Hugelkulter
 
As the logs rot, they slowly release  nutrients back into the soil. They also act like a sponge to hold extra moisture. A neighbor of mine has some hugel beds that have transformed her garden soil.
The first year, you might need to use extra nitrogen, as the logs will absorb (if that's the right word) it from the surrounding soil. Think of a "nurse log" out in the forest. Except it's under the soil.
I buried a lot of logs in my gardens last year. One is very sandy soil, the other heavy clay. I'll report on how it goes.