Undried samples of Marshall subsoil that contained 27 ppm exchangeable (NH4OAc extractable) K were dried to release K and then treated in various ways to maximize K reversion. This was done to determine the possibility of reducing the exchangeable K in dried soil to the level of the undried soil and, thus, of using dry samples with released K to estimate the amount of exchangeable K that was present in the soil before drying.
Much of the desired K reversion was attained by simply rewetting the dry soil. The rate and amount of this decrease in exchangeable K was enhanced by heating the rewet soil, but even so, the minimum values attained with rewet samples of air-dry and oven-dry soil were 54 and 62 ppm K, respectively. For complete reversion, it was necessary to mix the rewet soil with organic compounds and then oven-dry the mixture.
A decrease in exchangeable K was attained by drying the rewet soil, provided organic additives were present to prevent a release of K. Thus, the effects observed with various organic additives varied with the degree to which the additives were retained in the sample while the water was removed. Additions of high-boiling-point, normal alcohols; rewetting with a minimum of water; and slow drying gave the best results. When 10-g samples of oven-dry soil with 210 ppm exchangeable K were rewet with 7 ml of water, mixed with 7 ml of octanol and dried at 110C for 48 hours in a gravity-draft oven, there was only 30 ppm exchangeable K present. Oven drying did not enhance K reversion when sucrose or glycerol was present.