100 LESSONS IN CHEMISTRY. 



2d. That different plants, as has akeady been stated (55), 

 though requiring all the mineral substances described (42), 

 invariably select different proportions of particular kinds of 

 matter for their nourishment, the root crops, for example, 

 requiring large supplies of the alkalies, and the clover crops 

 of lime, &c. 



3d. That not only do our field crops exhibit a partiality 

 for certain matters of the soil, but that different parts of the 

 same plant select dissimilar quantities of these substances \ 

 thus the table shows us that while the straw of wheat yields 

 an ash containing so much as 65 parts in the hundred of 

 silica, the grain contains but a trifling amount of that 

 substance ; and that while the ash of the straw affords only 

 3 per cent of phosphoric acid, the inorganic matter of the 

 grain yields 49 per cent. If you glance at the composition 

 of the potato tuber, you will find that it is distinguished by 

 containing a very large proportion of the alkalme substance 

 potash, and a small amount of lime, while for the development 

 of the tops, a considerable quantity of lime is required. The 

 discovery of the unlike proportions in which the ingredients of 

 the soil enter into the structure of the different parts of plants 

 leads to important conclusions, and is capable of affording to 

 the practical man a rational explanation of many things 

 which appeared unaccountable to him in the growth of his 

 crops. It teaches him, that as the different parts of the 

 plant requu'e the materials of the soil in different proportions, 

 a crop may at one period of its growth thrive vigorously, and 

 at another droop away; that a crop of wheat, or any other 

 grain, may sprmg up with luxuriant growth, and promise a 

 rich return, yet form but a poor and starved-looking seed, 

 and again, how a plump and healthy head may be produced, 

 while the straw thrown up is weak and stunted. It enables 

 him also to understand how one soil, or one district of 

 country, may refuse to yield profitable returns of certain crops, 

 and yet afford ample food for plants of a different race. 



143. Modes adopted for improving the soil and maintaining 

 its fertility. There are three methods which have been adopted 

 for this purpose, viz : Fallowing^ the alternation of crops, and 

 the application of manures. The first of these, termed Fal- 

 lowing* consists in ploughing up the land into ridges at 



* The term fallow is supposed to be derived from a Latin word signify- 

 jng yellow, the land when in fallow presenting a j^ellow appearance, 



