THE SOIL IN RELATION TO PLANT GROWTH 129 



pastures does not (105). Lastly: potatoes grown in the Dunbar dis- 

 trict are remarkable for their quality, they will stand boiling and 

 subsequent warming-up without going black. The same varieties of 

 potatoes grown in the same way in the Fens blacken badly under the 

 same treatment, and consequently command a much lower price in the 

 market (8). Instances might be multiplied ; enough have been given 

 to show that the plant responds in a remarkable degree to variations in 

 soil conditions. Our knowledge of these variations is fragmentary and 

 wholly empirical, and would be much furthered by close and detailed 

 study, jointly by a botanist and a chemist, of the factors causing dif- 

 ferences in plant associations in two nearly similar habitats. 



The agricultural treatment of loams, as already indicated, admits of 

 considerable variety. The old plan was to apply a good dressing of 

 dung every third or fourth year and a smaller intermediate dressing ; 

 clover was also grown every fourth year, and, on light loams, the 

 root crop was eaten by animals on the land. At long intervals lime 

 was applied and sometimes bones. The modern movement is towards 

 specialisation, each man producing the crops he can best grow and 

 managing them in the way he finds most profitable, but the system 

 usually involves feeding a good deal of imported food to sheep and 

 cattle, either on the land or in yards, and utilising the excretions as 

 manure, buying nitrate of soda, sulphate of ammonia, and manufac- 

 turers' waste products (generally those derived from imported animal 

 or vegetable products) to supply more nitrogen, and buying also im- 

 ported phosphates and potassium salts. Thus the fertility of highly- 

 farmed countries like England tends to increase at the expense of new 

 countries that export large amounts of animal and vegetable produce. 

 But the transfer is prodigiously wasteful ; enormous losses arise in vir- 

 gin countries through continuous cultivation (p. 81), and at this end 

 in making dung (p. 89), and especially through our methods of sewage 

 disposal. It seems inevitable that these losses must make themselves 

 felt some day, unless the movement for the conservation of natural re- 

 sources ever becomes a potent factor in international life. 



Soil Fertility and Soil Exhaustion. 



From the preceding paragraphs it is clear that fertility is not an 

 absolute property of soils, but has meaning only in relation to particular 

 plants. Plant requirements vary ; a soil may be fertile for one plant 

 and not for another ; every soil might conceivably prove fertile for some- 

 thing. But in practice the agriculturist can only find use for a very 

 limited number of plants ; he, therefore, has to select those combining 



