Indiana Soils:Evaluation and Conservation Online Manual

III. SOILS, AGRICULTURE, AND ENVIRONMENT

Water is a component of soils, so soil and water properties go hand-in-hand and soil evaluation contestants decide: 1) whether to drain wet soils or preserve them as wetlands, 2) if irrigating droughty soils for crop production is feasible, 3) if a soil is suitable for structures that control water flow and soil erosion.

Wetlands

Wetlands are areas of soils that are saturated with water at or very near the surface for at least several weeks during the growing season and support water-loving plants. Wetlands filter water before people drink it; they temporarily store floodwaters to relieve flood pressure downstream; and they are a home for many birds and animals. When European settlers first arrived in Indiana, about 25% of the state consisted of wetlands, but practically all of them have been drained for agricultural production or filled for development. This same trend has occurred all over the U.S. In 1985, because of public concern for the environment, the U.S. Congress declared that the remaining wetlands of the country should be preserved. Federal laws forbid destroying practically all natural wetlands by drainage, constructing levees, filling, or other construction activities. If, however, it is necessary to drain or fill a wetland for vital development, the developer must create a new wetland, usually larger than the one destroyed.

Most land that was drained before 1985 and has been continuously farmed since then is exempt from these regulations. Drainage can be improved in these fields. It is to the benefit of both the environment and the farmer to maintain effective drainage systems in these soils. If their productivity can be improved, it will relieve pressure to drain other natural wetlands for food production. This puts drainage into an either-or position. Either drain the land well and strive for high crop yields or preserve the wetland in its natural state, but avoid the intermediate situation of a poorly functioning drainage systems and low crop yields.

Wetlands must have hydric soils. Soils are hydric either because they have a high water table during part of the year or because they flood during the growing season. Most hydric soils in Indiana have a high water table. Practically all poorly drained soils and a few somewhat poorly drained soils are hydric. For soil evaluation, all poorly drained soils are considered to be potential wetlands. Also, somewhat poorly drained soils on flood plains are potential wetlands because they tend to be on the lower parts of flood plains where flood waters may remain longer after a flood.

Usually both somewhat poorly and poorly drained soils respond to artificial drainage, so most of those soils are checked yes for drainage. The exception is somewhat poorly and poorly drained soils on flood plains. Some of these soils can be drained, but many have problems that make drainage impractical.

According to soil evaluation rules, a somewhat poorly drained soil on uplands or outwash landforms and under natural vegetation, such as in the woods, is checked NO for both “Preserve as wetland” and “Artificial drainage.” This means that its present land use is satisfactory. It can be preserved, but probably is not required to be preserved by law. Also, it might be drained, but because we have so few natural areas remaining, the landowner might wish to preserve the natural condition.

According to soil evaluation rules in this chapter, many agricultural practices may be marked YES for poorly and somewhat poorly drained soils. This assumes that the wet soils are drained and used for cropland. Of course, in actual situations, these practices would not be applied to wetlands or forest land.

Regulations about wetlands are complex. Landowners should check with the local Natural Resources Conservation Service (NRCS) office for assistance in dealing with all wetland issues.

Crop plants must have water and oxygen in their root zone if they are to thrive. Soils in capability subclass “w,” have so much water that oxygen in the air is excluded from the root zone, essentially suffocating the plant. Water must be removed from these soils to allow room in the pore spaces for air if they are to be used for efficient crop production. Drainage also allows for timely field operations and helps crops get an early start. One of the main benefits of drainage is that it decreases ponding in swales. In undrained fields, water may pond in low areas long enough to kill or greatly harm crop plants.

Drainage removes excess water from the surface and within the soil. Three kinds of practices are considered: 1) shallow surface drainage; 2) open ditch drainage; and 3) subsurface drainage. Most fields are drained by a combination of these practices. For example, tile drainage lines empty into open ditches, and although most of the area is drained by tile, some may be drained by the ditch itself. Also, some surface drainage is usually required to supplement a subsurface system. For example, a small closed depression could be opened to allow it to drain. According to current federal programs, drainage of wetlands not currently used for crop production could lead to the loss of federal benefits.

Surface drainage

Shallow surface drainage includes any system of grading the soil surface to facilitate removal of water by overland flow. For many soils, this practice is used to supplement subsurface and open ditch systems. For some soils it is the only system used. This includes soils that are shallow to bedrock and soils that have very slow permeability. Soils less than 40 inches to bedrock and poorly or somewhat poorly drained are rare in Indiana, however.

In some landscapes, the upland flats are so large that no outlets are available for subsurface drains. In other places, the soils formed in lake bed materials may lie so low in respect to stream level that it is impossible to use subsurface systems unless the drainage water is pumped. In practice, surface drainage might be the only system used for these soils.

Open ditch drainage

Open ditches are artificial, open drains constructed for the purpose of removing surplus water from wet land. They work effectively in deep soils with sandy subsoil or substratum textures. Open ditches can be supplemented by subsurface drains using special practices to ensure that sand is kept out of the drain tubes.

Subsurface drainage

Subsurface (tile) drainage removes subsurface water through a line or series of lines of clay or concrete tile or plastic tubing installed at a definite grade, often at depths of 36 to 42 inches beneath the surface. Subsurface drainage is an expensive practice, and a careful survey should be made first to insure that the land is suitable for tiling and that adequate outlets are available. These outlets can be open ditches or large tile, or sometimes drains may outlet directly to a stream channel or pump outlet.

Some soils are difficult to drain with a subsurface system because they have slow permeability in the subsoil. Tile drainage is not recommended for these soils. Other soils are on such large, flat areas that it is difficult to obtain outlets for the drains. This factor is not considered in soil evaluation, but it should be kept in mind when planning a system. A fiberglass fabric “sock” is fit over the plastic tubing lines to keep sediment out of them in some silty and sandy soils.

Effects of drainage

Drainage is necessary for efficient crop production on many soils of Indiana. It is beneficial for crop production, but it may have beneficial or adverse effects on the environment. Drainage of wetlands results in loss of wildlife habitat. It also may increase the chance of down-stream flooding because water leaves drained fields faster than undrained areas, and increases peak flow in streams that drain the fields.

Several studies have compared drained agricultural land with undrained agricultural land. These are some of the results.

Total discharge of water (overland flow plus subsurface drainage) is similar.
Up to 63% of the rain that falls on a drained field leaves the field through the drainage system.
Overland flow is 29% to 65% less in drained fields.
Sediment loss by erosion, caused by overland flow, is reduced by 16% to 65% in drained fields.
Phosphorus (P) loss is reduced by up to 45% in drained fields because much of the P moves with sediment.
Total nitrogen loss is reduced in drained fields because much of the N also moves with the sediment.
Loss of nitrate-N, a soluble N ion, is increased in drained fields because nitrate moves with water.

In summary, drainage improves water quality by reducing the amount of sediment and P in the water, but worsens water quality by increasing the amount of nitrate. Nitrate-N is present in subsurface drainage water at all times, but is usually most concentrated when water first begins to flow from the tiles after the growing season, usually in late fall or early winter. Often the nitrate level in tile outflow from fields is higher than allowed in drinking water. Many of the tile drains flow into streams that are tributaries of rivers that are used as city water supplies, so it is important to reduce nitrate additions to the rivers.

Small amounts of pesticides also move quickly through the soil into the subsurface drainage water. The amount removed in the water is usually less than 1% of the amount applied. Most of the pesticide is held and degraded in surface soil horizons. Typically, pesticides are degraded in May and June, a month or two after they are applied.

Nitrate content of rivers can be reduced in several ways. One is to maintain natural vegetation. In forests, for example, where no nitrogen fertilizer is applied, runoff and soil water are very low in nitrate, and contributions to stream flow dilute the nitrate content of stream water. Also, cropping systems in which cover crops are grown after crops such as corn, soybeans, and wheat have been harvested take up nitrate and prevent its loss to drainage water. These crops include forage crops in rotation and cover crops. Also, wetlands can be used to reduce nitrate loads. In wetlands, because oxygen is excluded from the soil, nitrates are converted by soil organisms to gaseous forms of N, such as N2 and N2O, and diffuse from the water to the air. Drainage water could be routed to a natural or constructed wetland before it reaches a stream. Alternatively, the outflow from the wetland could be held in a storage pond and used for irrigation during the next growing season. This system would also help recycle other plant nutrients such as P, and would promote biological degradation of pesticides that may be in drainage water. Controlling the level of water in the soil by controlling the level in the tile drainage system can also help reduce nitrate concentrations in soils.

Irrigation is the addition of water to soils to assist in production of crops. The extent of benefits from crop irrigation usually depends on the soil properties, types of crop, management skills of the farmer, and weather during the growing season. Some soils that hold very little water in the profile will respond to irrigation. Most of these soils are in land capability subclass “s.”

There are many kinds of surface irrigation systems. Some must be set up each time water is applied and require much labor. Others are more automated, and are permanently installed. They require less labor, but are more expensive to install. The center pivot circular system is a popular semi-automated system. It consists of a pipe with sprinklers mounted on wheels which rotates around a center pivot. The unit may be driven by water pressure, or by electric or hydraulic motors.

Another system of irrigation in Indiana is sub-irrigation. In this system the water table in nearly level, poorly and somewhat poorly drained soils is controlled by blocking water flow from drainage ditches. In some cases, water is also pumped from streams into the ditches. Field tile can also be installed in association with the open ditch and used for drainage in the spring, but then used for sub-irrigation during the growing season. Very sandy, poorly drained soils such as Maumee can be irrigated in this manner. Additional water can also be added with a surface system.

Irrigation can significantly increase crop yields on some soils. This is especially true on sandy soils that have low water holding capacity. Irrigation can also improve crop quality, especially in vegetable crops.

Three basic soil properties influence the likelihood of a soil to respond to irrigation: texture, structure, and depth. These are the properties that determine the soil’s water holding capacity, infiltration rate, and permeability. Generally, the sandier the soil texture and shallower the depth, the greater will be the crop response to irrigation.

Water-holding capacity is the major factor affecting suitability for irrigation. Soils vary greatly in their ability to hold available water, depending primarily on their texture and depth. Silty soils hold the most plant-available water. Deep silt loam soils, which are the best agricultural soils from a water-holding standpoint, are capable of storing as much as 12 inches of water in a 5-foot depth. Sandy soils, especially coarse sands, may hold as little as 2 to 3 inches of available water in a 5 foot depth. Clayey soils hold more water than silty soils but hold some of the water so tightly that plants cannot use it. Many Indiana soils are droughty because they are shallow. For example, soils formed from outwash may consist of 2 feet of relatively fine material like silt loam and clay loam over sand and gravel. These sand and gravel layers hold very little water.

Water infiltration is also very important in determining the suitability of soils for irrigation. Generally, infiltration is highest on coarse-textured (sandy) soils and decreases as texture becomes finer. Infiltration rate is also affected by condition of the soil at the time of rainfall or sprinkler irrigation. For example, a weakly-structured medium textured soil (loam or silt loam), when exposed to rainfall without protective cover, seals at the surface which greatly reduces water intake. When the same soil is protected by crop residue or a full crop canopy, however, this seal doesn’t form and intake remains high.

Good soil permeability is essential for successful irrigation. Lack of internal water movement can result in a buildup of water that will exclude oxygen from the root zone and restrict plant growth. Sandy soils are ideally suited for irrigation because they contain a high proportion of large pores and are rapidly permeable, but finer-textured soils may or may not have good permeability, depending on their structure.

Soils that respond significantly to irrigation are the sands and loamy fine sands, and the silt loams, loams, and sandy loams that are shallow or moderately deep over coarse sand and gravel. This is reflected in the soil evaluation rule below.

Some of the sandy soils suitable for irrigation are susceptible to wind erosion and need to be protected by winter cover crops or crop residue. Remember that a farmer will consider factors in addition to those listed in the rule to make a decision about irrigation. They include size of the suitable soil area, availability of water, location, type of crops to be grown, potential economic returns, and others. The rule for irrigation, below, assumes that the soil is used for cropland, even though other possible land uses may be marked YES on the scorecard.

A terrace is an embankment or ridge constructed across sloping soils on the contour or at a slight angle to the contour. The terrace intercepts surface runoff so that it can soak into the soil or flow slowly through an outlet. Terraces break a long slope into shorter segments. In the USLE they decrease the L factor.

Parallel terraces are used on much agricultural land. They are constructed parallel to each other and, where possible, parallel to the direction of field operations. These terraces eliminate production losses associated with point rows and minimize interference to farming operations when they are spaced at multiple widths of the planting and harvesting equipment. Some terraces are constructed with steep backslopes that are kept in grass. Most, however, are broad-based, having gently-sloped ridges that can be cultivated as a part of the field.

Terraces that are constructed in parallel and discharge runoff through subsurface drains are known as parallel tile outlet (PTO) terraces. With PTO terraces, water that accumulates behind a terrace ridge is discharged through a surface inlet into a subsurface drain (Fig. 26). The surface inlet, called a riser, has a restricted section to control the discharge rate, causing some of the runoff to be temporarily stored. This storage period is long enough for sediment to settle out of the water, but not so long as to damage the crop.

Water and sediment control basins (WASCOBs) are shorter versions of PTOs that are built across natural drainage ways in the field, mainly to control gully erosion (Fig. 27). Thus, a major distinction between the two kinds of terraces is their length. No-till or some other conservation tillage system is necessary to control sheet and rill erosion when terraces are part of the management system so that the basin created by the terrace does not rapidly fill with sediment.

Fig 26. Sketch of parallel tile outlet terraces (PTO). The terraces are wide enough so crops can be planted on most of them.

Fig 27. Sketches of a long drainageway before (a) and after (b) installing water and sediment control basins (WASCOBs).

The major benefit of a terrace system is the conservation of soil and water. Terraces reduce both the amount and velocity of water moving across the soil surface, which greatly reduces soil erosion. Terracing permits more intensive cropping than would otherwise be possible. PTO terraces and WASCOBs provide these added benefits:

The total area can be farmed since grassed waterways are usually not needed;
Elimination of grassed waterways also eliminates the inconvenience they cause when tilling or applying herbicides;
Peak discharges are reduced because runoff is temporarily stored; and
Sediment and other contaminants settle out behind the terrace ridge before polluting water in a receiving stream.

Fields with long, fairly uniform slopes that are not too steep (generally less than 8%, but up to 12%) are best adapted to terraces. The soil should be deep so that after some has been removed to build the terraces, enough of it remains to support a crop. Soils formed from weathered bedrock usually have bedrock fragments within the excavation zone that interfere with terrace construction.

The overall slope of a field being terraced can be improved by taking fill material from the “right” locations in that field. Topsoil should be removed from both the cut and the fill areas and stockpiled, especially when working with shallow soil. It can later be spread back over the terrace and borrow areas. Each basin has a surface inlet connected to a bottom drain. Terraces break up long slopes into shorter segments and trap sediment in each basin. Shorter slopes reduce erosion in the field and reduce the amount of sediment in road ditches and streams.

There are many farm ponds in Indiana, especially in the southern part of the state. They are discussed here because of the interest in them, but they are not included on the soil evaluation scorecard.

One kind of farm pond is an embankment built across a natural drainage-way to intercept runoff. The stored water evaporates, soaks into the ground, or overflows the structure. Ponds help to conserve soils lower in the landscape by slowing down-slope movement of runoff. Soils higher in the landscape should be protected against erosion so the pond will not fill with sediment. Ponds often furnish water for livestock in pastures, but some are used for fire protection, fishing, or other activities. Some ponds are designed to release water slowly through a small outlet after the pond is filled above the permanent pool level. Pond capacity above the outlet is available to temporarily store water that otherwise could cause severe erosion. An emergency spillway is provided to release flood water and prevent washout of the structure.

Another type of pond is a groundwater or pit pond. It is built in nearly level, poorly or somewhat poorly drained soils and is filled mainly by subsurface water flow. The water level in these ponds is about the same as the water table level in the surrounding soils.

Before building a pond, the area should be checked by a soil scientist. Some soils in suitable landscapes do not hold water. One example includes the Frederick and Hagerstown soils. They overlie fractured limestone bedrock and transmit water through cracks and caverns. Another example is soils that have thin layers in the substratum that conduct water readily. These layers are common in glacial till, but often are not continuous across the landscape. Finding them requires many borings.

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Soil Evaluation Rules—Water Management

Mark Preserve as Wetland YES if the current land use is natural vegetation and the soil has either of these characteristics:

1. natural drainage is poor (any landform), or
2. landform is flood plain and natural drainage is somewhat poor.

The rule to preserve wet soils is meant to point out that some soils can, or must, be preserved as wetlands. Be sure to check with the Natural Resources Conservation Service before considering any drainage project.

Mark Artificial Drainage YES if the soil has all three characteristics:

1. current land use is farmland or developed land, and
2. natural drainage is poor or somewhat poor, and
3. landform is not a flood plain.

Mark Irrigation YES for soils that have both of these sets of properties:

1. 6% or less slope, and
2. either:

a. subsoil is sandy or moderately sandy, or
b. soil has a coarse sand and gravel limiting layer within 40 inches.

Mark Parallel terraces YES for soils with all these properties:

well, moderately well, or somewhat poorly drained, and
more than 40 inches to any limiting layer, and
3% to 12% slope, and
medium or finer subsoil texture, and
parent material is not weathered bedrock.