The Use of Soil Maps

A. M. Hedge and A. A. Klingebiel.

Soils may be grouped into land capability classes, subclasses, and units to help us use them properly. Of the eight classes, which normally do not all exist on any single farm or ranch, classes I through IV are suited to cultivated crops, pasture or range, woodland, and wildlife.

Classes V through VIII are suited to pasture or woodland and wildlife and are not generally recommended for cultivation. Some kinds of soil in Classes V, VI, and VII may be cultivated safely, with special management, however.

Because several kinds of soil often occur in the same capability class on the same farm or ranch, the classes are divided into subclasses.

Four kinds of problems are recognized in the subclasses and are indicated by symbols: (e) erosion and runoff; (w) wetness and drainage; (s) root zone and tillage limitations, such as shallowness, stoniness, droughtiness, and salinity; and (c) climatic limitations. The subclass therefore provides more specific information about the kind and the degree of limitation for the use of soil than does the capability class.

The land capability unit is the most detailed and specific soil grouping of the capability classification. Soils that can be used in the same way and will give about the same crop yield are grouped into a capability unit. It is used most commonly for planning in specific areas, as it is a grouping of soils that are nearly alike in features that affect plant growth and in response to management.

Other interpretative soil groupings also are used in conservation planning. In extensive range areas, the mapping units are grouped into range sites, which give information about the kind and amount of vegetation the area will produce when it is in its best condition. This grouping, together with range condition, provides the basis needed for sound range planning.

On farms or ranches that are to be used for woodland, range, or pasture, and cropland, the soil map is interpreted to show the suitability of the land for those uses. For areas that are to be planned as woodland, the mapping units are grouped into woodland sites and interpreted in terms of kinds and amounts of wood crops that can be produced.

Many land capability units are adapted to a number of alternative uses and treatments. For example, on some deep, well-drained, sloping soils in the humid section of the country, the same results in conservation often can be achieved by a 3-year rotation with terraces, by a 4-year rotation with contour stripcropping, or by a 5-year rotation with contouring.

The operator therefore has three equally satisfactory choices of rotations and accompanying practices to control erosion. If he is short of good cropland and has plenty of land suitable for hay and pasture, it would be advantageous for him to choose the 3-year rotation with terraces. If he has enough good cropland to produce all the cultivated crops he needs and can utilize more hay or pasture, he may want to follow the 5-year rotation with contour cultivation.

Comparative costs will also be an important factor in choosing between the alternatives available in this example. It would be a wise choice if the cost of constructing and maintaining terraces with proper outlets can be avoided without sacrificing conservation of the soil or production of feed for the livestock. Other alternative uses include growing grass or trees.

A number of alternative systems of soil management are usually available on capability units composed of soils that are level, deep, moderately permeable, and well drained but have adequate soil moisture. The maintenance of adequate fertility and the control of weeds and insects are the major problems.

On these soils one level of management may include a 3-year rotation, consisting of 1 year of grass-legume hay. When the crop residues are returned to the soil, a green manure crop is plowed under, necessary plant nutrients are added, and sound cultural practices are followed, however, high crop yields can be expected even under a continuous cropping system. The more level productive soils on a farm may be used more intensively so that the steeper and more erosive soils can be utilized less intensively.

Some capability units consist of level or nearly level soils that require drainage for the successful production of cultivated crops. The decision to drain such soils must take into consideration the cost of the drainage and the potential crop production after drainage, as compared to the possibility of producing hay, pasture, or woodland on the undrained land.

Usually a number of alternative crops can be grown, if the land is drained for cultivation. Of course, the cropping system selected must maintain adequate tilth to insure good air and water movement in the soil.

Many capability units consist of well-drained sandy soils, some of which are droughty and subject to wind erosion. Sandy soils in the less humid areas often have a wind erosion hazard in addition to the problem of maintaining organic matter and fertility at a high level. Fortunately, the same management practices such as the return of crop residues, cover crops, and manure to the soil and the use of high rates of fertilizer help meet the needs for maintaining organic matter and fertility levels as well as offering protection against erosion hazards.

In areas of low rainfall where irrigation is contemplated, the land capability units reflect those soil differences that influence the frequency of irrigation, amount of water applied, length of runs, as well as the kinds and amounts of crops that can be grown.

To illustrate how the technology of conservation farming can be used by farmers, a conservation farm plan from a Southern State is presented. The two maps pertain to it.

CLASS II LAND has some limitation that reduces the choice of use or requires some conservation practice to keep it productive. It can be used for cultivated crops, pastureland, woodland, or wildlife land.

IIe2 These soils are gently sloping, deep, and are slightly to moderately eroded. They are well drained and water moves through them easily. Soil erosion and soil fertility are the main soil problems to overcome. These soils are suitable for growing all crops commonly grown in the area.

10B1 Tifton sandy loam, gently sloping, slightly eroded phase.

10B2 Tifton sandy loam, gently sloping, moderately eroded phase.

IIs1 Soils in this group are deep, sandy, and droughty. Water moves through these soils easily, and the maintenance of soil fertility is difficult.

1. A map of the soil and its capability. The symbols pertain to various aspects of soil and topography. For example: 10B1 refers to the kind of soil; the number 10 refers to the soil type; the letter B to steepness of slope; and number 1 to degree of erosion. The symbol IIe2 refers to the land capability unit; II designates the land capability class; e indicates the subclass; and 2 indicates the unit. Heavy lines on map indicate boundaries of a capability unit. See explanation in text for specific definitions.

2. A conservation plan map. The decisions made by the farmer concerning the use and management of the land, together with the field unit arrangement, are recorded on this map. These decisions were based on the soil and capability map shown on the opposite page. Needs of the farmer relative to his farm enterprise were also considered.