture content in the sample. While sampling and processing one has to maintain the same moisture content. Also as per extraction procedure one has extract  nutrients at certain soil  extractant ratio.If it is dry soil we can easily add appropriate amount of reagent to obtain right ratio .Air drying may not alter the nutrient content in soil especially cationic nutrients. While drying biological activity may get suppressed and when sample is wetted it may again increase. So drying may affect the nutrient mineralization especially incase of N,P,S.Also when soil solution ratio is wide  there is solubilization and exchange of nutrients between exchange phase and soil solution.Dilution may bring more nutrients into solution phase.

It seems that the moisture of the soil sample may affect extraction agent and can reduce its effectiveness, if there is soil hydrated. Namely EDTA and DTPA may behave very hydrophobic due their carboxyl group and it complain of metal binding  to their carboxyl groups from water. Determination of metals directly in EDTA and DTPA leaching can be problematic.

For determining the metal or cation, in any procedure there are a definite soil solution ratio. If the soil content high moisture, it will affect the soil solution ratio or to maintain a particular soil solution ratio, we should determine the moisture content in soil and adjust the extracting agent accordingly. However, the extraction of metal or cation by the extracting  agent is the reaction between solid and solution phase, definitely governed by the equilibrium constant. By changing the moisture content or soil solution ratio, it may affect the constant.

PFA

http://www.sciencedirect.com/science/article/pii/001670619190099F

https://books.google.co.in/books?hl=en&lr=&id=LTDOBgAAQBAJ&oi=fnd&pg=PA237&dq=Effect+of+soil+moisture+content+on+DTPA&ots=NKuP0e0upO&sig=2AEbuptYxTtvouNlfaf4j8xPezE#v=onepage&q=Effect%20of%20soil%20moisture%20content%20on%20DTPA&f=false

As all lab estimations are to be made on dry soil basis, therefore soil moisture does not effect the determination of DTPA or EDTA.

Determination of soil test values of dry soil for micronutrients using DTPA or EDTA as extractant is different from  sampling field capacity moist samples and determination of micro nutrients using same extractants. In the case of the latter the presence of excess moisture shall alter the soil  extractant ratio and the more  the dilution ,the more extraction of micronutrient s.So the problem is how to maintain soil  extractant ratio in field moist sample  collected from incubation study.