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acid is attended with equal difficulties. The small amount of less than 
one per cent, of these which is returned as existing in the soil may be 
often due to errors in the analysis, or may be greatly above or below the 
true amount. The amount of lime may be easily ascertained with suffi¬ 
cient accuracy, but lime is far from being the most important part of the 
compound. It is precisely those the determination of which is attended 
with the greatest difficulty that are the most bounteous sources of fertil- 
ty—upon which the growing plant feeds and fattens. 
A single instance only will illustrate the facts. The amount of guano 
usually applied to the acre is 200 pounds. The soil to the depth of one 
foot—and vegetation often reaches far deeper for its supplies— weighs 
3,920,000 pounds. The guano thus applied contains six pounds of pot¬ 
ash, 24 pounds of phosphoric acid, and 34 pounds of ammonia; or 1 
part in 600,000 of the first, 1 part in 150,000 of the second, and 1 part 
in 100,000 of the last—grains so minute as to be indistinguishable ex¬ 
cept to the nicest and most intricate analysis, if not even then beyond 
detection. 
Again, embedded in the earth at a greater or less depth, or lying on 
the surface—often turned over by the plow, broken by accident and de¬ 
composing in varying lengths of time, are substances harder than the soil, 
differing in size and in the properties that compose them. We call them 
rock, stone, gravel. We know not but they may have an important in¬ 
fluence on vegetation. We know that they do effect it to a certain extent 
by attracting or evaporating moisture in their cooler, or hotter, neighbor¬ 
hood. They are constantly undergoing a process of pulverization, and 
their smaller particles mingle with the soil. There may thus be chemi¬ 
cal agencies entering into the soil, which, though not great in bulk may 
possess quickening properties and principles. The extent of these opera¬ 
tions may be great—it may be small. It may be important—it may be 
trifling. The very doubts upon the subject are difficulties. When con¬ 
sidered at all, the subject is prolific of uncertainties that disturb and im¬ 
pair confidence in all the niceties of calculation. 
It may be that from the remote depths of the soil, which spade or plow 
never reaches, influences are exhaled that intimately affect the produc¬ 
tiveness of the surface. Perhaps volatile vapors and secreted gases circu¬ 
lating through the inner pores of the soil, shed propitious qualities that 
