12 51
Humus 21 17 33
Magnesium carbonate 11 33 20

NOTE III. (p. 76).

HYGROSCOPIC POWER OF SOILS.

Davy found the hygroscopic power of soils to be as follows. He found
that 100 parts by weight of three samples of different sands absorbed 3,
8, and 11 parts of water, respectively, in one hour; while three loams
absorbed similarly 1.3, 1.6, and 1.8 parts.

The following samples of soil were dried at 212 deg. Fahr., and exposed to
an atmosphere saturated with water and a temperature of 62 deg. Fahr., when
it was found they absorbed the following amounts in twelve hours'
time:--

Quartz sand 0.0
Limestone sand 0.3
Lean clay 2.1
Fat clay 2.5
Clay soil. 3.0
Pure clay. 3.7
Garden-loam 3.5
Humus 8.0

NOTE IV. (p. 81).

GASES PRESENT IN SOILS.

The air which we find enclosed in the pores of the soil is distinctly
_poorer_ in oxygen than ordinary air. Boussingault found the percentage
of oxygen in a sandy soil, freshly manured and wet with rain, to be as
low as 10.35 per cent; while the air in forest-soil contained 19.5 per
cent of oxygen, and .93 per cent of carbonic acid. The percentage of
oxygen in soils depends on the rate of decay of the organic portions.
The depth of the soil-layer also determines the quantity. This is owing
to the fact that diffusion takes place more slowly deep down than near
the surface.

NOTE V. (p. 90).

AMOUNT OF SOLUBLE PLANT-FOOD IN THE SOIL.

Two of the most reliable methods of ascertaining an approximation of the
quantity of soluble soil constituents are (1) by treating the soil with
distilled water, and (2) by analysing the drainage-water. With regard to
the former of these two methods, it has been found that even the amount
of fertilising matter dissolved out by pure distilled water varies. This
variation depends on the amount of distilled water used, as well as the
length of time the soil is left in contact with the solvent. By washing
the soil with different quantities of water, different amounts of
soluble soil ingredients will be found to have been washed out; for
although the first washings contain by far the greater portion of the
soluble matter, each subsequent washing will be found to contain further
quantities.

A number of experiments have shown that