AGRICULTURAL CHEMISTRY. 153 



manner that the eye at once makes the distinction into surface-soil and 

 sub -soil. In many soils, however, such distinctions are entirely ar- 

 bitrary, the earth changing its appearance gradually or even remain- 

 ing uniform to a considerable depth. 



Again, the surface soil may have a greater downward extent than 

 the active soil, or the tilth may extend into the sub-soil. 



Hard-pan is the appropriate name of a dense, almost impenetra- 

 ble, crust or stratum of ochery clay or compacted gravel, often under- 

 lying a fairly fruitful soil. It is the soil reverting to rock. The parti- 

 cles once disjointed are being cemented together again by the solu- 

 tions of lime, iron or alkali-silicates that descend from the surface soil. 

 Such a stratum often separates the surface soil from a deep gravel 

 bed, and peat swamps thus exist in basins formed on the most porous 

 soils by a thin layer of moor-bed-pan. 



With these general notions regarding the origin and characters of 

 soils, we may proceed to a somewhat extended notice of the pro- 

 perties of the soil as influencing fertility. These divide themselves 

 into physical characters — those which externally affect the growth 

 of the plant; and chemical characters — those which provide it with 

 food. 



Among the physical characters* we first notice the state of division 

 in which the soil is found. 



On the surface of a block of granite onty a few lichens and mosses 

 can exist; crush the block to a coarse powder and a more abundant 

 vegetation can be supported on it; if it is reduced to a very fine dust 

 and duly watered, even the cereal grains will grow and perfect fruit 

 on it. Thus two soils may have the same chemical composition, and 

 yet one be almost inexhaustibly fertile, and the other almost hope- 

 lessly barren. There are sandy soils in the Eastern States, which, 

 without manure, yield only the most meagre crops of rye or buck- 

 wheat; and there are sandy soils in Ohio, which, without manure, 

 yield on an average 80 bushels of Indian corn per acre, and have 

 yielded this for twenty to fifty years in unbroken succession. Ac- 

 cording to David A. Wells, (American Journal of Science, July, 

 1852,) these two kinds of soil yield very similar, practically identical, 

 results on chemical analysis, so far as their inorganic ingredients are 

 concerned. What is the cause of the difference of fertility ? Our 

 present knowledge can point to no other explanation than is furnished 

 by the different fineness of the particles. The barren sandy soils 

 consist in great part of coarse grains, while the Ohio soil is an ex- 

 ceedingly fine powder. 



It is true, as a general rule, that all fertile soils contain a large 

 proportion of very fine or impalpable matter. How the extreme 

 division of the particles of the soil is connected with its fertility is 

 not difficult to understand. The food of the plant must enter it in 

 a state of solution, or if undissolved, the particles must be smaller 



Q In treating of the physical characters of the soil, the writer employs an essay on this 

 subject, contributed by him to vol. XVI of the Transactions of the N. Y. State Agricul- 

 tural Society. 



