COMPOSITION OF SOILS. 



135 



existence of these compounds as definite acids is somewhat doubtful. 

 Humus absorbs ammonia, and it is slowly acted upon by the atmo- 

 sphere, so as to form carbonic 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. 



Silica, in greater or less quantity, is found in all soUs ; but it 

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

 is only when acted upon by alkalis in the soil that it forms compounds 

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

 minute quantities in soils, the proportion, according to Johnston, 

 varying from 0'16 to 0'84 in 100 parts, while the insoluble siliceous 

 matter varies from 6047 to 83"31 in 100 parts. Wiegman and 

 Polstorf found that plants took up sUica from a soil composed entirely 

 of quartz sand, from which everything organic and soluble had been 

 removed. The following table shows the plants which germinated, the 

 height to which they grew previously to being analysed, the quantity 

 of silica they contained when planted, and the increase : — 



Alumina exists abundantly in clayey soils, but it does not enter 

 largely into the composition of plants. It has the power of absorbing 

 ammonia and saline matters, and may prove beneficial in this way. 



Lime is an essential ingredient in all fertile soils. In 1000 lbs. 

 of such soil there are, according to Johnston, 56 lbs. of lime ; whUe a 

 soil is barren which contains only 4 lbs. The presence of phosphoric 

 acid in soils, in the form of phosphates of potash, soda, and lime, is 

 essential for the production of certain azotised compounds in plants ; 

 and sulphuric acid, similarly combined, is required for the formation 

 of others. Calcareous soils contain upwards of 50 per cent of lime. 

 The addition of lime to soils is often highly beneficial, by destroying 

 noxious weeds, and preventing disease in crops. Lime is a forcing 

 agent, and is useful in stiff clayey soils where it decomposes the silicate 

 of potash, forming silicate of lime, and liberating the potash which is 

 taken up by the plants. In marly soUs lime exists in the proportion of 

 5-20 per cent. In loamy soils lime is in smaller quantity. 



A rough way of estimating the general nature of a soil is thus 

 given by Professor Johnston : — 



