MANURES AND THEIR USES. 
203 
Besides furnishing the plant with its ash constituents, the 
root has the important function of supplying nitrogen, which it 
nearly always takes up in the form of nitrates. A plant is 
capable of making use of nitrogen in the form of nitric acid or 
ammonia. The faculty with which ammonia and other nitro- 
genous substances are converted into nitric acid in the soil is so 
great that nitrates become by far the most important source of 
nitrogen at a plant's disposal. 
Necessity for Manure. 
If we try to grow crop after crop on the same piece of land 
with no other preparation than digging or ploughing the land, 
and sowing the seed, we always find that after a greater or less 
number of years, according to the nature of the soil, the crop 
begins to show a great falling-off in yield, no matter how favour- 
able the seasons may be. Facts like these, which are well 
known to everybody, easily lead to the belief that crops take 
something out of the land. These substances — whatever their 
nature— are necessary to the fertihty of the land, and, far from 
being inexhaustible, the stock of fertility in an ordinary soil will 
not stand anything like a continuous drain. The fertility lost 
by the soil is actually due to the substances contained in the 
crop removed. This is made clear by the result of not removing 
the crop, but, instead, allowing it to rot on the ground or turning 
it in. When this is done, not only is the fertility of the soil not 
lessened, but it is generally increased, and this is one of the 
methods sometimes employed by gardeners and farmers to 
improve their land. 
For thousands of years it has been known to all people 
engaged in horticulture and agriculture that to restore the lost 
fertility to the soil it was not necessary to return the whole of 
the crops taken off, but that the manure of the animals which 
consumed the crops, if spread on the land and dug in, would 
suffice to make the soil nearly equal in productiveness to what 
it was before. By this it is very evident that the most im- 
portant ingredients given up by the soil to the crops are 
contained in the excreta of the animals fed upon it. 
From time immemorial it has been the custom to collect and 
to preserve the manure from all classes of farm animals, and to 
spread it upon the land periodically. 
