Well this was the principal of the old "French gardening" and later the
"Hot
bed" you took out around 6 to 8 inches of soil then built a heap of about
2

I did this the first year I had my current greenhouse. In the winter, I got
in tons of manure and stacked it around the insides of the greenhouse so
that it was about 2 foot high. It then generated lots of nice heat. Early
spring, I planted it with tomatoes, cucs, melons etc and got an excellent
crop. However, it soon got re-absorbed into the soil.

Best,

- h