THE LAWS OF PLANT GROWTH. 97 



27-} per cent. The dried tobacco plant has 24 per cent, of 

 ash while the whole wheat plant has but 3 per cent. It 

 must not be supposed that these peculiarities are of no im- 

 portance to the farmer, and that the fact that the ash of 

 beets, turnips, and carrots, including leaves and roots to- 

 gether, contains from 12 to 24? per cent, of soda, and from 

 6 to 11 per cent, of chlorine; while that of most other 

 plants contain a very insignificant quantity of these sub- 

 stances; or that the ash of clover contains along with the 

 large quantity of lime a considerable amount of sulphuric 

 acid, and that this acid exists in the ash of turnips, cabbage, 

 rape and kohl-rabi, mustard and other plants of the Oru- 

 cifercn family to the enormous extent of from 8 to 16} per 

 cent. For these facts explain the reason why an applica- 

 tion of salt (chloride of sodium) and of gypsum (sulphate 

 of lime) furnishes these elements to the crops mentioned, and 

 thus supplies necessary food without which they could not 

 grow. It results, in fact, that the soil must contain all these 

 substances, which are found in their ashes, in such quantity 

 and in such form as to yield easily to each crop as much of 

 each, as the plant specially requires. This is the first grand 

 law which controls the culture of farm crops. The second 

 is that the soil must be brought into such a proper condition 

 by tillage, as to enable the roots of plants to avail themselves of 

 the needed food which it contains. 



A special study should be made of the tables given in the 

 next chapter and specially placed by themselves that they 

 may attract the notice which they demand. For a third 

 law controlling the growth of plants is, that if one of these 

 necessary substances is wanting in the soil, or is existing there 

 in deficient quantity, the crop will prove a failure; it will 

 either be weak and diseased (for it is the weak and ill nour- 

 ished plants and animals equally which are subject to 

 disease) or it will fail to grow at all. 



The intelligent farmer will then naturally ask what are 

 these mineral or inorganic substances upon which plants 

 depend for their successful growth, and in what proportion 

 do they require them; and further, in what proportions do 



