PHYSICAL COMPOSITION OF SOILS. 87 



server to be hopelessly sterile sandy deserts, very commonly 

 prove to be even more productive than the more clayey lands of 

 the same regions. Examination of the sand shows, in these 

 cases, that instead of mere grains of quartz, the minerals of the 

 parent rock, partially decomposed, themselves constitute a 

 large proportion of the sandy mass. But in the regions of 

 deficient rainfall, as has already been stated, (p. 47) the 

 formation of clay (kaolinization) is exceedingly slow; hence 

 the decomposition of the rock powder results in the production 

 of predominantly pulverulent instead of clayey soils. But the 

 mineral plant-food is not on that account less available, pro- 

 vided other physical conditions necessary for the success of 

 plant growth are fulfilled. Among these moisture stands 

 foremost ; hence the relative proportions of the several grain- 

 sizes are of vital importance, since upon this depends to a great 

 extent the proper supply and distribution of moisture, without 

 which no amount of plant-food will avail. Moreover, the 

 finest and most highly decomposed powder is the portion from 

 which the roots draw their chief food-supplies. 



The point last mentioned is well shown in the results obtained by 

 Dr. R. H. Loughridge, from the analysis of each of the several grain- 

 si/es into which he had resolved a very generalized soil of the State of 

 Mississippi, representing a very large land area in that State as well as 

 in Tennessee and Louisiana. The details of this investigation are 

 given farther on ; but summarily it may be stated that he found prac- 

 tically the whole of the acid-soluble mineral plant-food accumulated 

 within the portion of the soil the fineness of whose grains was below 

 .025 millimeters (one-thousandth of an inch) ; ingredients so fine as 

 to be wholly impalpable between the fingers. Moreover, two-thirds of 

 the total amount was found in the portion described above as "clay." 

 It is thus readily understood why clay soils are in the regions of summer 

 rains commonly designated as "strong" lands. 



The corresponding later investigations of Kud/inski (Ann. Agr. Inst. 

 Moscow, Vol. 9, No. 2, pp. 172-234: Kxp. Sta. Record, Dec. 1904, 

 p. 245) and of Ma/urenko (Jour. Kxp. Landw. 1904, pp. 73-75 ; Kxp't 

 Stn. Record, Dec. 1004, p. 344) fully corroborate Loughridge's con- 

 clusions, for typical soils of Kuropean Russia. 



Ill the arid or irrigation regions, however, the case is dif- 

 ferent, for the reason that much of the decomposed rock- 



