double digging

noun
  1. British a method of digging ground in a series of trenches two spits deep, mixing the soil of the bottom spit with manure, and then transferring the soil from the top spit of one trench to the top spit of the preceding one
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for double digging

Historical Examples of double digging