AGRICULTURAL CHEMISTRY. 157 



wick — no more evaporation into the air can occur, and consequently 

 there is no longer any ascent of water; on the contrary, the water, by 

 its own weight, penetrates the soil, and if the underlying ground be 

 not saturated with moisture, as can happen where the subterranean 

 fountains yield a meagre supply, then capillarity will aid gravity in 

 its downward distribution. 



The most rational conclusion from all the facts at our command is 

 that all the mineral matters, as well as a portion of the organic bodies, 

 which feed the plant, are carried into it by water. So long as evapora- 

 tion goes on from the surface of the soil, so long there is a constant 

 upward flow of saline matters. Those portions which do not enter 

 vegetation accumulate on or near the surface of the ground; when a 

 rain falls, they are washed down again to a certain depth, and thus 

 are kept constantly changing their place with the water, which is the 

 vehicle of their distribution. In regions where rain falls periodically 

 or not at all, this upward flow of the soil-water often causes an accumu- 

 lation of salts on the surface of the ground. Thus in Bengal many 

 soils which in the wet season produce the most luxuriant crops, during 

 the rainless portion of the year become covered with white crusts of 

 saltpetre. Doubtless the beds of nitrate of soda that are found in 

 Peru have accumulated in the same manner. So in our western caves 

 the earth sheltered from rains is saturated with salts — epsom salts, 

 glauber salts, and saltpetre, or mixtures of these. Often the rich 

 soil of gardens is slightly incrusted in this manner in our summer 

 weather; but the saline matters are carried into the soil with the 

 next rain. 



It is easy to see how, in a good soil, capillarity thus acts in keeping 

 the roots of plants constantly immersed in a stream of water or moist- 

 ure that is now ascending, now descending, but never at rest, and 

 how the food of the plant is thus made to circulate around the organs 

 fitted for absorbing it. 



The same causes that maintain this perpetual supply of water and 

 food to the plant are also efficacious in constantly preparing new sup- 

 plies of food. As before explained, the materials of the soil are always 

 undergoing decomposition, whereby the silica, lime, phosphoric acid, 

 potash, &c, of the insoluble fragments of rock, become soluble in 

 water and accessible to the plant. Water charged with carbonic acid 

 and oxygen, is the chief agent in these chemical changes. The more 

 extensive and rapid the circulation of water in the soil, the more 

 matters will be rendered soluble in a given time, and, other things 

 being equal, the less will the soil be dependent on manures to keep 

 up its fertility. 



No matter how favorable the structure of the soil ma} r be to the 

 circulation of water in it, no continuous upward movement can take 

 place without evaporation. The ease and rapidity of evaporation, 

 while mainly depending on the condition of the atmosphere and on 

 the sun's heat, are to a certain degree influenced by the soil itself. 

 We have already seen that the soil possesses a power of absorbing 

 watery vapor from the atmosphere, a power which is related both to 



