154 
OF GARDENING AS A SCIENCE. 
for the siistentation of plants, whether native or exotic, it is invaluable, as may be 
proved by many instances. It would be well if, by chemical analysis, any light 
could be thrown on the matter. Perhaps it may he some simple hody^ which may be 
artificially collected and applied with less trouble, and equal effect, as more ponde- 
rous materials." {^See British Farmers Mag.^ Aprils 1841, p. 93.) 
If any of our readers can retrace the astonishing effects of a loam, raised by the 
spade in trenching from a depth of nearly two feet, and which had evidently 
reposed upon a substratum of chalky and flinty marl, or other coarse stuff, undis- 
turbed for centuries, upon a crop of any of the cabbage tribe, he will be satisfied 
with tlie truth of Mr. Main's remarks. 
On such a loam, yellow or brown orange in colour, void of any traceable fibre, 
and to a demonstration free from a particle of any substance that could bear the 
name of manure, a plot of potatoes, cabbages, broccoli, and the like, the latter 
planted in July within three days, or in detail, row by row, as the trenching has 
advanced, have thriven with a rapidity, verdure, and luxuriance that nothing could 
excel. What then, we inquire, chemically, has been and is the exciting agent ? 
Such a loam is composed chiefly of insoluble sand, of perhaps one fourth of alumine, 
or the matter of pure clay, of oxide of iron (the colouring material) and generally 
of a small per-centage of chalk (carbonate of lime). Now any, or all of these, how- 
ever varying in their proportions, are little soluble in water ; but the loam so 
constituted, if Liebig's authority be taken in proof, invariably contains a proportion 
of vegetable alkali, potassa, of which substance, wherever it be found in the vege- 
table organization, it is the sole source and parent. Here, then, Mr. Main's most 
valuable suggestion is realized ; for a body, though not absolutely simple, a 
chemical agent, is discovered, which being extremely soluble, is most energetic and 
potential in its effects upon the vegetable fluids. It also exists in its most 
available form and proportion in virgin loams, and therefore we are justified in 
concluding that to potassa may be ascribed those luxuriant results which are the 
subject of inquiry. 
But such loam, however invaluable to vegetable culture, producing that flavour and 
purity which can never be obtained from manure in any form, will not avail in 
pot-culture ; it is altogether too binding and intractable. The principle, never- 
theless, remains in full force, and therefore the judicious gardener has recourse to 
the turf taken off the purest loam of a common or grass pasture ; this he lays up 
in mass, turns, incorporates, and finally uses with its fibrous remains. Hence he 
obtains the best soil in a form and temperament that will give freedom to the pro- 
gress of the roots, and yet has not lost one particle of those salts which play so 
important a part in the economy of vegetation. 
But does his soil become paler during a course of culture ? does it in any way 
present signs of impoverishment? Certainly not; it acquires depth of tint, it 
gains humus, and, after a time, the colour is darkened by several shades, proving 
that carhon has been deposited — not abstracted. Yet new soil is soon required ; 
