CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 339 
ric acid, while the grain of the wheat yields 4 5 per cent, 
phosphoric acid and only 1 per cent, silica; hence silica is 
rapidly removed from the soil by the straw, and phosphoric 
acid by the grain. 
Now, if we grow wheat many years in succession, and make 
no return to the earth, it must sooner or later become en¬ 
tirely exhausted of these substances; or if only one, the 
straw, for example, the soil must in time become sterile from 
a want of soluble silica. 
Animal excrements are derived from the vegetables or 
grain on which they feed ; if these are rich in alkalies or phos¬ 
phates, we may call the manures rich, because they produce 
a large amount of certain crops. 
Guano, which is rich in phosphates, has been used with 
great success on some soils, while on others it is almost inert; 
on the former because the phosphates were exhausted, whilst 
on the soil less deficient it has but little power, especially if 
the other elements of plants are likewise deficient. 
There are lands in Peru which have become perfectly 
sterile from the constant use of guano; the alkalies, potash 
and soda, being completely exhausted, and these lands have 
again become fertile by their application (Playfair). In al¬ 
most all cases the continued application of only one or two 
ingredients must tend to impoverish the soil; in fact, the only 
means of keeping up the fertility of the soil is by replacing 
on the ground all the mineral ingredients taken out of it. 
The land is subject to losses by the exportation of grain 
and cattle, which contain principally phosphates , and many a 
farm has been completely exhausted of these ingredients, and 
in consequence become nearly sterile. But even if it were 
possible for plants to perfect their seeds or roots without the 
{f inorganic elements,” iron, soda, potash, lime, phosphorus, 
sulphur, &c., it is evident such seeds or roots would not supply 
the wants of animals ; for no blood could be formed without 
iron, nor bones without phosphorus and lime ; no bile with¬ 
out soda ; hence an all-wise Providence has so constituted the 
vegetables that they can arrive at perfection and maturity 
only in soils containing these inorganic elements. Thus 
wheat, corn, oats, or other plants yielding these articles of our 
daily food, cannot ripen a single seed or root in a soil totally 
deficient in any one of the following substances, viz., sulphur, 
phosphorus, lime, magnesia, iron, soda, or potash. 
We will now turn our attention from the vegetable to the 
animal kingdom. 
The following antithetical table has been drawn up by 
Dumas: 
