Soils that are naturally poorly drained and have inadequate aeration are generally high in organic matter content. But poor drainage makes them unsuitable for growing most crops other than a few water-loving plants like rice and cranberries. When such soils are artificially drained, they become very productive, as the high organic matter content provides all the good qualities we discussed in earlier chapters. Over the centuries, humans have converted swamps into productive agricultural land by digging ditches and canals, subsequently also combined with pumping systems to remove the water from low-lying areas. Aztec cities were supported in part by food from chinampas, which are canals dug in shallow lakes with the rich mud used to build raised beds. Large areas of Holland were drained with ditches to create pasture and hay land to support dairy-based agriculture. Excess water was removed by windmill power, and later by steamand oil-powered pumping stations (figure 17.12). Today, new drainage efforts are primarily accomplished with subsurface corrugated PVC tubes that are installed with laser-guided systems (figure 17.13). In the United States land drainage efforts have been significantly reduced as a result of wetland protection legislation, and large-scale government-sponsored projects are no longer initiated. But at the farm level, recent adoption of yield monitors on crop combines has quantified the economic benefits of drainage on existing cropland, and additional drainage lines are being installed at an accelerated pace in many of the very productive lands in the U.S. corn belt and elsewhere.
Benefits of Drainage
Drainage results in the lowering of water tables by removal of water through ditches or tubes (figure 17.14). The main benefit is the creation of a deeper soil volume that is adequately aerated for growth of common crop plants. If crops are grown that can tolerate shallow rooting conditions—like grasses for pastures or hay—the water table can still be maintained relatively close to the surface or drainage lines can be spaced far apart, thereby reducing installation and maintenance costs, especially in low-lying areas that require pumping. Most commercial crops, like corn, alfalfa, and soybeans, require a deeper aerated zone, and subsurface drain lines need to be installed 3 to 4 feet deep and spaced from 20 to 80 feet apart, depending on soil characteristics.
Drainage increases the timeliness of field operations and reduces the potential for compaction damage. Farmers in humid regions have limited numbers of dry days for spring and fall fieldwork, and inadequate drainage then prevents field operations prior to the next rainfall. With drainage, field operations can commence within several days after rain. As we discussed in chapters 6 and 15, most compaction occurs when soils are wet and in the plastic state, and drainage helps soils transition into the friable state more quickly during drying periods—except for soils with high plasticity, like most clays. Runoff potential is also reduced by subsurface drainage, because compaction is reduced and soil water content is decreased by removal of excess water. This allows the soil to absorb more water through infiltration.
Installing drains in poorly drained soils therefore has agronomic and environmental benefits because it reduces compaction and loss of soil structure. This also addresses other concerns with inadequate drainage, like high nitrogen losses through denitrification. A large fraction of denitrification losses can occur as nitrous oxide, which is a potent greenhouse gas. As a general principle, croplands that are regularly saturated during the growing season should either be drained, or revert to pasture or natural vegetation.
Types of Drainage Systems
Ditching was used to drain lands for many centuries, but most agricultural fields are now drained through perforated corrugated PVC tubing that is installed in trenches and backfilled (figure 17.13, right). They are still often referred to as drain “tile,” which dates back to the practice of installing clay pipes during the 1800s and early 1900s. Subsurface drain pipes are preferred in a modern agricultural setting, as ditches interfere with field operations and take land out of production. A drainage system still needs ditches at the field edges to convey the water away from the field, to wetlands, streams, or rivers (figure 17.13, left).
If the entire field requires drainage, the subsurface pipes may be installed in grids with mostly parallel lines (figure 17.16). This is common for flat terrains. On undulating lands, drain lines are generally installed in swales and other low-lying areas where water accumulates. This is generally referred to as random drainage (although a better term is targeted drainage). Interceptor drains may be installed at the bottom of slopes to remove excess water from upslope areas.
Is Drainage Really Needed?
Croplands with shallow or perched water tables benefit from drainage. But prolonged water ponding on the soil surface is not necessarily an indication of a shallow water table. Inadequate drainage can also result from poor soil structure (figure 17.15). Intensive use, loss of organic matter, and compaction make a soil drain poorly in wet climates. It may be concluded that the installation of drainage lines will solve this problem. Although this may help reduce further compaction, the correct management strategy is to build soil health and increase its permeability.
Fine-textured soils are less permeable than coarse-textured ones and require closer drain spacing to be effective. A common drain spacing for a fine loam is 50 feet, while in sandy soil drain pipes may be installed at 100-foot spacing, which is considerably less expensive. Installing conventional drains in heavy clay soils is often too expensive due to the need for close drain spacing. But alternatives can be used. Mole drains are developed by pulling a tillage-type implement with a large bullet through soil in the plastic state at approximately 2 feet of depth (figure 17.17). The implement cracks the drier surface soil to create water pathways. The bullet creates a drain hole, and an expander smears the sides to give it more stability. Such drains are typically effective for several years, after which the process needs to be repeated. Like PVC drains, mole drains discharge into ditches at the edge of fields.
COMMON TYPES OF DRAINAGE PRACTICES USED IN AGRICULTURE
- Subsurface drain lines (tile)
- Mole drains
- Surface drains
- Raised beds and ridges
Clay soils may also require surface drainage, which involves shaping the land to allow water to discharge over the soil surface to the edge of fields, where it can enter a grass waterway (figure 17.18). Soil shaping is also used to smooth out localized depressions where water would otherwise accumulate and remain ponded for extended periods of time.
A very modest system of drainage involves the use of ridges and raised beds, especially on fine-textured soils. This involves limited surface shaping, in which the crop rows are slightly raised relative to the inter-rows. This may provide a young seedling with enough aeration to survive through a period of excessive rainfall. These systems may also include reduced tillage—ridge tillage involves minimal soil disturbance—as well as controlled traffic to reduce compaction (chapters 15 and 16).
Concerns with Drainage
The extensive drainage of lands has created concerns, and many countries are now strictly controlling new drainage efforts. In the U.S., the 1985 Food Security Act contains the so-called Swampbuster Provision, which strongly discourages conversion of wetlands to cropland and has since been strengthened. The primary justification for such laws was the loss of wetland habitats and landscape hydrological buffers.
Wetlands are among the richest natural habitats due to the ample supplies of organic sources of food, and they are critical to migrating waterfowl that require food and habitat away from land predators. These wetlands also play important roles in buffering the hydrology of watersheds. During wet periods and snowmelt they fill with runoff water from surrounding areas, and during dry periods they receive groundwater that resurfaces in a lower landscape position. The retention of this water in swamps reduces the potential for flooding in downstream areas and allows nutrients to be cycled into aquatic plants and stored as organic material. When the swamps are drained, these nutrients are released by the oxidation of the organic materials and are mostly lost through the drainage system into watersheds. The extensive drainage of glacially derived pothole swamps in the north central and northeastern U.S. and Canada has contributed to significant increases in flooding and losses of nutrients into watersheds.
TO REDUCE RAPID CHEMICAL AND MANURE LEACHING TO DRAIN LINES
- Build soils with a crumb structure that readily absorbs rainfall and reduces the potential for surface ponding.
- Avoid applications on wet soils (with or without artificial drainage) or prior to heavy rainfall.
- Inject or incorporate applied materials. Even modest incorporation reduces flow that bypasses the mass of the soil.
Drainage systems also increase the potential for losses of nutrients, pesticides, and other contaminants by providing a hydrologic shortcut for percolating waters. While under natural conditions water would be retained in the soil and slowly seep to groundwater, it is captured by drainage systems and diverted into ditches, canals, streams, lakes, and estuaries (figure 17.19). This is especially a problem because medium and fine-textured soils generally allow for very rapid movement of surface-applied chemicals to subsurface drain lines (figure 17.20). Unlike sands, which can effectively filter percolating water, fine-textured soils contain structural cracks and large (macro) pores down to the depth of a drain line.
Generally, we would consider these to be favorable, because they facilitate water percolation and aeration. However, when application of fertilizers, pesticides, or liquid manure is followed by significant precipitation—especially intense rainfall that causes short-term surface ponding—these contaminants can enter the large pores and rapidly (sometimes within one hour) move to the drain lines. Bypassing the soil matrix and not filtered or adsorbed by soil particles, these contaminants can enter drains and surface waters at high concentrations (figure 17.21). Management practices can be implemented to reduce the potential for such losses (see the box “To Reduce Rapid Chemical and Manure Leaching to Drain Lines”).
Artificial drainage of the soil profile also reduces the amount of water stored in the soil and the amount of water available for a crop. Farmers who want to drain water out of the soil in case of excess rain but would like to retain it in case of drought play a game with the weather. Controlled drainage allows for some flexibility and involves retention of water in the soil system through the use of weirs in the ditches at the sides of fields. In effect, this keeps the water table at a higher level than the depth of the drains, but the weir can be lowered in case the soil profile needs to be drained. Controlled drainage is also recommended during winter fallows to slow down oxidations of organic matter in muck (organic) soils and reduce nitrate leaching in sandy soils.