10 
NATURE AND OFFICES OF EARTHS AND SOILS. 
degree of accuracy is required which can only be obtained by constant practice 
founded upon scientific principles. " The following is the analysis of a fertile soil 
in the neighbourhood of Bristol ; in 400 grains, there were of water, 52 ; silicious 
sand, 240 ; vegetable fibre, 5 ; vegetable extract, 3 ; alumine, 48 ; magnesia, 2 ; 
oxide of iron, 14 ; calcarious earth, 30 ; loss, 6. On the utility of analysis Dr. Ure, 
(Diet, of Chem.) observes, that " no system can be devised for the improvement 
of lands independently of experiment ; but there are few cases in which the labour 
of analytic trials will not be amply repaid by the certainty with which they denote 
the best methods of melioration, and this will particularly happen when the defect 
of composition is found in the proportions of the primitive earths. In supplying 
organic matter, a temporary food only is provided for plants, which is in all cases 
exhausted by means of a certain number of crops ; but when a soil is rendered of the 
best possible constitution, and texture with regard to its earthy parts, its fertility 
may be considered as permanently established. It becomes capable of attracting a 
very large portion of vegetable nourishment from the atmosphere, and of producing 
its crops with comparatively little labour and expense/' 
3. Of the Uses of Earths. Pure earths, " exclusively of organised matter and 
water, are allowed by most physiologists to be of no other use to plants than that of 
supporting them, or furnishing a medium by which they may fix themselves" in a 
situation favourable to their future growth. " But earths and organic matter, that 
is, soils, afford at once support and food." Thus the pure earths may be considered 
as the mechanical agents in the soil. They consist chiefly of metallic bases united 
to oxygen, not readily decomposable ; and consequently they cannot be reasonably 
supposed to be convertible into the elements of organised matter, which, as has been 
stated, are chiefly found to be oxygen, hydrogen, carbon, and azote. Plants, it is 
true, consume a small portion of the earths they grow in, as is discoverable by 
burning, for their ashes are found to contain earths ; but the quantity has been 
ascertained never to equal more than a fiftieth part of the weight of the plant con- 
sumed. <£ The earthy parts of the soil are chiefly useful in detaining water, so as 
to supply the proper proportions to the roots of the vegetables, and they are likewise 
efficacious in producing the proper distribution of the animal or vegetable matter." 
The earths, when duly mixed with such matter, prevent it from decomposing too 
rapidly, and regulate the supply of its soluble parts in proper proportions to the 
roots of the plants. The earths are also " necessary to the existence of plants, 
both as affording them nourishment, and enabling them to fix themselves in such a 
manner as to obey those laws by which their radicals are kept below the surface, 
and their leaves exposed to a free atmosphere." 
4. The due tenacity and coherence of the soil arise from the finely-divided 
matters of its constituent parts, " and they possess the power of giving those qua- 
lities in the highest degree, when they contain much alumina," (pure clay.) " A 
small quantity of finely-divided matter is sufficient to fit a soil for the production of 
turnips and barley ; and a tolerable crop of turnips has been produced on a soil con- 
taining eleven parts out of twelve sand ; a much greater proportion of sand, how- 
ever, always produces absolute sterility." Tenacity is obtained by certain propor- 
