Soil Structure

Soil structure is the naturally occurring aggregation of soil particles into units called peds. To determine soil structure you must carefully remove soil and shake it gently so that the soil falls apart naturally. Clay and organic compounds are the binding material that creates the peds that form soil structure. The size and type of soil structure is important because it affects water movement in the soil.

 

Figure 20. Structures found in Maryland include A. prismatic, C. angular blocky, D. subangular blocky, E. platy, and F. granular. Although not found in Maryland, B. columnar structure is similar to prismatic structure, but is rounded at the top.

 

Soil structure common to Maryland includes granular, angular blocky, subangular blocky, platy, prismatic, single grain, and massive (fig. 20). Granular structure is small and rounded, usually found in surface horizons. All other structures are usually found in subsurface horizons. Angular blocky has blocky peds that have sharp edges. Subagular blocky has blocky peds with rounded edges. Platy structure is flattened and can impede water movement through the soil. Prismatic is vertically elongated with flat tops. Single grain is found in sands and loamy sands and falls apart into loose grains. Massive is material that is held together, but does not fall apart naturally into any coherent structure.

 

Permeability

Permeability is the rate at which water and air move through the soil. Permeability is influenced by texture, structure, bulk density, and large pores. Soil structure influences the rate of water movement through saturated soil, in part, by the size and shape of pores. Granular structure readily permits downward water movement, whereas a platy structure requires water to flow over a much longer and slower path (fig. 21). Permeability is used in drainage design, irrigation scheduling, and many conservation practices. Permeability classes are shown in table 3.

 

Figure 21. Paths of water flow through soils with granular, prismatic, subangular blocky, and platy structure, respectively

 

Table 3. Permeability classes

Class

Rate (in/hr)

Very slow

<0.06

Slow

0.06-0.02

Moderately slow

0.02-0.6

Moderate

0.6-2.0

Moderately rapid

2.0-6.0

Rapid

6.0-20

Very rapid

>20

 

Depth

The depth of a soil is considerably important both for agricultural and nonagricultural uses. A shallow-rooted crop may produce equally well on either a deep or shallow soil. However, deeply rooted plants such as trees or alfalfa require deep soils for best growth. During droughty periods, crops on shallow soils usually are the first to show damage because of the lack of moisture. This results from a soil volume that cannot hold adequate water.

Houses with basements or septic systems should be built on deep, well-drained soils. The lack of deep soil may necessitate a house with a slab or shallow foundation, and a septic system may not be functional or permitted on such soils.

Shallow soils restrict plant growth by impeding root growth and provide only limited water and nutrient re-serves. The processes of soil formation may have been such that only a thin veneer of soil has formed over a very hard or resistant parent material. Erosion may have reduced the thickness of a once-deep soil. Coarse gravel and sand layers also can impede root penetration as can sustain high water tables. In addition, root-restricting horizons or pans may have been formed during soil formation.

Most of the better agricultural soils in Maryland have a thickness of at least 1 m (40 in.). These soils are considered deep for agricultural and judging purposes. A soil that has a thickness of greater than 1.5 m (60 in.) is very deep. Any soil possessing a root-restricting horizon at a depth of less than 0.5 m (20 in.) is considered shallow (fig. 22). Moderately deep soils are those between these two extremes.

In summary, the categories of soil depth are:

 

Figure 22. The soil on the left is a shallow soil with bedrock starting at 0.35 m (15 in.). The soil on the right is a very deep soil with no bedrock to a depth greater than 1.5 m (60 in.)

 

Reaction

Soil pH is an expression of the degree of acidity or alkalinity of a soil. It influences plant nutrient availability. A very acid soil (pH <5.0) typically has lower levels of nitrogen, phosphorus, calcium, and magnesium available for plants, and higher levels o availability for aluminum, iron, and boron than a net soil at pH 7.0. At the other extreme, if the pH is too high, availability of iron, manganese, copper, zinc, c especially phosphorus and boron may be low. A pH above 8.3 may indicate a significant level of exchangeable sodium.

 

Drainage

Some soils can be worked soon after heavy rains while others may remain saturated or ponded for long periods. Coarse-textured soils such as sands allow water to drain through the soil very rapidly if outlets are available. Moderately coarse-, medium-, moderately fine-, and fine-textured soils on similar landscape positions usually require correspondingly longer periods before they can be worked. Soils on extensive level areas or those in depressions commonly are poorly drained, and water tables may be at or near the surface for a long time.

Plants require good aeration as well as moisture for optimum growth. Soils that are excessively drained (such as sand) are well aerated but dry out quickly thus restricting crop production. Poorly drained soils that are not artificially drained retard crop production because long periods of water saturation starve roots of required oxygen. Also, these soils do not warm readily in the spring. Thus, the best agricultural soils are those that are deep and allow excess water to readily pass through the profile while retaining enough water to supply crops until the next rain. 

Soils that are deep, well drained, moderately coarse and medium textured are preferred for agricultural production because they have a very desirable air-water relationship for many crops. These soils are about half mineral and organic material and half pore space. Ideal conditions exist when approximately half of this pore space is filled with water and half with air. Of course, these proportions fluctuate with the rainfall pattern. Coarse-textured soils (such as sand) contain a much greater proportion of air than water in this pore space, and they must be irrigated for good crop production. On the other hand, fine-textured soils (such as clay) possess a higher proportion of water than air in the pore space. 

Well-drained soils also are preferred for many nonagricultural uses. Home sites and housing developments should be located in well-drained soils, especially if basements are to remain dry and septic systems are to function efficiently.

One of the best indicators of drainage class is soil color. The more redoximorphic features (mottling due to wetness) and gray in the subsoil, the poorer the soil drainage, the longer and higher the water tables stand in a soil profile, the more intense is the mottling and the higher it occurs within the profile. Soil scientists recognize six drainage classes in the field. Figure 23 shows the relationship between topography or position on the landscape and the resulting soil drainage. The water table, as indicated on the figure, is shown as it might appear during wet seasons.  

Figure 23. Maryland drainage classes and their location on the landscape.

 

Excessively drained. Water is removed from the soil very rapidly because of either coarse textures (such as sand and loamy sand) or shallow, porous profiles on steep slopes. Excessively drained soils are suited poorly to agriculture unless irrigation is practiced. No drainage mottles occur in these soils (fig. 24).

Figure 24. This soil profile shows an excessively drained soil. It is a sandy soil with no redoximorphic features.

 

Well drained. Good aeration occurs. Subsoil colors are bright and the profile lacks redoximorphic features above 1 m (40 in.) (fig. 25). Brown, yellowish brown and reddish brown colors are common.

 

Figure 25. This soil is a well drained soil with no redoximorphic features in the upper 1 m (40 in.)

Moderately well drained. In these soils, redoximorphic features are present above 1 m (40 in.) indicating that saturated conditions or water tables occur above this depth at various times during the year (fig. 26). Mottles are restricted to the 0.5 to 1 m (20 to 40 in.) zone for classification in this category. These soils may retard crop growth in wet years, but crops may do very well during drought periods. Artificial drainage may be beneficial during wet periods. Septic systems may experience periodic failure during saturated conditions.

 

Figure 26. This is a moderately well drained soil with redoximorphic features occurring starting at 0.75 m (30 in.)

Somewhat poorly drained. Redoximorphic features occur within the 10 to 20 in. zone, indicating prolonged periods of saturation or high water tables. Serious crop injury or failure may result during wet years (fig. 27). Unless artificial drainage is provided, crop production is restricted and septic systems commonly fail.

 

Figure 27. This is a somewhat poorly drained soil with a predominantly gray matrix starting at 0.35 m (15 in.)

 

Poorly drained. These soils have dark surface horizons and gray subsoils with redoximorphic features occurring above 25 cm (10 in.) (fig. 28). They have high water tables or are ponded for long periods or both. These soils usually occupy level areas or footslope positions and are productive only if they are artificially drained. Development of these soils for home sites should be avoided. 

 

Figure 28. This is a poorly drained soil with a predominantly gray matrix due to wetness occurring at the surface.

 

Very poorly drained. Water is removed so slowly that the water table remains at or on the surface much of the year (fig. 29). These soils usually occupy low-lying and concave or depressed positions on the landscape. They normally have very dark or black, thick surface horizons with relatively high organic matter contents. The subsoils usually are gray. These soils can be used for agriculture, but only if intensive drainage is practiced. 

 

Figure 29. This is a very poorly drained soil with a black surface due to organic matter accumulation underlain by a predominantly gray matrix due to wetness.

 

Available Water Capacity 

The available water capacity of a soil is closely related to texture. As mentioned previously, air and water occupy the pore space between the particles comprising the soil skeleton. The bigger the soil particles (such as sand or gravel), the larger the pores between them. Thus, water drains first and rapidly from these larger pores. This results in droughty soils because the plants are supplied only from the small amount of remaining moisture. Irrigation is necessary on these soils even in humid climates. 

As the particles become smaller, the pores between the grains also are reduced in size. This results in the retention of more water for plant use. Therefore, medium-textured and moderately fine-textured soils, such as loam, silt loam and clay loam have much higher available water capacities than coarse-textured soils. The moderately coarse-textured soils (such as sandy loam) are intermediate in those categories. Fine-textured soils (such as clay) have such small pores that plant roots are unable to obtain much more water from these fine soils than is available from medium-textured soils. Moderately coarse-, medium- and moderately fine-textured soils are, therefore, preferred for agricultural use because they provide good, available water capacity and aeration while being easily worked. 

To calculate the water available within the soil profile, consider only the first 1 m (40 in.) or to a root limiting layer if it occurs above 1 m (40 in.). The available water capacity of each horizon down to 1 m (40 in.), when added together, will give the total available water for the profile. A deep silt loam will hold more water than a soil with 0.25 m (10 in.) of silt loam surface soil and the remainder sand. Therefore, both the surface and subsoil must be considered in computing the available water

See Table 4 for a general guide when calculating the amount of available water in a 40 in. profile. The range and average water availability are presented in inches of available water per inch of soil depth. 

For example, if a soil consists of 20 in. of silt loam over loamy sand, the available water capacity would be affected by both textural classes. In determining the water in the 20 in. of silt loam or medium-textured material, simply multiply the depth (20 in.) by the amount of available water held by the silt loam textural class (0.23 in. of water per inch of soil). This calculation gives 4.6 in. of available water in the 20 in. zone. Now the remaining 20 in. (to complete the 40 in. profile) is loamy sand or coarse-textured material which holds only 0.05 in. of water per in. of soil. Multiplying 20 in. of loamy sand times 0.05 yields a total of 1.0 in. of available water. Therefore, the 40 in. soil profile has an available water capacity of 4.6 in. (silt loam) plus 1.0 in. (loamy sand) or 5.6 in. of available water. 

Table 4. Amount of available water by textural class.

Textural class

Available water (in. water/in. soil

 

Range

Average

Coarse (sand, loamy sand)

.02-.09

0.05

Moderately coarse (sandy loam, fine sandy loam

.09-.19

0.14

Medium (loam, sandy clay loam, silt loam

.19-.27

0.23

Moderately fine (clay loam, silty clay loam)

.10-.19

0.15

Fine (silty clay, sandy clay, clay)

.07-.19

0.13

     

Available Water Capacity Categories

Range in in. H20 per 40 in. soil

Very Low

Less then 2.5

Low

2.6 to 4.5

Medium

4.6 to 7.0

High

Greater then 7.0

 

Erosion 

Soils under their natural vegetative cover attain equilibrium with their environment. When this vegetative cover is removed and the soils are cultivated, this equilibrium is changed. At certain times of the year the soils are exposed to heavy rains with little or no vegetative cover to break the impact of the rain drops. As a result, soil particles are dislodged and runoff waters carry these particles downslope and deposit them on other parts of the landscape or carry them into streams. Wind also is an effective carrier of particles on sandy soils. Regardless of the process, the removal of soil is called erosion. 

Some soils in Maryland have been cultivated for hundreds of years and many of these soils are severely eroded. Often, the entire original surface horizon has been removed, leaving the subsoil exposed. In some parts of the Piedmont, it is estimated that from 60 to 90 cm (24 to 36 in.) of the soil have been lost. The degree or severity of erosion is an important soil property.  

The degree of past erosion can be determined by comparing the original soil depth, observed in virgin forests, with the present soil depth. The less surface soil, or the closer the subsoil is to the surface, the more severe the erosion problem. 

The amount of past erosion is estimated as a measure of the soil that remains in relation to the given original thickness. The following categories are used to define the severity or degree of erosion in Maryland. 

 

None to slight. Less than 7.5 cm (3 in.) of the original soil have been lost. No mixing of the subsoil into the plow layer is evident. 

Moderate. Between 7.5 to 20 cm (3 to 8 in.) of the original soil have been removed. Subsoil material may be mixed with the plow layer, but the plow layer remains darker than the subsoil. 

Severe. More than 20 cm (8 in.) of the original soil have been lost. Commonly, subsoil material is mixed with the plow layer, and the plow layer color closely resembles the subsoil color. Where the subsoil is exposed or gullies occur, the soil is severely eroded.

 

Erosion Potential

Erosion potential is determined by the steepness of the slope, length of slope, the nature of the soil (soil texture, infiltration rate and tilth) and the type of vegetative cover. A soil's susceptibility to erosion will influence greatly how the soil is used. Erosion potential can be determined primarily by evaluating factors such as slope gradient, slope length and soil texture.