Mine were also on sod ground to beans.  We planted them a little on the wet side and theory is compaction in the root zone not allowing the roots to get to the nuetrients.  Theory roight or wrong, I don't know.  Better believe these fields will get K before the upcoming season, probably chicken manure.  Beans in field right beside, same variety, same planting date, with chicken manure and  planted into bean stubble look good.