I have recently returned to gardening after a long hiatus. The plot I'm
using now is too small to justify my buying a tiller, so I'm turning it
over with a shovel and breaking it up with a hand cultivator.

My question relates to what to do with the weeds and grass turf that was
there when it was merely part of the back yard. I would assume that most
any plant is taking nutrients from the soil and concentrating them in its
tissues. I would also assume that if I removed the soil from these plants
and tossed them away, I would be losing a certain amount of nutrients that
had been in the soil.

I am tempted, during this first year, simply to turn over a deep
shovel-full of earth, break up the soil that was underneath the turf, and
leave the grass and weeds that were formerly face-up to the sun to rot
under the overturned earth. This way the soil would retain whatever
nutrients they possessed.

At least, that's the theory. Can anybody tell me if this is actually a
good idea? Or should I go ahead and shake all the soil out of the turf
layer and just get rid of the plant part? I have most of my garden
planted and already doing well, but I do have a bit more earth to break up
for some later crops.