116 COMPOSITION OF SOILS. 



Average of 5 years. 



(in the same plot 72-9 Ibs. tubers. 



Potatoes, -J in differem 1 ot8 92 . 8 - 



/same I 5 ' olbs - 



Flax {different * 



(same. 



Tfc 1 1 OtllllO.. ******** 



Barley, j different 46-5 



. . /same 104>0 



Turnips, | different 173-0 



~ t /same 28 ' 



Oat8 ' \different 32-4 



1 his shows a manifest advantage in shifting crops, varying from 1 to 

 75 per cent. ; the deficiency of inorganic matter being the chief cause 

 of difference. As this matter is shown to be of great importance to 

 plants, it follows that the composition of soil is a subject requiring 

 special notice. 



CHEMICAL COMPOSITON OF SOILS. 



233. Soils have been divided in the following way, according to the 

 proportion of clay, sand, and lime, which they possess : 



1. Argillaceous soils, possessing little or no calcareous matter, and above 



50 per cent, of clay. 



2. Loamy soils, containing from 20 to 50 per cent, of clay. 



3. Sandy soils, not more than 10 per cent, of clay. 



4. Marly soils, 5 to 20 per cent, of calcareous matter. 



5. Calcareous soils, more than 20 per cent, of carbonate of lime. 



6. Humus soils, in which vegetable mould abounds. 



Below the superficial soil there exists what is called subsoil, which 

 varies in its composition, and often differs much from that on the 

 surface. Into it the rain carries down various soluble inorganic mat- 

 ters, which, when brought to the surface by agricultural operations, as 

 trenching and subsoil ploughing, may materially promote the growth 

 of crops. 



234. Hamas, or decaying woody fibre, exists in soils to a certain 

 amount. This has been called also ulmine, or coal of humus. In a 

 soluble state it forms humic and ulmic acid. Humus absorbs ammo- 

 nia, and it is slowly acted upon by the atmosphere, so as to form car- 

 bonic acid by combination with oxygen. Peaty soils contain much of 

 this substance. When peroxide of iron is present in such soils, it 

 loses part of its oxygen, and is converted into the protoxide. 



235. Silica in greater or less quantity, is found in all soils ; but it 

 abounds in sandy soils. In its ordinary state it is insoluble, and it is 

 only when acted iipon by alkaline matter in the soil that it forms com- 

 pounds which can be absorbed by plants. Silica, in a soluble state, 



