49 
described, and in this way we could find out which of the sub- 
stances was necessary to the well-being of plants. By a series 
of such experiments we should find that the nitrogen and all the 
substances in the ash except the silica were absolutely necessary ; 
if even one of these was left out of the artificial soil the young 
plant would grow only so long as the food originally contained in 
the seed lasted ; after that was used up the plant would cease 
to grow, or if it put forth new leaves it would be by absorbing 
the substance of the old ones ; it would not increase in weight or 
size. This would occur in the case of the omission of any of the 
substances excepting the iron. The omission of the iron would 
cause a somewhat different appearance. The plant would grow 
to a considerable extent, even after it had used up all the food 
originally contained in the seed, but it would be white and colour- 
less, and would ere long perish. From such experiments we 
should obtain results which would show us that all the above 
substances except the silica must be presented to the plant 
through the soil, and if the soil does not contain sufficient of 
them they must be supplied in the form of manure. 
Shall we then have to arrange for the supply of these nine 
plant foods to the soil? If we have to do this then the art of 
manuring may become a somewhat complicated business. But 
fortunately the matter is very much simplified for us by the fact 
that some of these plant foods are already contained in all natural 
soils in practically inexhaustible quantities. Take, for instance, 
the oxide of iron: An ordinary crop would remove from an acre 
of land about 2 lbs. of this substance, but the amount naturally 
present in an acre of average soil 2 feet deep is from 50 to 250 
tons. Take, again, the sulphuric acid: An ordinary crop would 
remove 9 lbs. or 10 lbs. of this substance from an acre, but the 
amount naturally contained in an acre of soil 2 feet deep is from 
83 to 10 tons—sufficient to last for several centuries. But take, 
however, the case of nitrogen: The amount of this substance 
removed from the soil by an average crop is 80 lbs. per acre. 
Now, I have known many soils which have not contained more 
than 24 tons of nitrogen in an acre 2 feet deep ; and this, drawn 
upon at the rate of 80 lbs. a year, would be wholly exhausted 
in 70 years. But many years before it became exhausted the 
crops grown on such @ soil, unless the nitrogen were replenished 
from some other source, would, cease to be profitable—in fact, it 
may be taken as a general rule that unless the nitrogen in a soil 
exceed 5 tons per acre 2 feet deep crops cannot be grown profit- 
ably without manuring. : 
If we glance over the list of substances which enter into the 
composition of plants we find that those essential substances 
which it absorbs from the soil in greatest quantity are nitrogen, 
potash, lime, and phosphoric acid ; and of these four substances 
826. » 
