The only available material for a mulch in most instances is the soil itself. Experience has taught that when the top layer of soil, to a depth of two or three inches, is made fine and loose, the water beneath it cannot escape readily. It is partly for this reason that the smoothing-harrow should follow the roller after land has been plowed. The plow is used to break up the soil into crumbs that will permit air to enter. The loosening is excessive when the planting must follow soon, permitting rapid escape of water. The roller or plank-drag is employed to compress the soil, and to crush crumbs of soil that are too large for good soil conditions. The harrow follows to make a mulch of fine, loose soil at the surface to assist in prevention of evaporation.
A sandy soil will retain its mulch in effective condition for a longer time than a fine clay, if no rain falls. When the air is laden with moisture, clay particles absorb enough water to pack together and form an avenue for the rise of water to the surface, where the dry air has access to it.
Mulches of Foreign Material.—The truth that moisture is a leading factor in soil productiveness is evidenced by the value of straw and similar material as a mulch. A covering of straw around trees in an orchard, or bush fruits, or such plants as the potato, may give better results than an application of fertilizer when no effort is made to prevent the escape of water. People so situated that little attention can well be given to the fruit and vegetable garden obtain good results by replacing tillage with a substantial mulch that keeps the soil mellow, prevents weed growth, and retains an abundant supply of water.
In grain-producing districts where all the straw is not needed as an absorbent in the stables its use as a mulch on thin grass lands, or wheat-fields seeded to grass, is more profitable than conversion into manure by rotting in a barnyard. The straw affords protection from the sun, and aids in the conservation of soil water, when scattered evenly in no larger amount than two tons per acre, and a less amount per acre has value. The sod is helped, and as the straw rots, its plant-food goes into the soil.
Plowing Straw Down.—The practice of plowing straw under as a manure is unsafe, when used in any large quantity per acre. It rots slowly, and while lying in the bottom of the furrow it cuts off the rise of water from the subsoil which is a reservoir of moisture for use during drouth.
The Summer-fallow.—Bare land loses in total plant-food, but may make a temporary gain in available fertility. The practice of leaving a field uncropped for an entire season has been abandoned in good farming regions. Where moisture is in scant supply, and a soil is thin, there continue instances of the summer-fallow. In a crop-rotation containing corn and wheat, the corn-stubble land is left unbroken until May or June, and then plowed. In August it is plowed again, and fitted for seeding to wheat. The practice favors the killing of weeds, and the soil at seeding time may contain more water than would have been the case if a crop had been produced, because its mellow condition enables the farmer to hold within it nearly all the moisture that a shower may furnish after the second plowing.
The Modern Fallow.—The modern method of making a grass seeding in August partakes of the nature of the old-fashioned summer-fallow. The desire is to eradicate weeds, secure availability in plant-food, and fit the soil to profit by even a light rainfall. Thin soils lend themselves well to this treatment, which is described in Chapter VIII, and there is no better method for fertile land. The benefit of the fallow is obtained without serious loss of time.
CHAPTER XXIII
DRAINAGE