180 SORGHUM. 



analysis, does not appear any better than ordinary soils, and yet its 

 productiveness is such that, as has been said of it : 



The Russian proverb, " One can not distinguish the generous from the rich," 

 may be most appropriate]}^ applied to the tchernozeme. It appears rich be- 

 cause it is generous. 



It is commonly known, that the mineral matter, which composes^ 

 the larger part of every good soil, has been derived originally from 

 rocks which, during comparatively recent periods, geologically speak- 

 ing, have been disintegrated by one agency or another, and that those 

 supplies of mineral food necessary to the plant, in any soil, must have 

 previously existed in the rocks from Avhich this soil was produced. 



But those agencies, as frost, water, attrition, the carbonic acid and 

 oxygen of the air, which have in the past reduced these rock masses to 

 every degree of fineness, from small pebbles to impalpable powder, are 

 still operative ; and gradually, year by year, day by day, new supplies 

 of mineral food are being unlocked from these rock fragments and 

 made available to the plant. Besides, it will Ite seen, that, as this 

 pulverization goes on, the surface exposed to the action of these agen- 

 cies, above mentioned, increases in geometric ratio, and so, in conse- 

 quence, the disintegration becomes proportionately the more rapid. 



To illustrate: Suppose a block of granite 1 foot square to be broken 

 into cubes of B inches square, there would be obviously 8 cubes pro- 

 duced, and the surface exposed in the first case, being 6 square feet, 

 would be doubled. Let this process be repeated, and the surface be- 

 comes 24 feet square. Continue this operation for only 25 times, and 

 our block of granite 1 foot square, with 6 square feet of surftice, be- 

 comes resolved into minute fragments of quartz, feldspar, and mica, 

 about one three-millionth of an inch in diameter, and presenting an 

 aggregate surface of over 7 square miles ; and yet there is no reason 

 to suppose that the comminution ceases at such limits; indeed, there 

 is every reason to believe that the plant is incapaljle of assimilating 

 food which is not absolutely in the molecular condition. 



The calculations of Sir William Thompson show the size of the mole- 

 cule to be, at his largest estimate, only one hundredth the diameter, or 

 one millionth the bulk, of the fragments to wliich we have reduced our 

 block of granite in the above illustration. 



The importance, then, of assisting these agencies by good tillage is 

 obvious; indeed, an agriculturist, of long experience and distinguished 

 success, has declared that he would prefer to have an ordinary field 

 well i)lowxd without manure, than poorly plowed with it. 



So far as the partial mechanical analysis goes, it quite fails to throw 

 any light upon the cause of the very wide difference in the crops grown 

 upon the Rio Grande soils. 



