ROTATION OF CROPS 209 



yet be the intention to sow any seed. In these cir- 

 cumstances the land is called fallow land. 



"There is, however, one way to obtain an uninter- 

 rupted succession of crops from the same land un- 

 less the soil is very poor. All plants derive their 

 nourishment from the soil and the atmosphere; 

 but some take more from the atmosphere, others 

 from the soil. The plants that get their sustenance 

 chiefly from the air are those that have luxuriant 

 foliage. The potato is one of these. You know 

 that it is through their leaves that plants obtain the 

 carbonic acid gas diffused in the air. The greater 

 the spread of foliage, the more abundant will this 

 absorption be. The plants that depend almost 

 wholly on the soil are those with only a few small, 

 slender leaves, thus taking but little carbonic acid 

 gas from the air. Such is wheat. 



"Moreover, from the potato plant we take only 

 the tubers, which form but a small part of the whole, 

 and we turn under the stalks and leaves, which are 

 thereupon converted into humus. Thus the potato 

 has the property of enriching the soil at the expense 

 of the atmosphere, and it gives back more than it 

 takes. It is, then, one of the enriching rather than 

 impoverishing plants in respect to its action on the 

 soil. Cereals, on the contrary, are utilized by the 

 harvester both as to seed and haulm, nothing but 

 the meager roots being left in the ground ; and as, on 

 account of their very scanty foliage, cereals derive 

 almost their entire sustenance from the soil, they 

 take from it much more than they give back to it. 



