186 ANNUAL REPORT OF THE Off. Doc. 



was nevertheless adopted with the hope that it might at least solve 

 some of the soil problems, and is still retained, with very strict 

 definitions of the details of it^ operation, to aiford a rough approx- 

 imation to the total supply in a soil of material in condition for 

 plant use, or in such state that it will probably be made available 

 within a reasonable period by proper farm management. 



Pausing just to note that, by the action of the acid under the 

 accepted conditions of extraction, the soils' combined carbonic acid 

 is set free as a gas, the silica from decomposed silicates is largely 

 left as a gelatinous solid mixed with the undecomposed minerals, 

 and that the remainder of the mineral constituents are taken up 

 into the acid solution, let us consider a few of the results obtained 

 by this method of study. 



We note, in the first place, that the more siliceous rock surfaces 

 are weathered, the more fully are the weathered portions decom- 

 posed by acid. Thus Merrill found that of a certain gneiss, the 

 freshly exposed rock was decomposed by acid to the extent of 30.47 

 per cent, only while highly weathered portions were decomposed 

 to the amount of (>9.18 per cent.; also, that a fresh diabase from Bed- 

 ford, Mass., was decomposed to the extent of 3G.23 per cent., the 

 highly weathered rock 32.28 per cent., and the silty substances, sep- 

 arable from the latter by washing, to the extent of 77.52 per cent. 

 A similar diflference appears when the decomposition by acid of the 

 sand and clays, respectively, of the humid region is considered. The 

 air-dry sands usually exhibit 85 to 1)0 per cent, and upward of 

 matters not attacked by acid, the clays only 65 to 70 per cent. Thus 

 the Pennsylvania sandy loam soil of Northern Lancaster county, on 

 which our experiments in growing Sumatra wrapper leaf tobacco 

 have so satisfactorily progressed, show from 86 to 90 per cent, of 

 acid-resistant matters, while the Hagerstown loam soils from the 

 limestone region of the county, on which the greater pi'oportion of 

 the tiller tobaccos' are grown, show but 67 per cent. The similar 

 nature of these comparisons between rock and soil is at once rec- 

 ognized, when we recall that the clays represent the finer, more high- 

 ly weathered portions of the parent rocks. The least amount of acid- 

 resistant material found in any soil was that given by a Cuban lime- 

 stone soil, analyzed in the writer's laboratory, viz., 7.42 per cent. 

 This is very nearly the figure obtained from Longbridge in the an- 

 alysis of the finest clay particles washed from a heavy Mississippi 

 soil, viz., 15.96 per cent. 



Before speaking in further detail of the results obtained from the 

 acid analysis of Pennsylvania soils, several other points of funda- 

 mental importance in their interpretation must be considered. 



It is a matter of common experience that the finer the subdivision 

 of a substance, the more rapidly it dissolves. This is due to the 

 fact that fineness of subdivision greatly increases the extent of sur- 

 face exposed by a unit weight of the material to the action of the 

 solvent liquid. For example, a cube measuring two inches on a side 

 inch on a side, it will yield eight cubes, each having a superficial area 

 of 24 square inches; whereas if it be split into cubes measuring one 

 inch on a side, it will yield eight cubes, each having a superficial area 

 of six square inches, making a total of forty-eight square inches. 

 This effect is, however, one of rate, and but slightly influences the 

 final concentration of the solution. It might be supposed, at first 



