AGRICULTURE. 219 



soluble parts dissolved in water, and that water ad- 

 hering to the mass, and the whole mixed with larger 

 or smaller quantities of the remains of animals and 

 vegetables in different stages of decay. 



The Ingredients of Soil. 



Soils are extremely diversified in appearance and 

 quality, yet they all consist of different proportions of 

 the same elements. The substances that constitute 

 soils are certain compounds of the earths, silica, 

 lime, alumina, magnesia, and the oxides of iron and 

 manganese ; animal and vegetable matter in a state 

 of decomposition, and saline, acid, or alkaline com- 

 binations. 



Barren Soils. 



Pure silica or alumina, pure carbonate of lime, or 

 carbonate of magnesia, are incapable of supporting a 

 healthy vegetation, and no soil is fertile that contains 

 so much as 19 parts out of 20 of any single consti- 

 tuent. The soil of Bagshot heath, which is entirely 

 devoid of vegetable covering, contains less than -^th 

 of finely divided matter. 



Bulbous Roots. 



Plants that have bulbous roots require a looser 

 and lighter soil than such as have fibrous roots ; and 

 the plants possessing only short fibrous radicles de- 

 mand a firmer soil than such as have tap roots or 

 extensive lateral roots. A crop of turnips may be 

 raised in a soil containing 11 parts out of 12 of 

 sand, and some bulbous roots will thrive in a soil 

 containing 14 parts of 15 of sand ; but a much larger 

 proportion of sand will cause absolute sterility. 



In all cases the ashes of plants contain some of the 

 earth of the soil in which they were grown ; but it 

 never exceeds -fa of the weight of the plant con- 

 sumed. 



