188 Causes of Fertility or Barrenness of Soils, 
pliere uniting with the hydrogen set free in its nascent state. 
During this process it is probable that various organic acids are 
formed ; passing rapidly one into another, without entering into 
plants as such. Lime removes the acidity often found in vege- 
table soils, either by destroying the acid, or combining with it 
to form an organic salt. Were the action of lime restricted to 
vegetable matter only, it could not fail to pi'ove a most valuable 
application ; but its relation to the mineral matters in the soil is 
perhaps more important still. In most stiff soils the alkalies are 
found united with silica and alumina, in certain proportions, 
mostly insoluble and therefore useless. Rain water, containing 
carbonic acid, might gradually dissolve out portions, sufficient 
for a natural condition, but inadequate to the artificial require- 
ments of cultivation. Lime appears to possess the power of 
setting free the alkalies and magnesia from their insoluble con- 
dition, probably replacing them ; and what seems rather extra- 
ordinary, is, that when these substances are added to the soil and 
would pass away too rapidly and be lost, by some means not 
yet clearly understood lime possesses the property of fixing them 
as insoluble compounds, causing their union with those very 
substances from which it had previously displaced them. Any 
attempt at explanation of these remarkable changes would be 
out of place here ; but should the present discoveries be con- 
firmed by further investigation, a most important fact must 
follow, viz., the advantage of repeated applications of small 
quantities of lime, and the wastefulness of the old system of 
heavy dressings. Lime enters into the composition of most 
crops, but the quantity required for this purpose is so small, and 
the natural supply in most soils so abundant, that we can 
hardly attribute the effects of its application to this cause. From 
all these facts, we should expect to find limestone soils a very 
fertile class, and when the other essential elements of fertility 
are present, such is the case. We are not certain that lime as 
carbonate acts in the same manner as in the caustic state ; that 
its application to soils light and heav3', mineral and l)eaty, has 
been found beneficial is undoubted. Its influence may partly be 
ascribed to physical causes, making stiff clays more workable, 
sands more absorbent, and giving firmness to peaty soils. 
Iron, being found in most crops, is an important ingredient of 
soils. It generally exists as peroxide, giving a red appearance. 
The protoxide appears to exert an injurious influence on vege- 
tation ; it is found in the subsoils of stiff undrained clays, giving 
the yellow tinge. On exposure to the atmosphere it absorbs 
oxygen, becoming peroxide. When such subsoils have been 
brought to the surface too suddenly, great injury has resulted, 
which is generally attributed to the protoxide of iron. 
