New Mexico Master Gardeners
 

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 or 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.

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 surficial 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.

How the Rio Grande Valley and the Mountains around us were formed

Albuquerque spreads in the broad valley of the Rio Grande at the foot of the Sandia Mountains.  It is the home of Kirtland Air force Base, the University of New Mexico and  a half million residents. Here you can ski in the morning and play golf in the afternoon. Here the sun shines day after day, and the purple sage blooms on the open mesa and purple asters and yellow sunflowers color the roadsides in the fall. Here is a unique environment with the mountains on the east, the volcanoes guarding the west, the valley filled with gravels and sands. What created this place?

At one time millions of years ago, this place was covered by a shallow sea that stretched down to the Gulf of Mexico. In that sea small creatures lived, from the waters they collected calcium which they combined with carbon and oxygen o form their shells of calcium carbonate. Each small creature lived and died and added its shell to the debris at the bottom of the shallow sea, eventually forming a thick layer of limestone.

Later the land below the sea was lifted up and the waters of the sea drained away into the Gulf of Mexico leaving behind the thick layer of limestone studded with some of the shells of the small creatures.

As the land was lifted up a fracture formed from the north to the south. On the west side of the fracture or fault the land dropped down and on the east side it continued being uplifted forming the Sandia Mountains. Beneath the layer of limestone were granites formed by the slowly cooling igneous rocks of the mantle of the earth. As the igneous rocks cooled they formed crystals of quartz, feldspar and mica. Some areas cooled more slowly than others and the rarer minerals migrated into these areas where they formed pegmatites containing the rarer minerals and in some cases gem materials. The remains of these pegmatites stud the western slopes of the Sandias but most are now covered by housing developments. As the mountains were lifted up on the east and the land on the west of the fracture was dropped down, erosion began to fill the valley. Sands formed from the weathering of the quartz in the granite and clays formed from the decomposing feldspar were carried down to the fill the valley. The gravels were the first materials to be dropped by the rushing waters and then the sands and silt and finally as the waters formed lakes and pools the clays settled to the bottom forming thick layers. Since the lands to the north were also lifted up the rains and snows drained to the south and their natural course was along the fault line. Thus the Rio Grande was formed. Additional deposits of gravel, sand, silt and clay were brought down from the north by the river.

As the Sandia Mountains continued being uplifted they were continually eroded by wind and rain and the debris was spread out in broad fans at the foot of the mountains. These alluvial fans became the east mesa. On the west side of the fault as the land dropped down, the layer of limestone was buried by the sands and gravels. Now they lie as far below the valley as the Sandia Mountains are above the valley. From those deep limestone layers the waters dissolved calcium carbonate which was then carried upward and redeposited in the sands and gravels as caliche, cementing the particles together.

Hot magma below the fault later was pushed up and erupted as lava which spread out in thick layer upon the sands and gravels forming the west mesa and the west escarpment.

Still later sands were blown in from the west and northwest to partially cover the lava flows. In the Jemez Mountains to the north a monstrous eruption creating the Valle Grande Caldera spread volcanic ash for hundreds of miles. Some of that ash was dropped in the west along what is now known as the Rio Puerco. Additional volcanic materials in the form of obsidian were carried southward by the Rio Grande. The high silica content of the volcanic waters was deposited in wood buried in the sands and gravels crating petrified wood. There are still abundant on the west mesa and in gravel pits.

So the soil profiles from east to west are: solid granite, decomposed granite, shallow soils over granite, little or no ground water, gravels and sand gradually deepening ground water in areas where natural drainage from the mountains occur, clays with some ground water the valley with clays, sand, silt and gravels in layers. The valley were there is ground water but is no drainage because the river is higher than the valley. The river which is leveed to prevent flooding. Deep sands with ground water. Lava covering sands and gravels making the deep water inaccessible.

All of these soils in the area contain little or no organic matter and thus have very little nitrogen. Most of the soils and the water are slightly to very alkaline but contain good to adequate amounts of potassium. Phosphorus, the third mineral required in major amounts by growing plants is also in short supply while iron which is usually present in what might be adequate amounts is made unobtainable by the alkaline conditions.

The shape of the valley as it evolved also has a bearing on the current weather. Often winter storms pass through New Mexico bringing to a halt all traffic in every direction while Albuquerque itself basks in the winter sun. These same factors allow gardeners to take advantage of micro-climates created by block walls and northerly protections. Rainfall is more abundant near the foot of the mountains and spring and winter winds reach gale proportions on parts of the east mesa as they are funneled through Tijeras canyon. The cold air settles in the valley areas bring later spring frost and earlier fall frosts shortening their growing season as much as six weeks. So not only is Albuquerque unique, it also presents problems to the gardener that are unique. Only by understanding what can be done to modify the environment can one expect to become a successful gardener.

 Edited by R. Bronson. 11/16/2003

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