“When you are standing on the ground you are really standing on the roof top of a whole other world.” Dr. Jill Clapperton, rhizosphere ecologist at the Agriculture and Agri-Food Canada Lethbridge Research. (Clapperton, J. 2003.)
Since the introduction of synthetic (inorganic) fertilizers during the industrial revolution, most of the research has been focused on maintaining the nutrient balance in the soil. These nutrients include the six macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S); and the seven micronutrients: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn).
However, more researchers and agricultural producers are realizing that not only are the nutrients in the soil important, but also, the biological health of the soil.
“A healthy soil is the capacity of the soil to function as a vital system to sustain the productivity of animals and plants, maintain or improve the water and air quality, and health of the plants and animals within the limits of an ecosystem.” (Doram and Zeiss, 2000.)
We know that soil organisms break down organic matter, making nutrients available for uptake by plants and other organisms. The nutrients stored in the bodies of soil organisms also prevent nutrient loss from leaching. Microbes produce waste products that help to maintain soil structure, and earthworms help to distribute these beneficial waste products through the soil profile. However, we do not understand critical aspects about how these populations function and interact. The discovery of glomalin (a glycoprotein produced abundantly on hyphae and spores of mycorrhizal fungi in soil and in roots) in 1996 indicates that we lack the knowledge to correctly answer some of the most basic questions about the The chemical interactions that exist between the atmosphere, hydrosphere, lithosphere, and biosphere in soils. (Soil Biology Primer, SWCD, 2000.)
The linkages between soil organisms and soil functions are incredibly complex. The interconnectedness and complexity of this soil ‘food web’ means any appraisal of soil function must take into account interactions with the living communities that exist within the soil.
There are many different types of creatures that live in the topsoil. Each has a role to play. These organisms will work for the farmer’s benefit if simply managed for their survival.
An acre of living topsoil contains approximately 900 pounds of earthworms, 2,400 pounds of fungi, 1,500 pounds of bacteria, 133 pounds of protozoa, 890 pounds of arthropods and algae, and even small mammals in some cases. (Pimentel, D. 1995)
While a great variety of organisms contribute to soil fertility, earthworms, arthropods and the various microorganisms merit particular attention.
Bacteria are the most numerous type of soil organism: every gram of soil contains at least a million of these tiny one-celled organisms. One of the major benefits bacteria provide for plants is in making nutrients available to them. Some species release nitrogen, sulfur, phosphorus and trace elements from organic matter. Others break down soil minerals, releasing potassium, phosphorus, magnesium, calcium and iron. Still other species make and release plant growth hormones, which stimulate or inhibit root growth.
Several species of bacteria transform nitrogen from a gas in the air to forms available for plant use, and from these forms back to a gas again. A few species of bacteria fix nitrogen in the roots of legumes, while others fix nitrogen independently of plant association. Bacteria are responsible for converting nitrogen from ammonium to nitrate and back again, depending on certain soil conditions. Other benefits to plants provided by various species of bacteria include increasing the solubility of nutrients, improving soil structure, fighting root diseases, and detoxifying soil.
Actinomycetes (ac-tin-o-my´-cetes) are threadlike bacteria that look like fungi. While not as numerous as bacteria, they too perform vital roles in the soil. Like the bacteria, they help decompose organic matter into humus, releasing nutrients. They also produce antibiotics to fight root diseases. Many of these same antibiotics are used to treat human diseases. Actinomycetes are responsible for the sweet, earthy smell noticed whenever a biologically active soil is tilled.
Fungi come in many different species, sizes, and shapes in soil. Some species appear as threadlike colonies, while others are one-celled yeasts. Many fungi aid plants by breaking down organic matter or by releasing nutrients from soil minerals. Fungi are generally quick to colonize larger pieces of organic matter and begin the decomposition process. Some fungi produce plant hormones, while others produce antibiotics including penicillin. There are even species of fungi that trap harmful plant-parasitic nematodes.
The mycorrhizae (my-cor-ry´-zee) are fungi that live either on or in plant roots and act to extend the reach of root hairs into the soil. Mycorrhizae increase the uptake of water and nutrients, especially phosphorus. They are particularly important in degraded or less fertile soils. Roots colonized by mycorrhizae are less likely to be penetrated by rootfeeding nematodes, since the pest cannot pierce the thick fungal network.
Mycorrhizae also produce hormones and antibiotics that enhance root growth and provide disease suppression. The fungi benefit by taking nutrients and carbohydrates exuded by the plant roots they live in.
Fig. 1. Root heavily infected with mycorrhizal fungi (note round spores at the end of some hyphae). Photo by Sara Wright.
Nematodes are abundant in most soils, and only a few species are harmful to plants. The harmless species eat decaying plant litter, bacteria, fungi, algae, protozoa and other nematodes. Like other soil predators, nematodes speed the rate of nutrient cycling.
Arthropods are species of soil organisms that can be seen by the naked eye. Among them are sowbugs, millipedes, centipedes, slugs, snails and springtails. These are the primary decomposers. Their role is to eat and shred the large particles of plant and animal residues. Some bury residue, bringing it into contact with other soil organisms that further decompose it. Some members of this group prey on smaller soil organisms. The springtails are small insects that eat mostly fungi. Their waste is rich in plant nutrients released after other fungi and bacteria decompose it.
Earthworm burrows enhance water infiltration and soil aeration. Fields that are “tilled” by earthworm tunneling can absorb water at a rate four to 10 times that of fields lacking worm tunnels. This reduces water runoff, recharges groundwater, and helps store more soil water for dry spells. Vertical earthworm burrows pipe air deeper into the soil, stimulating microbial nutrient cycling at deeper levels. (Edwards, Clive and Bohlen, 1996).
Earthworms thrive where there is no tillage. Worm numbers can be reduced by as much as 90 percent by deep and frequent tillage. Tillage reduces earthworm populations by drying the soil, burying the plant residue they feed on and making the soil more likely to freeze. Tillage also destroys vertical worm burrows and can kill and cut up the worms themselves. Young worms emerge in spring and fall. They are most active just when farmers are likely to be tilling the soil.
As a rule, earthworm numbers can be increased by reducing or eliminating tillage (especially fall tillage), not using a moldboard plow, reducing residue particle size (using a straw chopper on the combine), adding animal manure, and growing green manure crops. It is beneficial to leave as much surface residue as possible year-round.
Soil organisms require an environment that is damp (like a wrung out sponge) but not soggy, and temperatures between 50 – 90 degrees F. Soil organism activity may be reduced due to dry soil conditions. However, avoid over-irrigation since waterlogged soils will be harmful to beneficial soil organisms. Any reduction in soil organism population will affect the availability of plant nutrients and formation of organic matter.
Creating a soil habitat is the first step to managing soil biological properties for longterm soil quality and productivity. This means using soil management practices that reduce soil disturbance, managing weeds and disease with crop rotation, mixed cropping and underseeding, and using high quality compost and composted manure.
For instance, unstructured soils with low organic matter content that have fine aggregates or clay within the plow-layer will take between three to five years to build the soil biological properties necessary to improve soil structure and stability depending on climate and previous soil management.
It is generally understood that the soil biological community benefits soil productivity, yet so little is known about the organisms that live in the soil and the functioning of the soil ecosystem. Continued research aimed at understanding the interactions between soil management practices and the soil biological, chemical and physical properties of soil will be the key to sustaining the soil, environment and future generations. Much work is ahead to gain a better understanding of how soil biological components affect the environment and planet they share with us.
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