Sentence:  Proceed as for the single dig but start by removing two <WSD>spits</WSD> of topsoil to the far diagonal corner and also one <WSD>spit</WSD> of subsoil. Turn the exposed subsoil from hole two into hole one. Incorporate organic matter. 
Meaning of the word marked with <WSD> tags:  The depth to which the blade of a spade goes into the soil when it is used for digging; a layer of soil of the depth of a spade's blade.