186 Causes of Fertility or Barrenness of Soils. 
up in the vegetable system, in union with organic matter ; we 
shall therefore consider each shortly. 
Silica, an oxide of the element silicon, possessing slightly 
acid properties, occurs pure in quartz crystals and many sand- 
stones ; forming the principal ingredient of all sandy soils, and 
being insoluble and destitute of all valuable physical properties, 
their poverty bears an exact ratio to purity. It is in combina- 
tion with various bases, such as alumina, magnesia, lime, and the 
alkalies, that silica becomes an important element in soils. The 
straw of cereal crops contains a large quantity, giving strength 
and hardness. When wheat is too frequently repeated on a poor 
soil, the straw becomes weak, and goes down before the corn is 
ripe, because sufficient available silica is not to be had. The 
silicates are generally insoluble, and it is probably by their de- 
composition in the soil, that the silica, soluble in its nascent 
state, is taken up by the moisture present in the soil, and pre- 
sented to the roots of plants. We say this is probable, because 
we can effect similar changes in the laboratory ; but we should 
always remember that there are many counteracting influences in 
the soil, which may interfere with the result. In the most 
fertile soils, we find the largest amount of silicates united with 
alkaline bodies, in which form it is most readily decomposed. 
Lime is supposed to possess the property of setting silica free ; 
whether this action is confined to quicklime only is still unde- 
cided. Chemistry here teaches the reason for rotation of crops, 
which practice had found necessary ; the wheat crop, which takes 
most silica, is seldom taken above once in four years on poor land, 
and we precede and follow it with crops which remove very small 
quantities of this important substance. Silicates exist more 
abundantly in clays than in most sandy soils, consequently we 
find the wheat crop more frequently repeated : wheat after the 
fallow, is followed by beans, succeeded by wheat again ; a rota- 
tion which would be impossible on sandy soils. Of course this 
difference is not to be traced to the silica only; many other sub- 
stances are equally required, but none are more important. 
Alumina is an oxide of the metal aluminium, possessing basic 
properties : it occurs abundantly in nature, forming, in a crystal- 
lized state, many of the precious stones ; but its principal form 
is common clay, which is a silicate of alumina. It is not known 
to enter into the composition of plants, yet it must be regarded 
as the most important constituent of soils ; partly due to its 
physical properties, and })artly to the extraordinary affinity it 
manifests for gaseous substances, especially ammonia and carbonic 
acid gas. Pure clay like pure sand would form a barren soil ; 
fortunately such do not exist in nature — clay being accompanied 
by various other substances, the relative proportions and combi- 
