MANURES AND THEIR USES. 
205 
chemical analysis alone, and there are very many cases in which 
it is the only safe guide a purchaser can have. 
Again, if any new substance is proposed or brought forward 
as a manure, and chemical analysis shows it to be destitute of 
particular fertilising ingredients, we know at once it will be of 
no use to buy it or try it, and in this way are saved from throw- 
ing away time and money and losing the crops as well. 
Botation of Crops. 
It has been before stated that when the same crop has been 
grown year after year without manure on the same land, the 
falling- off is very rapid, and some crops refuse to grow at all 
after two or three years of such treatment, when the soil is said 
to be " sick " of the particular crop grown. 
By growing other crops for a few years this sickness, what- 
ever be its cause, is removed, and the original crop can be taken 
again. A judicious rotation of crops enables the land to recover 
from the partial exhaustion produced by previous crops, and in 
the arrangement of a suitable rotation the greatest care should be 
exercised, not only on farm lands but in gardens as well. 
It is, of course, quite possible, by very careful cultivation and 
suitable manuring, to grow the same crop for many successive 
years on the same garden plot, as has been done in the case of 
wheat and barley and roots with such marked success by Lawes 
and Gilbert, of Rothamsted ; but gardeners who do not limit 
themselves to the production of one particular plant will benefit 
by arranging such a rotation that each succeeding crop may not 
require for its main support the same substances, or in the same 
proportion as were removed by the previous crop grown. Crops 
of the most opposite class ought, as much as possible, to 
alternate with one another, and each particular plant should be 
repeated as seldom as possible on the same plot of land. It is 
constant variety of crop, with suitable manuring, that will yield 
the largest total produce with the most economy, whatever the 
rotation may be. 
Classification of Manures. 
The various manures or plant-foods used in this and other 
countries for returning to the soil the ingredients lost by 
cropping have been classified by Drs. Stockhardt and Voelcker as 
follows : — 
F 
