Wood ashes are relatively high in potassium. If used in low or moderate rates, they can be good sources of this nutrient. Wood ashes are also high in several other salt products. For this reason, high application rates should be avoided as salt problems may occur. Bull E-2567, for recommended rates of potassium application for soybeans.
Soybeans seem to be relatively insensitive to the time of year of potassium application.
Research has shown that a direct application of potash to soybeans has a slight advantage over applying it the previous year to corn.