On old guy that we custom farmed for, once told us that the soil needed to rest.  Did not give a reason why it needed to rest, only thing I can see gained by resting the soil is giving it time to build up sub soil moisture.  Here most years you can grow a summer crop, plant rye in fall, kill the rye 1st of May and have full subsoil moisture by June 15th. 

I have the opposite opinion as that old guy did, and believe resting the soil is bad for it.

The soil that the river leaves behind after a flood is some where between 70 and 120 ppm of P and 260 to 400 ppm K. So catching as much of that soil as possible is more important then ever applying P and K.