II. SOIL AND LANDSCAPE PROPERTIES
Limiting Layer Next Section>>
In soil evaluation, four kinds of limiting layers are recognized: 1) bedrock, 2) dense till, 3) fragipan, and 4) coarse sand and gravel. Three depth zones to a limiting layer are also recognized: 1) 0 to 20 inches, 2) 21 to 40 inches, and 3) more than 40 inches or no limiting layer. Layers 1-3 have little pore space, so they restrict root penetration and conduct water or wastewater effluent very slowly. Few roots grow into coarse sand and gravel (layer 4) because it holds very little water. Also, this layer absorbs and transmits effluent so quickly that it is not purified by soil organisms. The effluent can then percolate to the ground water and contaminate drinking water supplies.
Depth to the limiting layer is important because it determines the amount of soil material favorable for plant rooting. A shallow soil limits the amount of water the soil can supply plants. For homesites, depth to a limiting layer largely determines the kind of wastewater soil absorption field that is suitable.
Depth and thickness requirements for all limiting layers:
1. Three classes of soil depth to a limiting layer are recognized:
a. 0 to 20 inches,
b. 21 to 40 inches, and
c. more than 40 inches (or no limiting layer).
2. The limiting layer must be more than 10 inches thick. If the layer or material meets the requirements listed below and extends to the bottom of the pit, assume that it is more than 10 inches thick, thus a limiting layer.
Bedrock gradually weathers to weathered bedrock parent material and then to various soil horizons. The guidelines below show when rock material is hard enough and continuous enough to be considered a limiting layer.
Bedrock—more than 80% of the layer is rock material so hard that:
1. it cannot be cut with a spade or dug into with a knife, and/or
2. roots cannot grow into it.
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The characteristics of till were listed in the Parent Material section of this chapter. The requirements of dense till, are the same with the additional requirement that the till has a density of 1.75 g/cm3 or more. Soil judgers can use several clues to identify dense glacial till. First, for soil evaluation, it must be calcareous. The site card will show if calcareous material is present in the pit and the depth to its upper boundary. Second, it must have a relatively high density. You can judge the density by “hefting” a sample and comparing it with samples from the A and B horizons of the same soil. Dense till will seem to be heavier. Third, few roots penetrate dense till, and those that do usually are confined to the cracks that go through the till in a widely spaced pattern. This pattern is something like that of a fragipan (Plate 16), except even larger in scale. Fourth, even when the whole soil profile is moist, dense till is difficult to dig into with a knife or spade. Fifth, it often has platy structure.
Note that not all till is dense. In some soils, the uppermost layer of calcareous till has brownish clay films on structural units or peds, and the till inside the peds is dense. Apparently water moved through the voids between peds to deposit these clay films. If peds with clay films are closer together than four inches, the layer is not considered to be dense till. In Plate 2 there appears to be such a layer a few inches thick around 3 1/2 feet deep, assuming that carbonates are present below 38 inches. Usually these horizons with clay films around dense till are less than 8 inches thick and are underlain by dense till with few or no clay films. Also, some tills have a density less than 1.75 g/cm3. Much of this till originated from material that was in, instead of under, the ice. This till is not dense.
From a standpoint of land use, dense till limits the depth to which crop roots grow. In addition, it is very slowly permeable and affects the type of septic system design recommended for home waste disposal.
Dense till meets all the requirements of till parent material. In addition:
1. It has bulk density of 1.75 g/cm3 or more (it feels heavy compared
with other soil material).
2. It has little pore space and few or no roots.
3. If clay films are present, they are more than 4 inches apart.
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Fragipans are firm, brittle subsoil horizons through which water moves very slowly and roots do not penetrate readily. Brittle means that a moist piece of soil about 1/2 to 1 inch across will rupture suddenly, or “pop,” rather than deform slowly when pressed between the thumb and forefinger. Fragipans are usually medium or moderately clayey in texture—near the boundary of silt loam and silty clay loam in (Fig. 15). Usually the upper surface of a fragipan is 20 to 50 inches below the surface, and the layer 10 to 30 or more inches thick.
Fragipans are most readily recognized by overall appearance. They consist of large structural units called prisms, which are taller than they are wide. The outside face of a prism is covered with gray coatings consisting mainly of silt in the upper part of the fragipan and of silt and clay in the lower part. These prism coatings are seen in Plate 12 just to the right of the tape at 3 feet as well as in the lower right hand corner. The shaved part of the soil profile cuts through some prism coatings at a right angle. In some soils, the gray material also forms a continuous layer just above the fragipan (Plate 12 at 3 ft.). This gray material tapers like a funnel into the gray streaks that make up the ped coatings. The interior of the prism is brownish, mottled with gray colors. In poorly drained soils, the gray colors may be dominant. The interior material is firm and brittle when moist.
The prism coatings appear as gray vertical streaks in a soil profile. When viewed from the top, they form a polygonal pattern (something like the skin of a giraffe or chicken wire, but with a less regular pattern) in a horizontal exposure (see Plate 16). The prisms must be more than 4 inches across on average. Roots grow downward in the gray silt between the prisms but not through the interior of the prisms. A horizontal surface through the prisms will be exposed in soil evaluation pits (see section on Site Selection and Preparation).
Fragipan soils present special problems for farming. In the spring, water is held up by the fragipan, and the soil horizons above the pabove the pan. Roots do not penetrate into fragipan prisms, so crop plants get very little water from fragipan horizons. In the winter, plants with taproots like alfalfa may be heaved out of the soil because of freezing and thawing cycles.
In Indiana, fragipans occur mainly in Soil Regions 5, 10, and 11, with a few in Regions 12 and 13 (Fig. 1) where the loess is fairly deep.
Fragipans—have these characteristics:
1. Prisms that, on average, are 4 inches or more wide.
2. Material in the prisms is brittle.
3. Few or no roots in the prisms.
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Coarse Sand and Gravel
Coarse sand and gravel, as it appears in the Ockley soil (Plate 6), limits root growth because of low water-supplying ability. Coarse sands hold very little water, while fine sands have relatively high water-supplying abilities. Therefore, a limit based on sand size must be established between those sandy materials that restrict plant growth and those that do not. This limit is between the medium sands and coarse sands.
In soil evaluation, “coarse sand and gravel” is defined as sandy material (Fig. 15) in which the sand size is mainly larger than 0.5 mm (the size of grains in 40-grit sandpaper). Most of these materials also contain much gravel, but some may contain none. Usually the materials, when moistened and squeezed, either will not form a cast or will form one that does not hold its shape when tossed up and down in the hand. Sandy materials that are not limiting layers include some water-deposited sandy materials and eolian sand deposits.
Thin bands of coarse sand and gravel are less of a limitation than thick deposits. For soil evaluation, the layer of this material must be more than 10 inches thick to recognize it as a limiting layer. It can be the lowest horizon in the soil pit or lie between finer textured layers.
Coarse sand and gravel layers are of little significance to water availability if the water table is permanently within reach of the plant roots. However, most poorly and very poorly drained soils in Indiana have a fluctuating water table, which becomes low enough in summer to make water holding capacity of the subsoil important. Therefore, in soil evaluation, coarse sand and gravel layers are considered to be limiting, regardless of drainage class.
1. Qualifies for the sandy texture group and the sands are mainly >0.5 mm in diameter (40-grit sandpaper).
2. Gravel is usually present but may be lacking.