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Soil biota multiply rapidly when organic material, roots, and plant litter, their food source, are available and the soil is moist and warm. Seasonal patterns of biological activity coincide with plant growth stages, litter fall, and root die-off. To be active, bacteria require films of water in soil pores, whereas fungi can function in drier conditions. When the soil is too dry, bacteria and fungi become less active or temporarily shut down, protozoa form dormant cysts, and the number of most other organisms declines. When the soil is saturated and anaerobic, the number of denitrifying bacteria increases. Organisms affect each other through predation and competition for food and space. Small soil pores can restrict the movement of large soil organisms. Different types of vegetation produce different types of litter and plant residue and thus provide different food sources for soil biota. Changes in the vegetation or the pattern of plant distribution affect the soil organisms.

Grazing.—Proper management of the plant community is the best strategy for maintaining the benefits of the soil food web. Plant production and the supply of organic matter can be maintained or enhanced by timely grazing, the proper frequency of grazing, and control of the amount of vegetation removed. If the plant community is overgrazed, a reduction in the amount of surface plant material and roots will result in less food for soil organisms. As biological activity decreases, a downward spiral of the important functions of soil organisms results in a lower content of organic matter and impedes nutrient cycling, water infiltration, and water storage. Heavy grazing also can reduce the abundance of nitrogen-fixing plants, causing a decrease in the supply of nitrogen for the entire plant community.

Erosion.—Erosion removes or redistributes the surface layer of the soil, the layer with the greatest concentration of soil organisms, organic matter, and plant nutrients. Runoff and wind erosion redistribute litter from one area of rangeland to a surrounding area. The loss of organic matter reduces the activity of soil biota in the areas from which the litter has been removed.

Compaction by grazing animals and vehicles.—Soil compaction reduces the larger pores and pathways, thus reducing the amount of habitat for nematodes and the larger soil organisms. Compaction can also cause the soil to become anaerobic, increasing losses of nitrogen to the atmosphere.

Fire and pest control.—Fire can kill some soil organisms and reduce their food source while also increasing the availability of some nutrients. Pesticides that kill above-ground insects can also kill beneficial soil insects. Herbicides and foliar insecticides applied at recommended rates have a smaller impact on soil organisms. Fungicides and fumigants have a much greater impact on the soil organisms.

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Soil biota, the biologically active powerhouse of soil, include an incredible diversity of organisms. Tons of soil biota, including micro-organisms (bacteria, fungi, and algae) and soil “animals” (protozoa, nematodes, mites, springtails, spiders, insects, and earthworms), can live in an acre of soil and are more diverse than the community of plants and animals above ground. Soil biota are concentrated in plant litter, the upper few inches of soil, and along roots. Soil organisms interact with one another, with plant roots, and with their environment, forming the soil food web.

As soil organisms consume organic matter and each other, nutrients and energy are exchanged through the food web and are made available to plants. Each soil organism plays a role in the decomposition of plant residue, dead roots, and animal remains. The larger soil organisms, such as millipedes and earthworms, shred dead leaves and residue, mix them with the soil, and make organic material more accessible to immobile bacteria. Earthworms can completely mix the top 6 inches of a humid grassland soil in 10 to 20 years. Ants and termites mix and tunnel through soils in areas of arid and semiarid rangeland.

Predators in the soil food web include scorpions, centipedes, spiders, mites, some ants, insects, and beetles. They control the population of soil biota. The smaller organisms, including mites, springtails, nematodes, and one-celled protozoa, graze on bacteria and fungi. Other organisms feed on dead roots, shredded residue, and the fecal by-products of the larger organisms. The smallest soil organisms, microscopic bacteria and fungi, make up the bulk of the biota in the soil. They finish the process of decomposition by breaking down the remaining material and storing its energy and nutrients in their cells. Algae and fungi are the first organisms to colonize rock and form “new soil” by releasing substances that disintegrate rock.


Visit the Ecomerge website to learn more about organic gardening soil and how to preserve soil at home.

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The development of objectives relative to soil crusts is an important part of rangeland management. Biological crusts protect the soil from water erosion and wind erosion. Physical crusts can protect soils from wind erosion as effectively as biological crusts, except on very coarse textured soils. In the more humid areas, it generally is desirable to break up physical crusts and thus improve seedling emergence and plant establishment; however, desirable biological crusts can be destroyed when the physical crusts are broken. Adequate organic matter, seeds of desirable species, and a period of rest are needed for successful establishment of plants after crusts are broken.

Recovery of biological crusts may take decades to hundreds of years. Therefore, preventing degradation by minimizing disturbance is important. Biological crusts that are in areas of low rainfall, are on coarse textured soils with low stability, and are in areas with a large amount of bare ground are most susceptible to frequent disturbances and have the longest recovery times. Biological crusts of all types are least susceptible to disturbance when the soil is frozen or is covered with snow. Biological crusts on sandy soils are less susceptible to disturbance when the soils are wet or moist, and the ones on clayey soils are less susceptible when the soils are dry. Trampling or grazing when the soil surface is very wet or ponded should be avoided because it can displace and bury the biological crust.

The following management strategies apply to land used for grazing, wildlife habitat, or recreation:

• Maintain the optimum amount of live vegetation, litter, and biological crust relative to the site potential in order to maintain the content of organic matter and soil structure and control erosion.

• In humid areas improve soil structure and plant establishment by incorporating organic matter into the soil while breaking up a physical crust.

• Defer grazing and recreational use during periods when biological crusts are most susceptible to physical disturbances.

• Use prescribed burning according to the needs of each site to prevent fuel buildup that can produce hot fires followed by severe erosion.

• Control the establishment and spread of invasive annual plants that can carry fire.



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Physical crusts form when organic matter is depleted from the surface layer, soil aggregates become weak, and raindrops disperse the soil into individual particles that clog soil pores, seal the surface, and form a layer that is dense when dry. A physical crust consisting of numerous thin bands can form when sediment from erosion is carried downslope and buries the soil surface. Physical crusts are more common on silty, clayey, and loamy soils and are relatively thin or weakly expressed, if present at all, on sandy soils. Soils with a high content of sodium disperse readily in water and are more susceptible to crust formation than other soils.

To examine a crust, lift the soil surface with a knife tip and look for cohesive layers or thin bands parallel to the soil surface. These layers have no apparent binding by visible strands of organic material, such as cyanobacteria. Fragments of physical crusts disperse or “melt” when placed in water. A vesicular crust is a type of physical crust with many small, unconnected air pockets or spaces similar to those in a sponge.

A biological crust occupies a large amount of the surface of calcareous and gypsiferous soils. Soil texture, moisture, temperature, season of precipitation, and history of disturbance largely determine the dominant organisms in the crust. For example, moss tends to be dominant in the Columbia Basin, whereas cyanobacteria and lichen are dominant in the Mojave, Sonoran, and Chihuahuan Deserts.

Prevent crust formation by keeping organic matter on your garden. Adding a healthy layer of organic matter to your garden from time to time will help prevent soil erosion.



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