Action of the Atmosphere iipon neicly -deepened Soil. 447 



Paris may bring down as much nitrogen as is contained in the 

 grain of an average crop of barley or wheat. As it must be sup- 

 posed that a good deal of ammonia and nitric acid will be absorbed 

 by tlie soil from the atmosphere, in addition to what is brought 

 down by rain, it appears quite conceivable that under favourable 

 circumstances the soil and plants growing upon it may obtain as 

 much nitrogen from the atmosphere as will suffice for many of the 

 crops usually grown, without the agency of other manure — a sup- 

 position which the results obtained by the Rev. Mr. Smith, at 

 Lois-Weedon, tend to justify. And the above calculations would 

 also point to the propriety of employing mostly minei'al manures 

 in the immediate neighbourhood of large towns, trusting to the 

 atmosphere supplying the greater part of the necessary nitrogen 

 and carbonic acid. 



Rain-water then, in its percolation through the earth, by virtue 

 of the oxygen which it holds condensed, eminently favours the 

 oxidation of the mineral and organic matters of the soil ; the 

 former, in consequence of this process, is brought into partial 

 decomposition, for a stone, says Dr. MacCulloch, is generally 

 disintegrated by the oxidation of the iron it contains, while the 

 latter, or the organic matter, is thereby made to pass through the 

 various stages of decay, evolving in the process much carbonic 

 acid and also ammonia. The rain also, by virtue of the carbonic 

 acid, ammonia, nitric acid, and salts, which it holds in solution, 

 dissolves gradually the mineral substances of the soil, and enables 

 them thereby to be taken up by the roots of plants, and it also 

 furnishes directly in itself much nitrogen in a form fit for being 

 assimilated by the vegetable organism, and it may further pro- 

 bably supply sensible quantities of salts of lime, magnesia, potass, 

 soda, &c., which are taken up bv the action of the Avind from the 

 waters of the sea. 



Tliis would seem to be the fittest place to take some notice of 

 the 



Absorptive Properties of Soils. — The absorbent powers of the 

 ingredients of the soil depend botii upon the kind of matter of 

 which it is constituted, and also upon the pliysical and mechanical 

 conditions under which that matter exists. As regards the absorp- 

 tion of gaseous substances, the quantity taken up will depend in 

 a great measure upon the amount of surface with which the body 

 to be aljsorbed can come in contact. Thus anything that tends to 

 solidifv the soil, or ])ring its j)articlcs together Into a comjiact 

 mass, will obstruct its powers in this direction ; and on the other 

 hand, tlio more j)()rous and lincly comniinutcd the soil is made, 

 the more surface will be l)rought to bear upon the gaseous matter. 

 Thus Mitscherlich calculates that a volume of one cubic inch 

 filled with globules not exceeding to ^jVirosth of an inch in 



2 11 -1 



