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New Mexico Soils

Factors of Soil Formation

The soil that develops in any area is the result of the interaction of five soil-forming factors: climate, vegetation, parent material, topography, and time. The first two are called "active" factors because they act on the soil parent material as conditioned by topography over varying periods of time.

Climate and vegetation frequently are considered together because climate is the major determinant of vegetation. Soils of the high mountains of northern New Mexico are commonly leached, well developed, and acidic because precipitation is relatively high, temperatures are low, and the dominant vegetation is coniferous trees, which are best suited to the climatic and soil conditions. On the other hand, grasses and desert shrubs are dominant in the hot, dry desert regions. Here, the soils are not leached, are less developed, and are neutral to very alkaline. Whether soil determines the kind of vegetation or whether vegetation determines the kind of soil is a much-debated point, but climate certainly is the deciding factor for both. 

In New Mexico, temperature and precipitation are related principally to land elevation. For example, in the relatively short distance between the Tularosa Basin and the ski run on Sierra Blanca, temperatures drop, precipitation increases, the vegetation changes from grasses to trees, and the soils change from calcareous and weakly developed to acidic and strongly developed. The growing (frost-free) season varies from about 210 days at the lowest part of the state near Carlsbad to fewer than 100 days in the Sangre de Cristo Mountains of northern New Mexico. Desert grasses and shrubs are the dominant vegetation in the sand plains of the south, whereas alpine vegetation occurs above the timberline on high mountain peaks. Precipitation distribution patterns differ from eastern New Mexico to western New Mexico. In the east, precipitation is lowest in winter and highest in summer. In the west, precipitation is at a minimum in April and May and a maximum in July and August. The average annual rainfall in the Albuquerque area is about 9 inches or less depending on the proximity to the mountains.

Parent material consists of the geologic material from which soils are developed. Soils on very young alluvium, such as in the valley of the Rio Grande or on the sand dunes of southern New Mexico, are essentially undeveloped, so their characteristics are similar to those of the parent materials. On the other hand, soils of the high mountains of northern New Mexico have characteristics that bear little relation to the parent material from which they developed. Climate and vegetation are the dominant soil-forming factors in humid areas.

Rocks of Cretaceous, Tertiary, and Quaternary ages dominate the surficial geology, but geologic formations dating as far back as Precambrian occur, mostly in the north-central part of the state. Evidence of volcanic activity can be seen throughout the state except in the southeastern quarter, where only sedimentary formations are found. Lava flows occurred as recently as about 1,000 years ago south of Grants. Limestone and sandstone are the principal sedimentary rocks for the state as a whole, but shale is locally important north and west of Tucumcari and in the northwest corner of the state.

Topography affects soils greatly. Thin, eroded soils are commonplace on steep slopes. In depressions, fine-textured, saline, poorly drained soils are a logical consequence of the topographical conditions. Soils on the south-facing slopes are subject to higher temperatures than their counterparts on the north sides of hills. The topography of the state is highly varied. The high plains of eastern New Mexico are relatively flat. The remainder of the state includes basins, plains, plateaus, mesas, mountains with their valleys, and floodplains.

The importance of time to soil formation arises from the fact that natural processes of soil development tend to reach an equilibrium which depends upon local environmental conditions. It takes thousands of years for a mature soil to develop from raw rock materials. The landscape of New Mexico is young, geologically speaking. Nearly all surface deposits from which soils have developed have been affected by climatic changes occurring in the last million years. Many of them owe their characteristics to the soil-forming processes operating during and since the last glacial period. 

Soil Classification

 Soil scientists use several systems to classify soils. These deal with the soil as a natural body and consider the volume of soil affected by biological activity, which usually extends to a depth of several feet. One such classification lists the five soil orders in New Mexico as Aridisols, Mollisols, Entisols, Inceptisols, and Affisols.

Aridisols are extensive in lower elevations over the southern two-thirds of the state but are replaced in the cooler and moister higher elevations by Mollisols. Aridisols dominate the lower elevations of New Mexico. Aridisols lack necessary moisture for mesophytic plant growth for long periods. Thus, Aridisols are not suitable for dryland agriculture. During most of the year the soil water is held at tensions above the wilting point for most plants. Generally, the soil horizons (distinct layers of soil) were formed under a more moist regime, as during former pluvial periods. The surface horizon (layer) is usually low in organic matter content and is thus light in color. The Aridisols are often calcareous from the surface downward and have a secondary accumulation of calcium carbonate (lime) and/or gypsum in the subsoil. Soil textures range from loamy sands to clays and consistency ranges from soft to extremely hard. Most of the surface is bare much of the time and in many instances a surface gravel pavement has formed by deflation of the finer windblown particles. The Aridisols are important resources but are easily misused. Both wind and water erosion are a constant hazard. Under agriculture, special fertility problems can exist because of unavailable micronutrients resulting from a high pH. In general, however, the Aridisols have a high content of bases needed for plant growth.

Mollisols are characterized by deep, dark surface horizons of high organic matter content. They occur in areas of New Mexico with more than 12-14 inches of rainfall (similar in rainfall to areas with Inceptisols and Alfisols). Mollisols are dominantly grassland soils but do occur in the forests of southern New Mexico where the base status is high and grass is the dominant understory. Mollisols are very fertile soils with a high supply of nutrients. Lime often accumulates in the subsoil. Mollisols, like most soils in New Mexico, are fragile when misused. Water erosion hazard is high in some areas. Most of the Mollisols are used to support grazing some in eastern New Mexico are used for crop production.

Entisols occupy the Rio Grande Valley from Santa Fe south to the Texas-Mexico border. The northern third of the state and the far eastern counties are dominated by Mollisols, Entisols, and Alfisols. Entisols can occur in any climate; however, most of these soils in New Mexico occur in an arid climate in association with Aridisols. Entisols have been exposed to the soil-forming processes for such a short time that no major soil horizons have formed. Examples of Entisols are soils on floodplains or soils frequently moved by wind erosion. They also occur on moderate to steep slopes where bedrock is shallow. In general, the Entisols express the properties of the parent material with little change. Their nutrient supplying capacity is generally high. Salinity and sodicity may be limited. Erosion hazard can be high, especially on the soils derived from wind-blown sediments. Most of the soils of the Rio Grande Valley in agriculture are Entisols.

Inceptisols are found in the highest elevations of the San Juan and Sangre de Cristo Mountains. Other materials of note include the gypsum sands of White Sands National Monument and the lavas of the Carrizozo, Grants, and other malpais (lava rockland). Inceptisols exhibit the initial sign of soil development: a color change in the subsoil. They occur in areas of more rainfall than those yielding Aridisols. In New Mexico this is mainly in mountains which receive more than 12-14 inches of rainfall. The Inceptisols are young, occurring mainly on steep slopes where erosion removes weathered sediments. They also occur in areas dominated by volcanic pumice where insufficient time has passed to allow the formation of a more weathered soil. In New Mexico, their base-supplying capacity is generally high.

Affisols also occur in climates more moist than those yielding Aridisols. Alfisols occur in the mountains of northern New Mexico and on the plains in eastern New Mexico. Organic matter accumulation in the surface horizon is greater than in Aridisols but is still low enough that the color is either light or dark to only shallow depths. Alfisols have been subjected to soil formation processes for long enough that clay has translocated and accumulated in the subsoil. Many of the Alfisols have sufficient moisture and nutrient supply to support dryland agriculture.

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