EXHAUSTION OF SOILS. 1*5 



gether. Let us assume that a certain soil contains a quantity of 

 silicates and of phosphates sufficient for 1000 crops of wheat, then, 

 after 1000 years, it must become sterile for this plant. If we 

 were to remove the surface-soil and bring up the subsoil to the 

 surface, making what was formerly surface-soil now the subsoil, 

 we would procure a surface much less exhausted than the for- 

 mer, and this might suffice to supply a new series of crops, but 

 its state of fertility would also have a limit. 



A soil will naturally reach its point of exhaustion sooner the 

 less rich it is in the mineral ingredients necessary as food for 

 plants. But it is obvious that we can restore the soil to its 

 original state of fertility, by bringing it back to its former com- 

 position ; that is, by returning to it the constituents removed by 

 the various crops of plants. 



Two plants may be cultivated side by side, or successively 

 when they require unequal quantities of the same constituents, 

 ;it different times; they will grow luxuriantly without mutual in- 

 jury, if they require for their development biffeufxt ingredients 

 of the soil. 



The experiments of Saussure, and of many other philosophers, 

 have shown that the seeds of beans, of Phaseolus vulgaris, of peas, 

 and of garden cresses, germinate and even grow to a certain ex- 

 tent in moist sand or moistened horse-hair ; but when the mine- 

 ral substances contained in the seeds no longer suffice for their 

 further growth, then the plants begin to droop ; they may even 

 perhaps blossom, but they never bear seeds. 



Wiegmann and Polstorf allowed plants of various kinds to 

 vegetate in a white sand previously treated with aqua regia, and 

 freed from the acid by careful washing,* Barley and oats grow- 

 ing in this sand, on being properly treated with water free from 

 ammonia, reached a height of H foot; they blossomed, but did 



*This sand contained in 1000 parts: — (Preisschiukt, p. 96. ,i 



