THE SOIL IN RELATION TO PLANT GROWTH 149 



nutrient is absent from these soils, or whether the adjustment of the 

 air and water supply is wrong, is not known ; but the limitation of 

 yield arising from this unsuitability of soil conditions is one of the most 

 serious problems of our time. Another instance may be given : in 

 Romney Marsh pastures commonly occur carrying a vegetation of rye 

 grass and white clover, with crested dog's-tail and agrostis, easily 

 capable of fattening sheep in summer without any other food. All 

 round these pastures are others, with the same type of vegetation, but 

 the plants grow more slowly, produce more stem and less leaf, are less 

 nutritious and incapable of fattening sheep. The soils are identical in 

 mechanical analysis and in general water and temperature relation- 

 ships, although certain differences have been detected (123^). Again: 

 grass grown on Lower Lias pastures in Somersetshire and Warwick- 

 shire causes acute diarrhoea (" scouring ") in cattle, whilst grass on 

 adjoining alluvial pastures does not (105). Lastly : potatoes grown in 

 the Dunbar district are remarkable for their quality, they will stand 

 boiling and sebsequent 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 differences 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 



