There is always a current of air passing from the lower to the upper end of a well constructed drain; and this air is always cooler in warm weather, when it issues from, than when it enters the drain. Its lost heat is imparted to the soil.
How does it hasten the decomposition of roots and other organic matter in the soil?
How does it accelerate the disintegration of its mineral parts?
Why is this disintegration necessary to fertility?
This heating of the lower soil renders it more favorable to vegetation, partially by expanding the spongioles at the end of the roots, thus enabling them to absorb larger quantities of nutritious matters.
4. Under-draining hastens the decomposition of roots and other organic matters in the soil, by admitting increased quantities of air, thus supplying oxygen, which is as essential in decay as it is in combustion. It also allows the resultant gases of decomposition to pass away, leaving the air around the decaying substances in a condition to continue the process.
This organic decay, besides its other benefits, produces an amount of heat perfectly perceptible to the smaller roots of plants, though not so to us.
5. Draining accelerates the disintegration of the mineral matters in the soil, by admitting water and oxygen to keep up the process. This disintegration is necessary to fertility, because the roots of plants can feed only on matters dissolved from surfaces; and the more finely we pulverize the soil, the more surface we expose. For instance, the interior of a stone can furnish no food for plants; while, if it were finely crushed, it might make a fertile soil.
Any thing, tending to open the soil to exposure, facilitates the disintegration of its particles, and thereby increases its fertility.
How does under-draining equalize the distribution of the fertilizing parts of the soil?