68 GEOLOGY OF AGRICULTURE. 



ble III.) as phosphates, and is essential to the produc- 

 tion of the cereals, and of all of the sweet, nutritive 

 grasses. 



104. Carhonic acid {CO"^, Table I.) can hardly be said 

 to be an ingredient of the soil, and yet it exists in 

 nearly all soils in combination with some of the bases 

 (Table III.) as carbonates ; and all cultivated soils are 

 always producing it. Whenever vegetable matter 

 burns, its carbon combines with oxygen and forms car- 

 bonic acid. The same happens when vegetables decay 

 in such circumstances that air has access to them. Ve- 

 getable matter in the soil is thus constantly giving off 

 carbonic acid. A portion of this may be supposed to 

 combine with the bases in the soil, to form carbon- 

 ates. Much of it goes to feed plants, entering their 

 roots, dissolved in water, or ascending to be taken in 

 through the pores of their leaves. When land lies in 

 fallow through the heat of summer, it cannot be 

 doubted, that much of it escapes into the air and is 

 lost, at least to the owner of that field. 



105. Silicic acid (SiO^), or Silica, (quartz, flint, 

 sand) constitutes generally from 60 to 90 per cent, of 

 good soils, and often as much as 95 per cent, of sandy 

 soils. Silica is insoluble in water, but is rendered sol- 

 uble by alkalies. One effect of ashing land, is to ren- 

 der the silica soluble, so that it can be taken up by 

 the roots of plants. Its office seems to be to afford 

 the stiffening material, for the stalk, straw, husk, and 

 other parts which require to be firm in order to sup- 

 port or protect the seeds. 



