228 
INVESTIGATION OF SOILS. 
in water. Now, heath-mould contains little else than sand and vegetable mattei 
reduced from the plants which itself produces ; and therefore leaf-mould becomes 
inaj^propriate to the heath-tribes, which cannot generally flourish in richer soils, i 
In analysing heath and leaf-moulds, the latter will be found far the mort! 
complicated, but both are richer in humic substances than loams ; while the 
inorganic constituents of the latter, existing in the ashes after ignition, are verj; 
different from the simple white sand of the former ; and this circumstance, when 
fairly proved by experiment, will satisfy the inquiring gardener that he cannot in 
all cases substitute the one for the other ; though practice will clearly point oulj 
which of the two must be applicable to individual plants. 
There is one substance that cannot be deemed either earth or manure, bu 
which offers many advantages to pot-culture, and now begins to be appreciated 
Charcoal is this substance : it is quite insoluble ; it changes not, and is not liable 
to decay or putrefactive fermentation ; but it absorbs ammoniacal and other gaseSj 
to the extent of many times its own bulk, becomes also saturated with water, anc 
retains moisture pertinaciously ; hence, it forms the best sort of drainage, supplying 
the root with aqueous and gaseous matter, suitable to their habits, yet never 
producing change or decay. It is at once the most innocuous and useful o 
negative appliances, and may doubtless be added to soils in small fragments, as L 
preventive of aridity. ' j 
It is decomposed by ignition, and then is converted into carbonic acid gas I 
this gas is also developed constantly during the slow decay of vegetable remains 
and thus becomes a fertile source of nutriment to the roots, and perhaps to th( 
whole plant through the pores of the leaves. 
From what has been stated, we hope that some idea will be formed of th(| 
compound nature of soils, and of the necessity which exists for the rigid invest!' 
gation of every species of plant in order to obtain correct views of cultivation. 
It is now known, that, besides the earthy matters, soils contain four, if no 
five species of vegetable organic substances soluble in water, and the three alkalies 
these are with certainty derived naturally from vegetable decaying matter, thoug| 
in varying quantities ; they are termed by chemists, crenic, and apocrenic, humic 
geic, and ulmic acids : but if there be a deficiency of inorganic elements, th( 
material wanting must be confined by art, otherwise a plant which requires chalk 
cannot thrive in earth destitute of or ill-supplied with that earth ; another whicl 
thrives in soil containing phosphate of lime or soda will languish under opposifi 
circumstances. | 
These facts are decisive of the utility of chemical appliances, and they also 
account for the constant complaint which we hear of loam^ and of the difficult)! 
which one gardener meets with in cultivating some tribes that his neighbour oifi 
acquaintance raises in luxuriant perfection ; there are causes for all things, anc! 
some of these chemistry has discovered, and continues still to discover. 
We dwell more particularly upon the necessity of unremitting, strict researches 
