Sentence:  Soil of the usual depth may be trenched two <WSD>spit</WSD> (spadeful) deep; and if this is done every third year, it is evident that the surface which has produced three crops will rest for the next three years; thus giving a much better chance of constantly producing healthy and luxuriant crops, and with one half the manure that would otherwise be requisite. 
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.