60 BOTANY. [CHAP. n. 



of certain gases in solution, consequently if the same crop 

 was grown for several years in succession on the sauie 

 portion of ground, and the produce removed, as is usual 

 in the case of cultivated crops, the supply of special food 

 required by the particular plant would be exhausted ; 

 whereas, other substances that have been slowly dis- 

 solved, but of no value to the particular plant grown, 

 would be wasted, the principle of rotation of crops over- 

 comes this difficulty, and supplies each particular kind 

 of plant sown with its own special kind of food. As 

 an illustration of the above : 



Wheat, barley, and oats always contain a considerable 

 amount of silica or flint in their ash, and this substance 

 may be considered as the predominant inorganic food 

 required by the above plants; peas, beans, and clover 

 require lime, whereas potatoes and turnips require potash 

 as their speciality. Now if any one of the above crops 

 was sown for many years in succession on the same land, 

 and the entire crop removed, it can be readily under- 

 stood that the special food required would become scarce, 

 whereas if wheat is sown one season and turnips the next, 

 the latter crop not requiring silica, this substance would 

 slowly dissolve and accumulate, and thus be present in 

 sufficient quantity when the turn for sowing wheat came 

 round. 



In some virgin soils, rich in the various food con- 

 stituents required by special plants, the same crop may 

 be grown many years in succession, but eventually 

 exhaustion takes place and the crop becomes deficient. 

 The fact of forests and tracts of heath, etc., occupying 

 the same position for centuries appears to contradict the 

 above statement respecting the rotation of crops, but it 



