216 



SCIENCE 



[Vol. LVI, No.' 1443 



plying itlie method of storing linotype bars and 

 using them as needed, for example, in the 

 cumulation decade by decade of the Royal 

 Society's International Catalog. This work 

 coiild be organized for the future in three years 

 and printing of the tirst itwo decades of the 

 twentieth century finished in five or less. I am 

 less sure that it will come than I am confident 

 of the future provision in ihe way of books. 

 But if and when the key to the record does 

 exist, then no budding scientist may fail of 

 confidence in his start, of the help in his 

 labors as they go on year by year which comes 

 from knowing what has been done and what 

 is being done by his fellows. We shall have 

 them ready at hand — not alone the record in 

 print of human efforts to comprehend the uni- 

 verse, but also such an effective and useful key 

 to that record that we may reverse the old 

 saying, and affirm he who reads may run. 



Wm. Warnee Bishop 

 Mat 26, 1922 



THE DEPLETION OF SOILS BY 

 CHEMICAL DENUDATION 



The rate of chemical denudation of soil and 

 rock material has been of vital interest to geol- 

 ogists and soil scientists. The geologist has 

 been interested in an attempt to establish a 

 unit of time for estimating the age of the 

 ocean, the time periods of geologic processes 

 and incidently the time periods of the life of 

 man, animals and other fomis of life on the 

 globe. He has been ably assisted by hydro- 

 graphers and oceanographers. The soil scien- 

 tist has been interested in relation to time 

 measurements of soil productivity and of meth- 

 ods to be taken to prolong the life and the en- 

 durance of the soil for the agricultural needs 

 of the people. 



Unfortunately 'the vast amount of informa- 

 tion thait has been collected has been based 

 mainly upon the translocation of material in 

 true solution, disregarding all material in col- 

 loidal solution. The results of the methods 

 used have shown surprisingly little silica, 

 alumina and iron lost from the soil in com- 

 parison, for instance, with the amount of 

 potash lost. In fact, the soil scientists have 

 assumed that the three first named elements. 



which form by far the larger proportion of the 

 soil, remain in the soil because of their slight 

 solubility. There has been a lack of evidence 

 of any considerable loss of these through solu- 

 tion. 



The hydrographers have determined, from 

 the average composition of the soluble salts 

 carried to the sea, that SiO, constitutes 8.60 

 per cent., Al^Og and Fe^Og together constitute 

 0.64 per cent, and K^o" 2.13 per cent. This 

 gives a ratio of one part of potash to 4.4 parts 

 of silica, alumina and iron. As the ratio of 

 potash to silica, alrunina and iron in igneous 

 and in shale rocks is about 1 : 25, there is an 

 apparent selective loss of potash; that is, the 

 loss of potash is relatively much greater than 

 the loss of the main soil constituents. From 

 this fact, together with the fact that plants 

 appear to have the same selective power of ab- 

 sorption of potash in much greater proportion 

 than of the original proportion of the main 

 soil constituents, soil scientists have taken a 

 very pessimistic view of the length of time the 

 soil will remain fit for agricultui-al use. 



Opinions have been advanced by some that 

 our most productive soils will only last for one 

 hundred and fifty years or so without replace- 

 ment, because of this selective loss of plant 

 food material due to their greater solubility 

 and assuming further that the silica, alumina 

 and iron are removed in very small proportions 

 from the soil because of their relative insolu- 

 bility. Such is briefly an outline of the situa- 

 tion up to a short time ago. 



If silica, alumina and iron are removed from 

 the soil in proportion to the loss of the so- 

 called more soluble salts the losses could not 

 be determined by chemical analysis any more 

 than the removal of a cartload of soil would 

 affect the chemical composition of the material 

 that remains. Recent investigations of the soil 

 colloids by this bureau suggest that this may 

 be actually what takes place. It would appear 

 that, in the breaking down of the silicates to a 

 point where the potash goes into solution, silica, 

 alumina and iron also go into solution in the 

 same proportion as they bear to the potash 

 content in the original material. There is, 

 however, this very important difference which 

 has not heretofore been recognized or properly 

 appreciated and that is that the solution of 



