The Science of Building Soil is contained in the PTE's. You can build OM no matter if it is continuous broadleaf or grass plants ie. soybeans; wheat or corn. Yes OM can be increased with continuous soybeans! ... Cereal rye cover crop allowed to head out before terminating growth does result in OM buildup about as fast as continuous corn (C:N Ratios... rye straw 75:1 corn stalks 40:1 rye has higher lignin content... slower to reduce to OM and lasts longer... more stable.)
Soil Building Rules:
Rule 1: Do not destroy OM with any tillage... direct plant crop by only minimal soil disturbance to open slot, insert seed and cover up trench. "Stubble is for the soil... grain is for the farmer." Crovetto
Rule 2: Feed the soil to feed the crop. C HOPKNS CaFe MgB Mn CuZn MoCl for a start. (These applied nutrients also help the "soil biology" (AM fungi, bacteria... ie. actinomycetes, etc. to thrive; decompose and store carbon from previous crop residue.) If you know the nutrition of mushrooms you've got the answer. Clay must be saturated with calcium at higher levels than most know and/or recognize or understand in order for other cation nutrients to exchange from the clay lattice structure into the soil solution through AMF then into plants.

http://wvlc.uwaterloo.ca/biology447/modules/module8/soil/chap2c.htm

Adding manure is good and a quick answer, but is sometimes difficult to achieve.