THE SOIL AS A SOURCE OF MLNERAL ELEMENTS 405 



effectiveness. The order of the retentive capacity of the micelles for cations 

 is H+>Ca+ + >]Vlg+ + >K+>NH4+>Na+. In other words, of the 

 ions in the above series, the H^■ ions are most tenaciously bound to the col- 

 loidal particles and are the most difficult to displace, while the converse is 

 true of the Na+ ions. 



Of the five principal anions found in the soils — Cl~, SO4 , NOs", 

 HCOs", and PO4 — only the last is retained by the soil particles as 



such in any appreciable quantities. The other four anions usually leach 

 out of soils rather rapidly. IVIicro-organisms may utilize nitrates and sul- 

 fates in the synthesis of organic compounds and thus fix them in the soil in 

 the form of organic nitrogen compounds but this is an entirely different 

 phenomenon from the retention of ions in the soil by a purely physico- 

 chemical mechanism. On the other hand, even in soils which have received 

 heavy treatments of phosphates, only small quantities of PO4 ions 



are found in the drainage waters. Evidently the PO4 ions are tied up 



by the soil particles in some manner, but the mechanism by which this is 

 accomplished is not entirely understood. It is generally considered that the 

 phosphates are precipitated as insoluble compounds by a reaction with some 

 of the soil constituents, and that in this precipitated form they are not 

 readily available to plants. 



The discovery of the phenomenon of cation exchange has made neces- 

 sary a revision of the older concept of the soil as a rather inert medium 

 containing a soil solution from which plant roots obtain all of the mineral 

 salts which they absorb. Roots not only absorb the freely diffusible ions 

 in the soil solution but can also liberate cations adsorbed on clay micelles 

 which in turn can be absorbed by the plant. In general it appears that the 

 cations which enter plants come largely from the outer layer of the clay 

 micelles while the anions come largely from the soil solution. 



The region of the root in which intake of water occurs is also, in gen- 

 eral, the zone in which absorption of ions takes place, and the pathway 

 which solutes follow in passing from the soil to the stele is the same as that 

 followed by water (Fig. 70). The root tips are, under favorable condi- 

 tions, not only rapidly growing organs, but centers of high metabolic ac- 

 tivity. Carbon dioxide resulting from respiration is continuously being re- 

 leased into the soil in which it reacts with water, forming carbonic acid. 

 Around each root tip there will usually be, therefore, a localized zone of 

 high carbonic acid content. In this zone H+ ions from the carbonic acid 

 may displace adsorbed cations on the clay micelles. This is especially likely 

 to happen to the micelles in intimate contact with the root tips. The cat- 

 ions released as a result of this ionic exchange can then be absorbed by the 



