9. The rhizosphere
Plant
Root
• The zone of soil that is
significantly influenced by living
roots
• Usually extends about 2mm out
from the root surface
• The rhizosphere is enriched in
organic material due to root
exudates and sloughed off root
cells.
• Microbial activity in the
rhizosphere may be 2 – 10 greater
than in the bulk soil.
24. Soil is a habitat
Soil particles
Plant roots
Water
Notas del editor
Specifically we want to look at
The different kinds of organisms that live in our soils
The kinds of functions they perform and how they make a living
How abundant they are
Some examples of how they interact with each other, and finally
How our soil management practices affect soil life
This slide gives you just a small glimpse of the enormous diversity of organisms in the soil.
Soil organisms come in a great variety of sizes.
Macro or large organisms are those with a diameter greater than about 2 mm (1/10 in). They are easily visible to the human eye. Examples would include earthworms, plant roots (yes, plant roots are soil organisms), mice, voles, snakes, beetles, and millipedes to name a few.
Meso are the mid-sized organisms that range from about 2 mm down to 0.2 mm in diameter. These include mites, springtails, and smaller worms. Some of these critters are visible to the naked eye, but many of them are difficult to see without some magnification.
Finally, the micros are the small ones. These are less than 0.2 mm in diameter. In general, these can only be seen using microscopes, though large masses of fungal filaments can sometimes be seen. In fact, some scientists claim a single soil fungus that is spread over many acres in Michigan’s upper peninsula is actually the largest living organism in the world. Most of these organisms are truly miniscule such as the yeasts, actinomycetes, algae, and bacteria. Bacteria, for example, range from 0.5 to 5 um (1/50,000 to 1/5,000 in) in diameter. To put that into perspective, about 4,000 of the smaller bacteria could line up head to tail across the head of a pin. Of course bacteria have neither heads nor tails.
The focus in this program is going to be on the small end of the scale, the organisms we don’t see and often forget about, the microbiology of the soil.
A healthy soil contains a very large number of different kinds of species. Each of these species has a different function in the soil.
These include animals, many of which are very familiar to us because we see them all the time. Among these are:
All the familiar soil-dwelling mammals and snakes. These animals are also near the top of the food chain. They feed on plants and smaller animals.
The arthropods which include spiders, insects, and insect larvae.
The annelids which are all the various types of worms.
The mollusks which include animals such as snails and slugs.
The arthropods, worms, and mollusks are mostly herbivores and detritovores, meaning they feed on plants and parts of dead animals and plants. They perform an important function in the decay process in that they mix these materials into the soil. They also break apart large pieces of material to make them more accessible to other degraders.
And finally, the nematodes.
These are very small roundworms, 4 to 100 um (1/500 in) in diameter and up to a few millimeters in length (1/20 – 1/10 in). We usually hear about nematodes because they can be significant crop pests. Some will pierce the cells of crop roots to feed. This allows other plant pathogens to invade and cause infections that may severely damage or kill the plant.
Most nematodes, however, are beneficial. They feed on insect larvae, fungi, and bacteria, all of which could be plant pathogens. Since bacteria contain more nitrogen than the nematodes can use, their feeding serves to release plant available nitrogen into the soil. Nematode feeding may account for as much as 30-40% of the organic N released in some soils.
Another interesting, almost microscopic soil dwelling animal is the water bear. This water bear should not be confused with…
In addition to adding organic matter to soil, plant roots also have a great influence on the soil biology in the volume of soil immediately adjacent to them. This volume of soil is known as the rhizosphere and usually extends about 2 mm (1/10 in) out from the surface of living roots. Plant roots exude organic materials into this zone as well as dead cells sloughed from the growing roots. These sources of organic carbon greatly increase soil microbial life in the rhizosphere compared to the bulk soil. The net effect is beneficial for plant growth since the microbial activity tends to increase nutrient and water supply to the root. Rhizosphere activity also appears to increase root soil contact and to lubricate root extension through the soil.
Now lets turn our attention to the smaller and perhaps less familiar soil organisms.
The fungi are another large group of organisms that include yeasts, mildew, molds, and rusts. Although some fungi cause significant crop diseases, many soil fungi play very important functions in overall soil and crop health. The AM fungus shown here is an arbuscular mycorrhizae, a fungus that benefits higher plants. We will talk more about this kind of fungus in a few minutes. Mushrooms are the fruiting structure of some fungi. The beautiful red balls are the fruiting structure of a slime mold. Also shown here is a red yeast.
The fungi are an extremely important group of degraders. They are able to degrade parts of plants and animals that bacteria have a hard time with. Materials like cellulose, starch, and lignin. The fungi are very important in the process of humus formation and in nutrient cycling. The thread-like strands of fungi, called hyphae, also help to stabilize soil structure. Some fungi are predators on other organisms such as nematodes. Many fungi release chemicals into the soil that may be toxic to plants, animals, and bacteria. The first modern antibiotic drug, penicillin, was obtained from the soil fungus, Penicillium.
The protists are a large group of single celled organisms. These organisms are highly mobile and “swim” about in the soil pore water. The protists are mostly predators that feed primarily on bacteria. Consequently they have a large influence on soil bacteria population. This feeding contributes to nutrient cycling by releasing nutrients that were contained in the bacteria. Examples shown here are an amoeba, a ciliate, and a flagellate.
Several soil environmental factors affect the growth and activity of soil microorganisms. Some of these are factors that are altered by soil management.
In general as soil organic matter increases, so too does microbial growth and activity. The type of organic matter will have some effect on the type of microbial community in the soil. The residue of a single crop may be favored by a certain microorganism. In a monoculture of that crop, the favored microorganism will predominate.
Microorganisms are sensitive to oxygen status. Most microbes that are beneficial to crop production are aerobic, or require oxygen. Aerobic organisms will have a hard time thriving in a soil that is frequently flooded.
To thrive, microorganisms require adequate moisture and are most active at temperatures ranging from about 65 – 100 F. If the soil is too dry, too cold or too hot, microbial activity will slow considerably.
Finally soil fertility, especially adequate calcium and near neutral pH will favor growth of most desirable microorganisms.
