THE WASTE AND CONSERVATION OF PLANT FOOD. 219 



of absorbing and retaining materials which are sohible in water nnder 

 ordinary conditions. It mnst not be forgotten also that tlierain water 

 Avhich descends npon the earth is not pnre. Rain water brings to the 

 earth a certain amount of valuable plant food. Ii^ot only does it absorb 

 and hold in solution ammonia and nitric acid, which maybe formed by 

 the electrical discharges in the air, but it also collects and brings to 

 the surface of tlie earth vast quantities of meteoric dust containiug 

 valuable fertilizing principles. Thus we have constantly entering the 

 soil water which contains more or less of the materials necessary to 

 plant growth. Even the drainage waters, which leave an arable field, 

 may not reach the sea without giving up much of this material. The 

 drainage waters in passing underneath the earth's snrface take devions 

 conrses and are often l)ronght near to the surface again or are poured 

 upon soils which are (piite diflerent in their texture from those fur- 

 nishing the materials in solution. Sterry Hunt' has pointed out how 

 such waters sooner or later come upon permeable strata by which they 

 are absorbed and in their subterranean circulation undergo impor- 

 tant changes. Especially when these waters reach argillaceous strata 

 their content of neutral, soluble salts may snfl'er great changes. Snch 

 waters, charged witli organic and mineral materials, contain, usually, 

 large amounts of potassium salts and notable quantities of silica and 

 phosjdiates and in nnmy cases ammoniated salts and nitrites or nitrates. 



The experiments of Way, Yoelcker, and others have shown that in 

 contact with argillaceous sediments these waters give up their potash, 

 ammonia, silica, phosphoric acid, and organic matter, which remain in 

 combination with the soil; on the other hand, soda, magnesia, sulphuric 

 acid, and chlorine are not removed from the drainage waters. Eichorn 

 attribntes this power of selective absorption in the soil chiefly to the 

 action of hydrated double aluminum silicates, and supposes that the 

 process is one of double exchange, equivalents^ of lime or soda being 

 given up for the potash retained. By this power of selective absor])tion 

 the mineral matters required for the growth of plants tend to be retained 

 in most soils, while those not required for the growth of plants are 

 removed. Nevertheless, nuich of the valuable mineral material m solu- 

 tion nuist escape absorption and finally find its way into the streams, 

 rivers, and seas. 



From the foregoing summary of the methods of waste of i)lant food 

 it has been seen that in spite of all the precautions of the farmer and 

 the chemist, and in spite of the selective absorption of the soil, immense 

 (piantities of valuable plant food are carried into the sea, where appar- 

 ently they are lost to agriculture forever. But this is only an apparent 

 loss. The economies of nature are so happily adjusted as to provide a 

 means of gradually returning in some form or other to the power of the 

 farmer the plant food which has been apjiarently destroyed. It is true 

 that this return will probably not be to the locality where the waste 



1 Chemica] tiud Geological Essays, page 95. 



