115 
this refers only to the carbonates found in the ashes. Alkaline 
bases, existing in the ashes of plants, must therefore be neces- 
sary to their growth, since if this were not the case, they 
would not be retained. 
All grasses contain the acid silicate of potash. And in 
com fields this salt does not vary much, because it is carried 
bacWn the putrefying straw. But this is not the case in a 
meadow, and hence we never find a luxuriant crop of grass 
on sandy and calcareous soils, which contain little potash, 
evidently because one of the constituents indispensable to the 
growth of grasses is wanting. Wheat grown in a rich soil 
of mouldered wood never comes to perfection, but droops 
prematurely. The cause is this : the strength of the stalk 
is due to silicate of potash, and the corn requires phosphate 
of magnesia, neither of which a soil rich in humus can 
afford. 
Phosphate of Magnesia, in combination with ammonia, is 
an invariable constituent of the seeds of all kinds of grasses. 
I have already spoken of potash, when treating of soils, 
which may be replaced in many cases by soda, magnesia, 
or lime; but there are other substances besides alkaline 
required to sustain the life of plants, and one is phosphoric 
acid, which has been found in the ashes of all plants hitherto 
examined, and always in combination with alkalies or alkaline 
earths : it is a constituent of all land capable of cultivation, 
and plants yield it to animals, to be used in the formation of 
their bones. Some plants, however, extract other matters 
from the soil besides silica, potash, and phosphoric acid, 
which are essential constituents of the plants ordinarily cul- 
tivated : these are common salt, sulphate of potash, nitre, 
chloride of potassium, and other matters as necessary consti- 
tuents. 
The leguminosoe, turnips, and potatoes, require only small 
quantities of potash. 
