48 THE FERTILITY OF THE SOIL [CH. 



than three or four grain crops should be taken; 

 during the remaining time the land grows grass or 

 leguminous crops, so that it may gain organic matter. 

 It is not unremunerative during this period; on the 

 contrary, these crops are distinctly valuable. Much 

 the best for the purpose are clover, either alone or 

 mixed with timothy, and lucerne ; these collect gase- 

 ous nitrogen as well as carbon dioxide to form 

 nitrogenous organic matter in the soil. It is an 

 indispensable part of the method that limestone 

 should previously be added to the soil, or the clover 

 or lucerne may fail. When the soil has been thus far 

 improved the supply of phosphates present may be 

 insufficient for the crop that can be produced, and 

 this limiting factor has therefore to be removed by 

 the addition of rock phosphates. The crop now 

 increases till it is limited by some new factor. It 

 may happen that the supply of potassium salts in 

 the soil constitutes this new limiting factor, in which 

 case addition of potassic fertilisers becomes desirable. 

 The fertility may then rise higher than it was at first 

 in the virgin soil. 



The whole process, it will be observed, consists in 

 the successive removal of the limiting factors. 



The exhaustion of the virgin lands constitutes the 

 simplest case because everything is removed from t*he 

 soil and nothing is put back. It could only arise under 

 conditions of cheap transport facilities between the 



