14. It keeps off the effects of cold weather longer in the fall.
15. It prevents the formation of acetic and other organic acids, which induce the growth of sorrel and similar weeds.
16. It hastens the decay of vegetable matter, and the finer comminution of the earthy parts of the soil.
17. It prevents, in a great measure, the evaporation of water, and the consequent abstraction of heat from the soil.
18. It admits fresh quantities of water from rains, etc., which are always more or less imbued with the fertilizing gases of the atmosphere, to be deposited among the absorbent parts of soil, and given up to the necessities of plants.
19. It prevents the formation of so hard a crust on the surface of the soil as is customary on heavy lands.
How does under-draining prevent drought?
1. Under-draining prevents drought, because it gives a better circulation of air in the soil; (it does so by making it more open). There is always the same amount of water in and about the surface of the earth. In winter, there is more in the soil than in summer, while in summer, that which has been dried out of the soil exists in the atmosphere in the form of a vapor. It is held in the vapory form by heat, which acts as braces to keep it distended. When vapor comes in contact with substances sufficiently colder than itself, it gives up its heat—thus losing its braces—contracts, and becomes liquid water.
This may be observed in hundreds of common operations.