542 
On the Food of Phiiifs. 
plant ; otherwise we must make it into carbonate, the use of which 
will be extremely wasteful. 
The principal mixed manures are : — 
Bones — an exceedingly valuable manure, although of necessity 
costly : bones contain nearly half their weight of animal matter 
exceedingly rich in nitrogen, which slowly decays, giving rise to 
ammonia ; the rest being phosphate of lime with a little carbonate. 
Hence, when observing the wonderful effects of bone-dust on cer- 
tain crops, we can be at no loss to account for what happens. 
The use of this substance has been mostly confined to turnips; 
so far as I have heard, its effects on other plants, such as wheat, 
which requires large quantities both of azote and phosphoric acid, 
are by no means so remarkable. Is this fact connected with some 
acid excretion from the roots of the turnip, by the aid of which 
the phosphate is dissolved ; and which does not take place to so 
great an extent in other plants ? and could we not, as Liebig 
himself proposes, treat the bones first with dilute mineral acid, • 
and so increase the effect ? 
Fish have been used in many localities by the sea- side with a 
good result ; they contain much albuminous matter and oil, the 
use of which is not however very clear. Rape and linseed cake 
and " graves," or the refuse of the tallow-melter, bits of mem- 
brane impregnated with fat, belong to the same list. Hair, 
woollen rags, feathers, dried blood, animal flesh, and such like 
substances, are all extremely valuable, but difficult to get in any 
quantity ; they diflcr very much in the rapidity with which they 
undergo decomj)osition, the effect of the two last-named being 
much more sudden and transient than that of the former. 
When a soil is deficient in humus, the use of which seems to 
be to furnish by its ceaseless decomposition a source of carbonic 
acid to the roots of the plant, more particularly valuable in the 
early part of its life, peat earth, especially when brought into a 
state of decay by mixing it with a portion of stable -manure, will 
prove an useful addition. It has often struck me that the dead 
i leaves which encumber towards winter our parks and woodlands, 
collected in the latter j)art of the autumn, and either ploughed 
into the ground at once, or first made into a compost heap, would ^ 
j)rove of great service ; they would furnish nitrogen, and humus, 
and silicate of potash, and many other salts, all highly useful. 
There now remain but two other manures which deserve parti- 
cular notice, namely, the "guano" of the South Seas and the 
nitrates of potash and soda. The first-named substance is the 
])artly decomposed excrement of birds, accumulated in beds of 
almost incredible thickness on some of the uninhabited coasts and 
islands of the Southern Pacific, consisting, according to an old I 
analysis of MM. Fourcroy and Vauquelin, of uric acid, partly 
