1248. When the particular earths that a plant requires have been exhausted in any soil by a crop of that particular plant having been grown in it, it is evident that another crop of the same plant cannot be grown in the same soil till it has recovered a sufficient quantity of that substance which had become exhausted; but it is equally evident that another crop requiring a different substance may be grown in the same soil the following year. Thus plants that require potash, such as the beet, the mangold wurzel, and the turnip, may succeed plants that require lime, such as beans, peas, &c.; and thus the same result is obtained as was proposed by the former hypothesis, with this difference, that the real cause why a rotation of crops is advisable being now known, the necessity for it may be avoided by supplying the soil after each crop with the mineral substances which had become exhausted, and thus the same crop may be grown on the same soil for twenty years in succession.
