ITS IMPORTANCE TO VEGETATION. 296 



wide plains, in the hollows of deep valleys, and on the bottoms of ancient 

 lakes. Such an incrustation, probably so formed, is the bed of nitrate of 

 soda in Peru, from which all our supplies of that salt are drawn — such 

 are the deposits of carbonate of soda (urao) extracted from the soil in the 

 South American State of Colombia. 



5°. Contraction of the soil on drying. — Some soils in dry weather di- 

 minish very much in bulk, shrink in, and crack. Thus, after being 

 soaked by rain, pure clay and peaty soils diminish in bulk about one- 

 fifth when they are again made perfectly dry — while sand has the same 

 bulk in either state. The more clay or vegetable matter, therefore, a 

 soil contains, the more it swells and contracts in alternate wet and dry 

 weather. This contraction in stiff clays can scarcely fail to be occa- 

 sionally injurious to young roots from the pressure upon the tender fibres 

 to which it must give rise, while in light and sandy soils the compres- 

 sion of the roots is nearly uniform in all weathers, and they are undis- 

 turbed in their natural tendency to throw out off-shoots in every direction. 

 Hence another good quality of light soils, and a less obvious benefit 

 which must necessarily result from rendering soils less tenacious by ad- 

 mixture or otherwise. 



III. RELATIONS OF THE SOIL TO THE ATMOSPHERE. 



Power of absorbing oxygen and other gaseous substances from the 

 air. — 1°. The importance of the oxygen of the atmosphere, first to the 

 germination of the seed, and afterwards to the growth of the ])lant, Ihave 

 already sufficiently insisted upon. It is of consequence, therefore, that 

 this oxygen should gain access to every part of the soil, and thus to all 

 the roots of the plant. This access can be facilitated by artificially 

 working the land, and thus rendering it more porous. But some soils, 

 in whatever state they may be in this respect, have been found to absorb 

 oxygen with more rapidity, and in larger quantity, than others. Thus 

 clays absorb more oxygen than sandy soils, and vegetable moulds or 

 peats more than clays. This difference depends in part upon the natural 

 porosity of these different soils, and in part also upon the chemical con- 

 stitution of each. If the clay contain iron or manganese in the state of 

 first or ^ro^-oxides, these will naturally absorb oxygen for the purpose of 

 combining with it, — while the decaying vegetable matter will in like 

 manner, in such as contain it largely, drink in much oxygen to aid their 

 natural decomposition. 



2°. Besides the gases, oxygen and nitrogen, of which the air princi- 

 pally consists, the soil absorbs also carbonic acid from the atmosphere, 

 and portions of those various vapours, — whether of ammonia and other 

 effluvia which rise from the earth, or of nitric acid formed in the air,— 

 and these, in the opinion of some chemists, contribute very materially to 

 its natural fertility. This, however, is very much a matter of conjec- 

 ture, and no experiments have been made as to the relative capabilities 

 of different soils thus to extract vegetable food from the surrounding air. 

 One fact, however, seems to be clearly ascertained, that all soils, namely, 

 absorb gaseous substances of every kind most easily and in the greatest 

 abundance when they are in a moist state. . The fall of rains, or the de- 

 scent of dew, therefore, will favour this absorption in dry seasons, and it 

 will also be greatest in those soils which have the power of most readily 



