102 LESSONS IN CHEMISTRY. 



rendered by their loss incapable of supporting crops requiring 

 a large supply of these matters, such as turnips and clover, 

 and yet be capable of affording sufficient nourishment to 

 plants which are found to select chiefly materials of a differ- 

 ent kind. Experience taught that whilst crop after crop 

 of the same plant materially exhausted the soil, the injury 

 produced by changing the crops grown was not so great; 

 and even before chemistry had enabled us to understand the 

 effects produced by the growth of plants, farmers in many 

 advanced districts, were induced to put a limit to the number 

 of crops of the same kind grown in succession. 



145. In considering the effects which the different kinds of 

 plants exert upon the soil, it is necessary to recollect what has 

 been stated, that not only do the different plants of the farm 

 give a preference to particular kinds of food, but that the 

 different parts of the same plant require different proportions 

 . of these materials for their growth. Such being the case, it is 

 obvious that the exhausting effect which the production of any 

 crop exerts upon the soil, will be influenced in a great degree 

 by the purpose for which it is cultivated. Thus, plants like 

 wheat, oats, barley, beans, peas, and flax, cultivated for their 

 seeds, which are collected and used for food or sent to market, 

 require a large supply of mineral materials and especially of 

 phosphoric acid, a substance which you are aware is con- 

 tained in very small amount in even the most fertile soils, 

 and must therefore produce a different effect from the crops, 

 such as potatoes, tm*nips, mangel-wurtzel, and clover, which 

 we grow for the sake of their roots or fohage, or of flax when 

 pulled before the seeds come to maturity. The study of the 

 food of plants, therefore, points out to us the propriety of 

 alternating with the wheat, and other crops which require for 

 the formation of their seeds a large amount of nitrogen and 

 phosphates, others, like the foliage and root crops, which do 

 not contain the same amount of these substances, and are 

 besides capable of condensing from the atmosphere a larger 

 amount of its materials. In many districts in Ireland the 

 farmers yet act with the same kind of thoughtlessness* that 

 distinguished the early settlers in North America ; they 

 scourge the soil with crop after crop of grain until it will 



* Improvements in practice are of slow progress. Even in the County 

 of Antrim, which can boast of so many intelligent farmers, I have been 

 sliown fields producing a seventh crop of oats. 



