This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
In Attempting to accomplish the object sought in this work - i.e., that of embodying in one publication as nearly as possible all that is available and valuable information in the potato field - the subject of drainage is held to be of such importance that considerable space is devoted to it. The following article is made up of extracts from "Bulletin 199" of the Agricultural Experiment Station of the University of Wisconsin and is by E. R. Jones, one of America's foremost drainage experts:
Of the several conditions which influence the growth of crops none is more important than the amount of water in or on the soil. While water in a thin film around the soil grains is absolutely essential to plants, an excess is as bad as a deficiency. The removal of this excess is known as land drainage. Surface drainage deals with the surface runoff, and under-drainage with the water which occupies the spaces between the soil grains. Most land has some natural drainage, but many acres have it to such a limited degree that an improvement therein is found profitable. "Too much water is detrimental because: "1. It makes areas so soft that they cannot be cultivated. When these soft areas are long and narrow in form, they cut the upland into irregular pieces that cannot be cultivated conveniently.
2. It delays cultivation, particularly in the spring.
3. It makes soils cold: (a) because in the spring more than half of the heat that the soil receives is used to warm this unnecessary water; (b) because its evaporation consumes heat that the soil could otherwise retain; and (c) because its presence in the soil prevents the entrance and downward movement of rainwater, which in the spring is usually warmer than the soil.
4. It crowds out the oxygen from between the soil grains, thus hindering the necessary decomposition of organic matter in the soil.
5. It prevents all crop growth where it stands on the soil to a sufficient depth. Where it stagnates only a few inches from the surface of the soil it prevents healthy root development below that depth. The shallow root system thus developed limits the depth from which the plant may get water, and with it plant food material.
The occurrence of an excess of water in a soil or on an area is an indication that some source supplies water faster than it can be removed. The water is either coming too fast or it is going too slowly. Areas at the foot of uplands from which numerous small or large springs run during the greater portion of the year owe their wetness to the excessive seepage from the upland; while retentive clays, due to the fineness of the soil, and flat muck or peat marshes, due to lack of fall, are too wet primarily because the water is very slowly removed from them.
It is evident that the drainage conditions on an area may be improved either by hindering the entrance of damaging water upon one side, or by facilitating its removal from the other. To accomplish one or both of these improvements drains must be constructed which will give gravity a better opportunity to remove surplus water.
When the drainage needs of our lands are analyzed it is evident that those that have good drainage owe it to:
1. Some natural condition that prevents the entrance of an excessive amount of water;
2. A valley or ravine to serve as an outlet for the water that does enter;
3. A surface slope to allow the escape of surface water to the outlet, or
4. A subsoil sufficiently porous to admit of some under-drainage.
Consequently, improvements in drainage conditions consist of:
1. Protection ditches in the absence of natural protection;
2. Outlet ditches where there are no valleys or ravines;
3. Surface ditches to aid the limited slope in removing surface water, or
4. Covered under-drains to facilitate the removal of damaging water from wet subsoils. Some areas need only one of these types, while others need them all.
When water oozes into the dead furrows and shallow ditches until they are kept wet almost continually it is an indication that the land needs tiling. The water table, instead of extending horizontally from a tile, bends upward at a slope that increases with the retentiveness of the soil. It is evident that laterals may be farther apart in sand than in clay, and that the deeper the laterals are the farther apart they may be. Four rods is a common interval in clay subsoils and eight rods in open subsoils. In muck or peat it is frequently best to put them eight rods apart at first, and if that does not prove to be close enough together an intermediate line may be put in later in each space, making them eventually four rods apart. In rare cases of springy soils it has been found necessary to have lines of tile two rods apart.
A single line of tile in a wet sag is frequently sufficient, but if more than four rods wide, two lines are better, each to be located as near to its side of the sag as seems necessary. In this way the centre of the sag, unless it is exceedingly low, will be protected from the seepage of the adjacent upland.
Mains are generally located parallel to a natural water course - a little to one side if there is danger of washing by the surface runoff. Sub-mains should be so laid out as to give the laterals a sufficient gradient without an excessive depth.
Instead of permitting each lateral to discharge directly into the outlet ditch, it is best to put in a main perhaps four rods away from the ditch and parallel to it, to receive the discharge from the laterals. The expense is but little greater because of the saving in the length of the laterals, and there is an advantage in having only one outlet - that of the main - to look after.
The water in an outlet ditch should be enough below the banks to afford an outlet for a line of tile which may be laid to it from any part of the marsh of which the ditch is the direct outlet. This line of tile is entitled to a depth of 3 feet at the head, and a gradient of 0.1 foot in 100 feet. On a marsh exactly level and with no part more than half a mile from an outlet ditch, this means that the 3 feet of depth at the head, added to the 2.64 feet of fall in a half mile requires a depth a little more than 5 1/2 feet deep at the outlet. With an allowance of 2 1/2 feet for the depth of the water in the outlet ditch, it is evident that, under these conditions, it should be dug 8 feet deep. Where there is a surface slope toward the ditch, its depth may be decreased by an amount equal to the fall causing the slope. Following this rule, the depth may, in rare cases, be reduced to 4 feet. Tile outlets may be submerged for a short time during flood flow without serious results.