IV] THE LOWER LIMIT 47 



be the history of many farms elsewhere so long as 

 continuous wheat culture is adopted. It is futile to 

 speak of land as inexhaustible: fertility is no more 

 inexhaustible than any other form of capital. The 

 pitiful thing is that so much of the loss is sheer 

 waste: about one third of the plant food goes into 

 the crop, the rest is lost beyond hope of recall as 

 gas into the atmosphere or as saline matter in the 

 drainage water and the streams. It is not the crop 

 that exhausts the land but the continuous cultivation. 



Fortunately recovery is by no means impossible, 

 though it may be prolonged. It is only necessary to 

 leave the soil covered with vegetation for a period of 

 years when it will once more regain much of the 

 nitrogenous organic matter it has lost. But it does 

 not wholly recover. The phosphates and potassium 

 salts removed in the crops, and the calcium carbonate 

 leached out, are not regained ; for want of them the 

 gTowth of recuperative vegetation may suffer. 



The problem has been investigated with charac- 

 teristic energy in the United States, and a remedial 

 scheme has been evolved by Dr Cyril Hopkins, 

 Director Thorne and others, based on experience 

 in the older countries and on careful experiments 

 in the new. The central feature is that continuous 

 tillage must stop, and for one third to one half of its 

 time the land must lie untilled and covered with 

 vegetation, i.e. in the course of six years not more 



