236 CAUSES OF FERTILITY AND STERILITY [chap. 



and 0107 per cent, of nitrogen in the top 4 feet of 

 soil successively. Many of these deep rich soils appear 

 to be wind-borne : in all cases they are of very uniform 

 texture, and represent the accumulated residues of ages 

 of previous vegetation in a form that is capable of 

 decay and nitrification so as to become available for 

 subsequent crops. In a peat bog there is equally an 

 accumulation of organic matter and nitrogen, but the 

 mass is infertile because of the acid character of the 

 humus (which causes the absence of the valuable bacteria, 

 such as those fixing nitrogen and nitrifying ammonia), 

 the deficiency of mineral plant foods, and the bad 

 mechanical condition which affects the supply of air 

 and water. In the main, then, a fertile soil is one rich in 

 the debris of previous vegetation, one which has been so 

 sorted out by running water, wind, the agency of worms, 

 etc., as to possess a very uniform texture, adapted to 

 satisfy the needs of the plant for air and water. 

 Mechanical texture is of fundamental importance : in 

 this country many soils owe their value to this property 

 alone ; for example, the Thanet Sand formation in East 

 Kent (a very fine-grained sand or silt), though it con- 

 tains but little plant food, yet carries some of the best 

 fruit and hop plantations in the kingdom, and farms on 

 it command a high rent 



Condition. 



The question of condition has equally its chemical 

 and its mechanical side; it is well known that on 

 any but the lightest soils continued cultivation makes 

 the texture better and renders it easier to obtain a 

 seed bed. On clay soils the effects of bad manage- 

 ment are very persistent; any ploughing, rolling, or 

 trampling when the soil is wet will so temper the 



