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AGRICULTURAL METEOROLOGY. 145 
ably; the deeper layers, in this case, drying more slowly, according to 
their degree of consistency, and to their greater or less contraction on 
drying. Dry soils, with a large proportion of clay, exhibit this variation 
in an especially striking manner. ; 
Diminution of bulk on drying.—Most ‘soils contract on drying, ard 
cracks and fissures ensue, which have an injurious effect on vegetation, 
as the finer roots, which frequently supply the bulk of nourishment, are 
either bared or torn asunder. 
1, Gypsum diminishes its volume in an inconsiderable degree. 
2, Fine carbonate of lime loses but little bulk, while clay exceeds it, 
and humus exhibits a remarkable degree of contraction. 
3. The addition of sand, or carbonate of lime to clay, diminishes this 
- property of contraction. 
4, Many kinds of mar! fall into small pieces on drying, because of the 
great difference which clay and lime, the elements of marl, experience 
in their diminution of bulk on drying, after having been moistened, 
these individual parts changing their volume in different degrees, and 
thus causing a more easy disintegration. 
5. Humus experiences, on drying, the greatest diminution of bulk, 
contracting at least one-fifth, and expanding again under the action of 
moisture. This is why the upper surface of the earth, in damp, turf 
bottoms, containing much humus, frequently rises or sinks several inches, 
according as the soil is penetrated with more or less water. The eleva- 
tion of these soils is more remarkable during a sharp frost, after wet 
weather, the freezing, by its expansion, still further increasing the vol- 
wine of the particles of water within the turf. Hence, too, the reason 
why these turf bottoms have, in their wet state, a remarkable elasticity 
if heavily trodden upon. 
Property of the earths to absorb moisture from the atmosphere.—Most of 
the earths which are commonly found in soils have the property, in their 
dry state, of absorbing moisture from the atmosphere, and this influ- 
ences, considerably, their different degrees of fertility. 
1. All soils, excepting siliceous sand, thus absorb moisture. Slaty 
mnarl, similar to the sands in some conditions, surpasses them in this 
respect; clay soils, especially those containing humus, absorb most 
freely. 
2. Humus, although possessing the greatest power of absorption, ex- 
hibits degrees of difference according to its kinds; purely vegetable 
humic acid absorbs moisture more freely than that obtained from animal 
manure. i 
3. The degree of absorption lessens as soils become saturated with 
moisture, which generally occurs in a few days. A portion of the ab- 
sorbed moisture becomes vaporized by the action of sunlight, and thus 
is performed a natural operation which exerts a very beneficial effect 
upon vegetation. The earths absorb, at night, moisture which they par- 
tially give off during the day. ‘ 
4, While fertile arable soils absorb moisture freely, this is not an in- 
fallible test of their properties, and the test requires much modification 
in its application. 
Property of carths to absorb oxygen gas from the atmosphcere.—Alexander 
Vou Humboldt, many years ago, pointed out this property of the earths, 
and experiment contirms it, always providing that the earths are in a 
moist condition. 
1. All the earths lose this property upon drying, and regain it as soon 
as they are moistened. 
2. aS exhibits it in the greatest degree ; the clays approach near- 
A 
