MINERAL NATUEE OE SOILS 368 



by ordinary weathering agencies. Although no special investi- 

 gations along these lines have been carried on by the United 

 States Bureau of Soils, it is nevertheless stated^ that alumina or 

 aluminum hydrate is but seldom, if ever, a normal constituent 

 of soils. It is evident, therefore, that further investigations are 

 necessary before the matter can be regarded as definitely decided. 

 Since the work of Lemberg was made public,^ it has been very 

 commonly assumed that various minerals of the zeolitic group 

 were present and exercised an important function in the con- 

 servation of soil fertility. Notwithstanding the somewhat en- 

 thusiastic endorsement by Hilgard, of this idea, as set forth 

 in the previous pages, the writer can but feel that too much has 

 been assumed, both regarding their actual presence and their 

 possible utility. 



One must not lose sight of the fact that the actual occurrence 

 of zeolites in soils is as yet not proven. Their presence is inferred 

 from the fact that weak acids, such as are known to be capable 

 of decomposing zeolitic minerals, will extract from the soil cer- 

 tain constituents which are characteristic of minerals of the 

 zeolitic group ; and it is assumed, purely for lack of a better rea- 

 son, that these elements are those thus combined. Even if this 

 be true, their efficacy as potash holders may well be questioned, 

 since potash is not as a rule an element of great importance in 

 zeolitic minerals. Out of the 23 known species of zeolites (in- 

 cluding apophyllite), in but five is potash considered an essential 

 constituent. These five, as already noted on p. 29, are apo- 

 phyllite, ptilolite, mordenite, phillipsite, and harmotome, of 

 which phillipsite alone carries upwards of 6% (theoretically), 

 the other smaller amounts, the average for the five being about 

 4%. Now assuming that all the zeolites in the soils belonged 

 to these five groups and none to the 18 potash-free varieties, 

 and that 10% of any soil consisted of zeolitic material, even then 

 we have thus combined only 0.4% of KgO. 



It must be remembered, further, that the zeolites are invariably 

 secondary minerals, as already noted, and as such are com- 

 monly regarded as decomposition products. This does not 

 necessarily mean, however, that they are products of superficial 



iBull. 30. 



2 Zur Kenntniss der Bildting und Umbildting von Silicaten, Zeitsehrift der 

 Deutsehen Geolisclien Gesellschaft, Vols. XXXVII and XXXVIII, 1885 and 

 1887. 



