_Influence on plant food in the soil._ Covered or under drains check losses of plant food that occur with surface and open ditch drains. They render available more plant food, for lowering free water and increasing ventilation: Deepen the feeding area of the roots. Aid the process of nitrification. Aid chemical changes which make plant food available. Check denitrification. LOCATION OF DRAINS As gravity is the force that is to take the surplus water from the soil, the outlet of the drainage system should be at the lowest part of the area to be drained. [Illustration: FIG. 83.--CROSS-SECTIONS OF STONE-DRAINS.] [Illustration: FIG. 84. _A._ Cross-section of a pole-drain. _B._ Cross-section of a tile-drain.] [Illustration: FIG. 85.--A COLLECTION OF DRAINAGE TOOLS.] [Illustration: FIG. 86. _A_ represents a poorly laid tile-drain. It is poorly graded, and has partly filled with soil. It has lost more than half its water carrying capacity. _B_ was properly graded, and has kept free from sediment.] The main drains should be located in the lowest parts of the fields, indicated by courses taken by water after a rain or by small streams running through the farm. The lateral drains, if surface or open ditch drains, should run across the slopes; if under drains, they should run up and down the slopes. _Grade or slope of the drain._ The grade of the drain should be sufficient to cause a flow of the water. In the case of open ditches it should not be steep enough to cause too rapid a current and a consequent serious washing of the banks of the ditch. Large, deep ditches will carry water with a grade of one inch to a hundred feet. _Tile drains._ Covered or under drains are made of brush, poles, planks, stones, tiles, etc. (Figs. 83-84). Where tiles can be obtained at reasonable prices they are considered best. Tiles are made of clay and are burnt like brick. They are more lasting than wood and are easier and cheaper to lay than stone, unless the stone must be gotten rid of. The most approved form of drain tile is the round or circular form. These are made in sizes ranging from two and one-half to six and eight inches in diameter, and in pieces one foot in length. The size used depends on the length of the drain, the amount of water to carry, the frequency of heavy rainfalls and the character of the soil. The distance apart varies from twenty-five feet in heavy soils to over two hundred feet in light soils. The usual depth is about three feet, though the farther apart the deeper they are put. A lateral tile drain should enter a main at an acute angle to prevent too great a check in the current. In putting in a drainage system the first thing to be done is to make a plan of the ground and determine the slope of the land and the grade of the drain. The ditches are then staked out and the digging proceeds. In digging the ditches plows are sometimes used to throw out the top soil, then the work is finished with spades and shovels.