It is thus seen that the percentage of the important plant-foods in stable manure are minute when compared with those commonly found in “commercial” fertilizers. Nor are they so much more available for plant absorption than the latter; a very large proportion is not utilized at all the first year, and unless the amount applied is very large it hardly carries the supply needed for the usual crops.
It is now well understood that its efficacy is largely due to the important physical effects it produces in the soil. It helps directly to render heavy clay soils more loose and readily tillable. If well “rotted” or cured it also serves to render sandy, leachy soils more retentive of moisture; and the humus formed in its progressive decay imparts to all soils the highly important qualities discussed later on ([chapt. 8]). More than this, the later researches have shown that stable manure acts perhaps most immediately upon the bacterial activity in the soil, greatly increasing it not only directly by the vast numbers of these organisms it brings with it, but also in supplying appropriate food for those normally existing in the soil ([see chapt. 9]). In so doing it serves indirectly to render the soil ingredients more available, and to impart to the soil the loose condition required in a good seed-bed—a “tilth” which cannot be brought about by the operations of tillage alone.
The only possible substitute for the use of stable manure is found in green-manuring with leguminous crops conjointly with the use of commercial or mineral fertilizers. Unless this is done the use of the latter, alone, ultimately leads to a depletion of humus substances, which renders the acquisition of proper tilth by the seed-bed impossible, and causes a compacting of the surface soil which no tillage can remedy.
Proper method of using stable manure in humid and arid climates.—In the humid region it is a common practice to spread the stable manure on the surface of the fields and leave it there without any special operation to put it into the soil; trusting to the rains, earthworms and subsequent tillage for its being brought into adequate contact with the roots; it is rarely plowed in. In the arid region this mode of using it is impracticable; it would remain on the surface indefinitely without advancing in its decay because of the dryness, and unless plowed in very deep the ordinary, strawy manure would ruin the seed-bed by rendering it too pervious to the dry air, thus preventing germination. Much of this valuable material has therefore been, and to some extent is still being burnt, thus causing a severe depletion of the land, both of humus and of mineral plant-food. The best way to deal with stable manure in the arid regions is to thoroughly rot or cure it before putting it on the land, and then plowing it in. To do this of course it must be put in piles and wetted regularly; a procedure which at the high prices of labor is thought to be too expensive, but which in the end would be found eminently profitable, unless green-manuring is regularly done. The very small proportion of humus generally present in arid soils renders this precaution indispensable, if production and proper tilth is to be maintained. The saving of stable manure and of all composting material, even if less needful as a means of supplying plant-food in the rich soils of the arid regions, is fully as essential in order to maintain the humus supply.
(B.) MINERALS UNESSENTIAL OR
INJURIOUS TO SOILS.
The minerals heretofore mentioned contribute to soil formation either one or several ingredients, important to plant growth either by their mechanical or chemical action. It remains to consider some not intrinsically desirable, but frequently present in certain soils, which should be known to the farmer in order that he may be enabled to counteract or remove their injurious effects. Leaving aside such as are of only casual or rare occurrence, the following may be mentioned as among those which not unfrequently affect soils desirable for culture to such extent as to make them unavailable for general farming purposes:
Iron Pyrite; sulphid of iron containing two molecules of sulphur to one of iron; a mineral exceedingly common in deposits of metallic ores, and whose deceptive gold-like color has caused it to be mistaken for gold so often as to cause it to be designated as “fool’s gold” among miners. While it frequently does contain some gold and is often associated with valuable ores, it is practically valueless when occurring outside of mineral veins, in rock-masses; and more especially in sedimentary rocks, such as sandstones, limestones, shales and clays.
When present in soils it sometimes becomes a source of trouble to the farmer, because in contact with air it is soon transformed into ferrous sulfate or copperas, which, like the carbonate referred to above, is injurious to plants. Sometimes indeed iron pyrite is actually formed in badly-drained soils alongside of the carbonate of iron, when much sulfate (such as gypsum) is present; and then its injurious effects subside more slowly than do those of the carbonate ([see above, p. 46]).
Recognition of Iron pyrite.—The mineral is easily recognized by its golden or brass-yellow tint; the latter color being the one most commonly shown in the “sulphur balls” occurring in marls or soft limestones. A very easy test is to pulverize it and then heat it on a shovel over a fire, when it will soon itself take fire, burning with a blue sulphur flame, and upon more complete roasting, leaving behind a red powder, viz., “Venetian red” or red ochre; that is, ferric oxid. In clays it commonly occurs in large, well-defined cubes, which do not readily form copperas but rather become covered with a crust of limonite or brown iron ore.
When a subsoil is found to contain pyrite, or when “sulfur balls” have been accidentally introduced with dressings of marl, the remedy is thorough and persistent aeration of the material. In the case of marls nothing more need be done; but in that of ill-drained subsoils it is best to add lime in moderate dressings, to accelerate the transformation into ferric hydrate or iron rust, and gypsum; whereby the copperas becomes not only innocuous but adds two beneficial ingredients to the soil. The same policy will render available manure or other materials which have been disinfected by means of solution of copperas.