274 CAUSES OF FERTILITY AND STERILITY [chap. 



and 0-107 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. Even more famous examples 

 are the silt soils of Boston and Wisbech on the seaward 

 side of the Fens, or the famous red soils of Dunbar. 

 Both are light and easy working and show no great 

 richness from the chemical point of view except as a 

 result of continuous and heavy manuring. But they 

 are happily tempered and of the right texture for the 

 free movement of water in the soil, with the result that 

 they respond to good cultivation and carry some of the 

 best crops in the country. 



