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Get The Dirt: Understand the Ecosystem of Your Soil

Soil is the foundation upon which all agricultural activities are built. And therefore, it is the crux of the environmental movement to make sure our most precious resource is preserved (and created) sustainably. The first step to doing so, however, is understanding its ecology. Take it away, experts!

The following is an excerpt from The Soul of Soil: A Soil-Building Guide for Master Gardeners and Farmers by Grace Gershuny and Joe Smillie. It has been adapted for the web.

The first step toward effective ecological soil management is an appreciation of the complex, living system known as soil. And to understand soil is to be aware of how everything affects and is affected by it. We are all part of the soil ecosystem.

Soil fertility can be described as its capacity to nurture healthy plants. Sustainable agriculture aims to protect the soil’s ability to regenerate nutrients lost when crops are harvested—without dependence on “off-farm” fertilizers. This regenerative capacity, in turn, depends on the diversity, health, and vitality of the organisms that live, grow, reproduce, and die in the soil. Through the activities of soil microbes—which can number in the billions in every gram of healthy topsoil—the basic raw materials needed by plants are made available at the right time, and in the right form and amount.


The basic aim of ecological soil
management is to provide hospitable
conditions for life within the soil.

Your farm is both the product and producer of soil. Consider your farm to be a living organism that achieves its greatest long-term productivity when its natural cycles and processes are enhanced. Shortcutting these cycles for short-term control or economic gain will eventually bear out the ecological maxim, “The creature that wins against its environment destroys itself.”

The place to start is where you are. Thousands of soil types have been named, classified, and described. Knowing their names can tell you a lot about their general characteristics; but, like any living creature, each individual is unique. Find out what soils live in your area, how they are classified and described by soil scientists, and how that
compares with what you observe about them yourself.

Soil classification schemes organize soils according to their different qualities, based on the kinds of minerals they contain, how they were formed, and various physical characteristics. The individual character of any soil arises from a combination of factors inherent to its particular geographic region (see table 1).

Table 1:

Climate. Temperature and precipitation affect the rate of organic matter accumulation and the presence of soluble soil minerals. For example, more organic matter accumulates where decomposition is slow due to cooler temperatures, while high rainfall leaches mineral nutrients from topsoil.

Native vegetation. Grasslands, forests, and transition zones each affect soil development in a different way. Leaf litter from pine forests, for example, increases soil acidity. Soil particles developed under grasslands are usually bound into stable aggregates by the activity of the plentiful microorganisms and roots found there.

Parent material. Underlying rock types from which it was formed determine a soil’s mineral content and basic textural qualities. Limestone bedrock, for instance, helps counteract soil acidity. Red soils indicate that the parent material
and derived soil is rich in iron. Volcanic ash eventually produces soils heavy in amorphous clays.

Topography. Soil may be eroded from slopes and deposited in lowlands. The legendary fertility of river valleys such as the Nile resulted from deposits of rich sediment carried from the highlands, while mountain farmers all over the world have problems holding onto precious topsoil.

Time. The availability of minerals and the extent of humus development in soil is also influenced by how long the native rock has been subject to weathering. Young soils, such as those in Hawaii and other areas of volcanic activity, may be low in clay, which is produced by the chemical effects of weathering on parent rocks.

Glaciation and geologic activity. In the north-temperate region, the advance and retreat of glaciers, most recently a mere 12,000 years ago, has had a significant effect on soil formation and quality. Volcanic activity has left nutrient-rich lava deposits in many areas.

Soils worldwide have been classified into ten major orders (see table 2). In humid temperate regions such as the northeastern United States, where forests are the predominant natural vegetation, the soil order of spodosols is most common. These soils are generally formed from coarse-textured parent material, and tend to be quite acidic and low in mineral nutrients. Prairie soils, which have developed under flat, grass-covered areas with modest rainfall, are classified as mollisols. They are among the most naturally productive soils, with high native organic matter and mineral content. In tropical regions with very high seasonal rainfalls, the heavily leached ultisol soils also tend to acidity. The Sahara, Gobi, and Turkestan Deserts, as well as South and Central Australia and the American Southwest are largely comprised of aridisols. If irrigated they can be productive, but great care must be taken to prevent toxic accumulations of soluble salts.

Each order is further broken down into suborders, great groups, and subgroups. Beyond this, soils are described in terms of families, associations, and series, which provide more information about their plant growth characteristics, organic and mineral content, structure, drainage, and color. Series are often named after the places—towns, rivers, or counties—where they are located.

Your local Extension or Soil Conservation Service office can probably give you a soil map for your land. They can also show you your county’s soil survey, which provides detailed information on local soils and their best uses, as well as helpful climatological data.

Table 2:

Entisols: Recently formed mineral soils with little evidence of horizon formation. Found in a wide range of climate zones, including the Rocky Mountains, the Sahara Desert, Siberia, and Tibet. May be highly productive, but
most are relatively barren.

Vertisols: Mineral soils with a high content of swelling-type clays, which in dry seasons cause the soils to develop deep cracks. Found in some areas of the southern U.S., India, Sudan, and eastern Australia. Their physical
properties make them difficult to till and cultivate.

Inceptisols: Young soils with limited horizon formation. May be very productive, as those formed from volcanic ash. Found in the Pacific Northwest (U.S.), along the Amazon and Ganges Rivers, North Africa, and eastern

Aridisols: Mineral soils found mostly in dry climates. Productive only if irrigated, and may become saline. Found in the southwestern U.S., Africa, Australia, and the Middle East.

Mollisols: Characterized by a thick, dark surface horizon, they are among the world’s most productive soils, with high natural fertility and tilth. Generally found under prairie vegetation, such as the Great Plains (U.S.), Ukraine, parts of Mongolia, northern China, and southern Latin America.

Spodosols: Mineral soils characterized by distinct horizons, including subsurface organic matter, and aluminum and sometimes iron oxides. Coarsetextured, readily leached, and tending to be acid, they occur mostly in humid, cold temperate climates, generally under forests. Can be very productive if properly fertilized.

Alfisols: Moist mineral soils with high base status and presence of silicate clays. Found mostly in humid regions under deciduous forest or grass including parts of the U.S. Midwest, northern Europe, southern Africa, and Southeast Asia. Highly productive, good nutrient levels and texture.

Ultisols: Moist soils that develop under warm to tropical climates. Highly weathered, acidic, with red or yellow subsurface horizons. Found in the humid southeastern U.S., Southeast Asia, and southern Brazil. Can be highly productive, with good workability.

Oxisols: The most highly weathered soils, with a deep subsurface horizon of iron and aluminum oxides. High in clay, commonly deficient in phospho-surfaces. Not well adapted to mechanized farming, they have been poorly researched.

Histosols: Organic soils that have developed in a water-saturated environment, with at least 20 percent organic content. Can be very productive if drained, especially for vegetable crops.

SOURCE: adapted from Nyle Brady, The Nature and Properties of Soils, 10th ed.

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