Soils which have been saturated with water for a prolonged period of time may show an increased soil test pH value. The main reason for the change is the denitrification of soil nitrate to nitrogen gas which occurs under anaerobic conditons. For each atom of nitrate nitrogen which is converted to nitrogen gas, 6 atoms of acidic hydrogen are neutralized by forming water molecules as part of the bio-chemical reaction. The amount of pH change may be an increase of several tenths of a pH unit. Since this change in soil pH is due to a non-reversible bio-chemical reaction, we suggest that farmers follow the soil test lime recommendations from samples collected from these flooded soils and make no adjustments to the recommended lime application ra te.
Manganese will become more soluble under flooded conditions but will return to its normal level of availability as the soil dries out for Spring planting. Likewise, the manganese in the soil sample will revert to its normal level of availability as th e sample is dried and analyzed by the Agricultural Service Laboratory and will reflect the amount of available manganese in the field.
A portion of some nutrients such as potassium and magnesium may have been leached out of the plow layer by excessive rainfall. If there is no appreciable hard pan and the subsoil is within 15 to 20 inches of the surface, the crop will still have acces s to these nutrients later in the growing season. If leaching is suspected, it would be worthwhile to obtain a soil analysis from a subsoil sample. Samples should be collected from the top 4 inches of subsoil clay. Several cores should be collected with in a field and combined to make one composite sample. The test results can be used to reduce the recommended rates of potassium and magnesium if high levels are found in the subsoil.