green manure crops do build humus. if you have all this spent straw and sawdust then you could use that as your mulch around the trees, far better than tillage. it has been proven that with proper managment pastures can build an inch of soil a year, that would be very hard to beat.
these three guys in these videos are amazing innovators and are leading the way in soil building and soil health, the concepts they bring up are mind blowing and far beyond anything bill mollison wrote on soil building.
carbon is far more important than nitrogen in healthy soil.
the first video is colin seis who invented pasture cropping, this is the direction i would go in to change your soil and prepare it for the forest to come.
remember if you use animals in a food forest you can virtually eliminate the need for nitrogen fixers.