CHAPTER VIII 



ROTATION OF CROPS 



Active and 

 dormant 

 constituents 

 of soils. 



Fertility 

 sometimes 

 restored to 

 soils by 

 fallowing. 



Different 

 plants re- 

 quire food 

 in different 

 proportions. 



It has been shown in the preceding pages that only a 

 small portion of the soil can be dissolved out in water, and 

 that this soluble portion is the only available one for plant 

 food. Other and much larger portions are, however, in a 

 dormant condition, and are, therefore, after a time, by the 

 action of water and air, capable of being eaten up by the 

 plant. But, when successive crops from one kind of plant 

 are taken off the soil, the alteration of the dormant consti- 

 tuents cannot keep pace with the drain caused by the 

 growing plants — which then become weak, sickly and un- 

 productive. By resting the land or allowing it to fallow for 

 a time, some of the dormant constituents become active, and 

 fertility is restored ; but, in certain cases where not much 

 land is available, the loss of a year's crop is a very serious 

 matter, and means have been devised to overcome the 

 difficulty. One of these means is the application of manure. 

 But then manure costs money, and it may not always be 

 possible to obtain it. So the second system, called rotation 

 of crops^ is employed with success in many cultivations. 



The ashes of all plants are not the same in composition ; 

 and, therefore, plant food is not always required in the same 

 proportions. Some plants, as shown by their ashes, require 

 a large proportion of potash and phosphoric acid ; others 

 consume much siHca and require less potash and phosphoric 



