19. _The formation of crust on the surface of the soil_ is due to the evaporation of water, which is drawn up from below by capillary attraction. It arises from the fact that the water in the soil is saturated with mineral substances, which it leaves at its point of evaporation at the surface. This soluble matter from below, often forms a very hard crust, which is a complete shield to prevent the admission of air with its ameliorating effects, and should, as far as possible, be avoided. Under-draining is the best means of doing this, as it is the best means of lessening the evaporation. The foregoing are some of the more important reasons why under-draining is always beneficial. Thorough experiments have amply proved the truth of the theory. [What kinds of soil are benefited by under-draining?] The _kinds of soil benefited by under-draining_ are nearly as unlimited as the kinds of soil in existence. It is a common opinion, among farmers, that the only soils which require draining are those which are at times covered with water, such as swamps and other low lands; but the facts stated in the early part of this chapter, show us that every kind of soil--wet, dry, compact, or light--receives benefit from the treatment. The fact that land is _too dry_, is as much a reason why it should be drained, as that it is _too wet_, as it overcomes drought as effectually as it removes the injurious effects of too much water. All soils in which the water of heavy rains does not immediately pass down to a depth of at least _thirty inches_, should be under-drained, and the operation, if carried on with judgment, would invariably result in profit. [What do English farmers name as the profits of under-draining? What stand has been taken by the English government with regard to under-draining?] Of the precise _profits_ of under-draining this is not the place to speak: many of the agricultural papers contain numerous accounts of its success. It may be well to remark here, that many English farmers give it, as their experience, that under-drains pay for themselves every three years, or that they produce a perpetual profit of 33-1/3 per cent., or their original cost. This is not the opinion of _theorists_ and _book farmers_. It is the conviction of practical men, who know, _from experience_, that under-drains are beneficial. The best evidence of the utility of under-draining is the position, with regard to it, which has been taken by the English national government, which affords much protection to the agricultural interests of her people--a protection which in this country is unwisely and unjustly withheld.