84 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 



behind then consists of an argillaceous, loamy mass, variously coloured by iron and 

 very similar in appearance to the clay formed from the decomposition of felspar. 

 According to the quantity of the substances mixed with the carbonate of lime in 

 the rock, the loamy earth formed from limestone is either abundant or only in 

 restricted layers, bands and pockets lying on, or intercalated within, the unde- 

 com posed debris of the stone. Chemical analysis has resulted in the discovery 

 that there are, as a rule, in loamy earth of this kind the same ingredients avail- 

 able for plants as have been identified in earth produced from silicates; and we 

 are led to believe that earths, collected in widely different places and covering 

 rocks of most various kinds, are much more uniform qualitatively than has been 

 supposed. Only, the relative proportions of the substances forming the mixture 

 are usually different. Silica and the alkalies are less conspicuous in earth derived 

 from limestone, and carbonate of lime in that which is formed from silicates. 

 This difference is particularly striking in instances where the rock consisted 

 almost entirely either of quartz and mica or of nearly pure carbonates of lime 

 and magnesium. In these cases the earth formed is not argillaceous, but of loose 

 consistence, very abundant, and composed, according to the kind of rock, of 

 quartzose sand and mica scales or calcareous and dolomitic sand. 



The conversion of rocks into earths by the action of water from the atmosphere 

 containing carbonic and nitric acids is, besides, materially modified by the disrup- 

 tions which ensue from changes of temperature, more particularly by the freezing 

 of water within the pores of rocks. It is also affected, though more remotely, by 

 the mechanical action of water and air in motion, and, lastly, by the plants them- 

 selves, which penetrate with their roots into the narrowest crevices and mingle their 

 dead remains with the portions of the rock that are decomposed, broken up, or 

 abraded by chemical and mechanical agencies. The substance produced from a 

 rock in the manner explained is called earth-mould, or simply earth. The matter 

 resulting from the decomposition of plants and animals is designated by the term 

 " humus." Earth which includes an abundance of decomposed fragments of plants, 

 i.e. has a large admixture of humus, is called vegetable mould. 



Every kind of earth, but especially earth rich in humus and clay, has the power 

 of retaining gases, and especially water and salts. When water containing salts in 

 solution is poured over a layer of dry vegetable mould, it percolates into the spaces 

 between the particles of earth, and speedily drives out of them the air which has 

 but slight adhesion, and which then ascends in bubbles. It is not till all the inter- 

 spaces are full of water, whilst a fresh supply is constantly maintained from above, 

 that any of the liquid oozes out from beneath the stratum of earth. The water 

 remaining in the interstices is held there by adhesion to the particles of earth, and 

 we must conceive each of these particles as surrounded by an adherent film of 

 water. The inorganic salts, infiltrating with the water, are held with still greater 

 energy. The water which trickles from the bottom of the earth always contains a 

 much smaller proportion of salts in solution than that which was poured on above, 

 whence we conclude that the latter are in part absorbed by the earth. 



