318 SOIL CONDITIONS AND PLANT GROWTH 



Some of the South African grazing lands tend to give the 

 animals a serious disease, Lamziekte, which is attributed to a 

 harmful quality in the herbage and not to any particular 

 plant or organism. 1 Lastly : potatoes grown in the Dunbar 

 district 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 (jd). 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 frag- 

 mentary 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 some- 

 times bones. The modern movement is towards specialisation, 

 each man producing the crops he can best grow and manag- 

 ing 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 manufacturers' waste products (generally those 

 derived from imported animal or vegetable products) to 

 supply more nitrogen, and buying also imported 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 ; 



a See Ingle, Journ. Agric. Sci., 1908, 3, 22-31; A. Theiler, also H. H. 

 Green, $th and 6th Veterinary Reports, Union of South Africa, 1918. 



