At first glance i thought it was mainly the nitrogen thing, but after reading this part  " When walking in them today (rained 3/4 inch last night like it seems to every day) the new field was smeary and hard, the old field didn't stick to my boots at all." there is something else going on also. I am thinking that the old field has a little more organic matter and water holding capacity and infiltration rate, in other words better soil health.
 Dig up some roots and see if there is any difference between the two fields and examine any difference in soil moisture.