52 THE PHYSIOLOGY OF PLANTS 



§ 8. Why and how should we replace the chief nutritive 

 substance in the soil? 



(a) Falluu: 



The harvests which are taken from the ground in cultivat- 

 ing the soil consist either of fruits, seeds, or inflorescences, 

 in which case the remainder of the plant is returned to the 

 soil, or else the whole plant may be withdrawn from the soil 

 for economic purposes. No soil of ours can endure for any 

 length of time the loss of mineral substances, which form part 

 of the crop, without being replenished, for no soil contains 

 sufficient substances to cover the loss. The soil will therefore 

 become impoverished more or less rapidly, according to the 

 nature and requirements of the various crops. A certain 

 number of substances, such as hydrogen, oxygen, and carbon, 

 are replaced by the natural wetting and aeration of the soil. 

 Of the mineral substances, lime and iron can be renewed for 

 the new crops by the natural processes of decomposition taking 

 place in the soil ; but potassium, phosphoric acid, and nitrogen, 

 which are taken up in large quantities from the soil into the 

 younger organs (as in vegetables), or into the ripened storage 

 tissues (fruits and seeds), must be replaced in some way or other. 

 In former times, when agricultural methods were less exact- 

 ing and exhausting, the method employed to restore the pro- 

 ductive activity of exhausted soil was to let the soil lie fallow, 

 the natural means which the soil possesses to recoup its losses 

 by rest. Under the same climatic conditions, fallow land is 

 moister and warmer than cultivated soil ; but a greater moisture 

 and higher temperature increase and accelerate the processes of 

 decomposition which are going on in the soil, which is indicated 

 by the fact that fallow land is richer in carbonic acid than land 

 in cultivation. Now the more rapid decomposition of organic 

 substances on the one hand, and on the other hand the 

 greater amount of carbonic acid thus formed, which causes a 

 more rapid solution of the mineral constituents of the soil, 

 increase the amount of soluble substances by which the next 

 crop will be benefited. As fallow land has also a smaller 



