in.] DEMANDS UPON THE SOIL. 25 



removed from the land. It would be difficult to 

 remember all these figures, but they represent certain 

 general facts which should be remembered. 



43. We see that different parts of the same 

 plant contain very different quantities and 

 varieties of inorganic matter : for instance, the silica 

 in the corn of wheat is about i Ib. for each acre grown, 

 whilst in the straw there is 100 Ibs. and when you 

 examine the straw of wheat you see the bright glassy 

 coating which requires this silica. You also see that 

 beans only require about 12 Ibs. of silica per acre, 

 whilst wheat required 102 Ibs., and one lesson this 

 teaches is that different crops require different 

 kinds of food. If you notice the requirements of 

 the turnip crop, you will see that an acre of turnips 

 requires about 200 Ibs. of potash, and nearly 40 Ibs. 

 of soda, whilst a crop of wheat only requires about 

 26 Ibs. of potash, and scarcely 2 Ibs. of soda. You 

 must not lose sight of the fact that as different 

 crops require different kinds of food, they therefore 

 draw from the land different kinds of in- 

 organic matter. By the removal of our various 

 crops from the land, we remove in them large quan- 

 tities of those inorganic matters which are necessary 

 for keeping the land fertile, and one of the great 

 objects to be accomplished in successful farming is to 

 be able to do this, and at the same time make the 

 land more productive every year. It is, however, 

 quite possible for soils to be rendered unpro- 

 ductive by the plant-food they contain being removed, 

 and the land thereby becoming exhausted. 



44. Some soils are unable to grow crops by reason 

 of their having some injurious matter present, such as 

 some of the lower compounds of iron, salt, and acrid 

 organic matter, all of which prevent healthy vegetable 

 growth. 



45. Other soils are unproductive because their 

 mechanical condition is unfavourable for vege- 



