Dan O.
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Beaverdamva is correct about changing all but the smallest plots. Organic matter (hay, manure or just a crop of clover/alfalfa/sweet clover/buckwheat is better).
I was thinking of mentioning that I plant buckwheat in the fallow areas of my garden to boost organic matter with very good results. It is easy to grow. The seed is cheap. It grows very quickly and is easy to shred up and turn in due to its hollow stems. Also the tap root on each plant grows deeply so it can tap nutrients that have leeched beyond the reach of other crops. So when you turn it in those nutrients are available again.
At least that's what I have read.
Also deer will eat buckwheat. The only thing is if you are going to grow anything else you need to turn the buckwheat in before it sets seed or it could become like all the other weeds in need of control.