The importance of soil organic matter
Soil microorganisms have had another direct importance for humans—they are the relationship with fungi that increases the contact of roots with the soil. The Relationship between Micro-organisms and Soil. Aggregation. BY R. J. SWABY. Department of Soil Microbiology, Rothnmsted Experimental Station. Soil organisms can also interact with other organisms, and, with the Relationships where there is mutual benefit but no obligation on the part of either .
Organic matter can also be restored to the soil through growing green manures, cuttings from agroforestry species and the addition of manures and compost.
Soil organic matter is the key to soil life and the diverse functions provided by the range of soil organisms. Increased production of green manure or crop biomass aboveground and belowground increases the food source for the microbial population in the soil.
Agricultural production systems in which residues are left on the soil surface and roots left in the soil, e. In one year experiment in Brazil, such practices resulted in a percent increase in microbial carbon biomass and a percent increase in microbial N biomass Figure A2.
Balota, Andrade and Colozzi Filho, In undisturbed soil ecosystems, e.
Fine roots are the primary sites of mycorrhizal development as they are the most active site for nutrient uptake. This partly explains the increase in mycorrhizal colonization under undisturbed situations as rooting conditions are far better than under conventional tillage.
Venzke Filho et al.
Gassen and Gassen, Another consequence of increased organic matter content is an increase in the earthworm population. Earthworms rarely come to the soil surface because of their characteristics: Soil moisture is one of the most important factors that determine the presence of earthworms in the soil. Through cover crops and crop residues, evaporation is reduced and organic matter in the soil is increased, which in turn can hold more water.
Residues on the soil surface induce earthworms to come to the surface in order to incorporate the residues in the soil. The burrowing activity of earthworms creates channels for air and water; this has an important effect on oxygen diffusion in the rootzone, and the drainage of water from it.
Furthermore, nutrients and amendments can be distributed easily and the root system can develop, especially in acid subsoil in the existing casts. The shallow-dwelling earthworms create numerous channels throughout the topsoil, which increases overall porosity, and thus bulk density Figures A2.
The large vertical channels created by deep-burrowing earthworms increase water infiltration under very intense rainfall conditions. The ability of organic matter to retain cations for plant use while protecting them from leaching, i. Many acid-forming reactions occur continually in soils. Some of these acids are produced as a result of organic matter decomposition by microorganisms, secretion by roots, or oxidation of inorganic substances.
In particular, ammonium fertilizers, such as urea, and ammonium phosphates, such as monoammonium and diammonium phosphate, are converted rapidly into nitrate through a nitrification process, releasing acids in the process and thus increasing the acidity of the topsoil Figure A2. When acids or bases are added to the soil, organic matter reduces or buffers the change in pH.
This is why it takes tonnes of limestone to increase the pH of a soil significantly compared with what would be needed to simply neutralize the free H present in the soil solution.
All of the free hydrogen ions in the water in a very strongly acid soil pH 4 could be neutralized with less than 6 kg of limestone per hectare.
However, from 5 to more than 24 tonnes of limestone per hectare would be needed to neutralize enough acidity in that soil to enable acidsensitive crops to grow. Almost all of the acid that must be neutralized to increase soil pH is in organic acids, or associated with aluminium Al where the pH is very low.
However, with large values of soil organic matter, the pH will decrease less rapidly and the field will have to be limed less frequently.
In comparing conventional and conservation tillage in Brazil, the highest values of soil CEC and exchangeable calcium Ca and magnesium Mg were found in legume-based rotation systems with the highest organic matter content Figure A2. Organic matter releases many plant nutrients as it is broken down in the soil, including N, phosphorus P and sulphur S.
Leguminous species are very important as part of a cereal crop rotation in view of their capacity to fix N from the atmosphere through symbiotic associations with rootdwelling bacteria. After nine years, no tillage in combination with the intensive cropping system had resulted in a percent increase in soil N compared with conventional tillage. Although N uptake by plants was less in no-tillage systems, probably because of N immobilization and organic matter building, the maize yields under the different tillage systems did not differ.
As the no-tillage system was more efficient in storing soil N from legume cover crops in the topsoil, in the long term this system can increase soil N available for maize production Amado, Fernandez and Mielniczuk, Calegari and Alexander noted that the P content both inorganic P and total P of the surface layer cm was higher in the plots with cover crops after nine years.
Cover crops were shown to have an important P-recycling capacity, especially when the residues were left on the surface. This was especially clear in the fallow plots, where the conventional tillage plots had a P content 25 percent lower than the no-tillage plots. Depending on the cover crop, the increase was between 2 and almost 30 percent.
Even more important is the effect of land preparation on the increase of P availability in the soil Figure A2. Phosphorus content of soil after 9 years of conservation agriculture, compared with conventional tillage Note: Three to five years after initiating an intensified production system, both P and potassium K can be accumulated in the topsoil.
On the other hand, where direct seeding is practised and the crop residues are left on the surface, percent of the nutrients were concentrated in the top layer of the soil.
Plant residues that cover the soil surface protect the soil from sealing and crusting by raindrop impact, thereby enhancing rainwater infiltration and reducing runoff.
Increased organic matter also contributes indirectly to soil porosity via increased soil faunal activity.
Biology Life in Soil | Soils 4 Teachers
Earthworms pass both soil and organic matter through their guts, in the process aerating the soil, breaking up the litter of organic material on its surface, and moving material vertically from the surface to the subsoil. This is extremely important to soil fertility, and it develops the structure of the soil as a matrix for plants and other organisms. It has been estimated that earthworms completely turn over the equivalent of all the soil on the planet to a depth one inch 2.
Some vertebrates are also in the megafauna category; these include all sorts of burrowing animals, such as snakes, lizards, gophers, badgers, rabbits, hares, mice, and moles.
One of the most important roles of soil organisms is breaking up the complex substances in decaying plants and animals so that they can be used again by living plants. This involves soil organisms as catalysts in a number of natural cycles, among the most prominent being the carbon, nitrogen, and sulfur cycles. The carbon cycle begins in plants, which combine carbon dioxide from the atmosphere with water to make plant tissues such as leaves, stems, and fruits.
Animals eat the plants and convert the tissues into animal tissues. The cycle is completed when the animals die and their decaying tissues are eaten by soil organisms, a process that releases carbon dioxide.
Proteins are the basic stuff of organic tissues, and nitrogen is an essential element of all proteins. The availability of nitrogen in forms that plants can use is a basic determinant of the fertility of soils; the role of soil organisms in facilitating the nitrogen cycle is therefore of great importance.
When a plant or animal dies, soil organisms break up the complex proteins, polypeptides, and nucleic acids in their bodies and produce ammonium, ions, nitrates, and nitrites that plants then use to build their body tissues.
There are no known diseases of turfgrass as a result of protozoa. Millions of nematodes can inhabit a few square feet of soil.
Nematodes reproduce by eggs and are most prevalent in warm, moist, sandy soils. They are essential in the soil food web, especially in the recycling of soil nutrients.
Like many other microorganisms, nematodes can positively affect soil. However, some species can be detrimental to plants. Of the thousands of nematode species so far identified, only about 50 feed on turfgrass roots.
Sting, ring, stubby root, lance, root-knot, spiral and others are important parasitic nematodes on turfgrass roots.
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Detrimental populations are most likely found in irregular patches of the rootzone. This is to be expected, because most parasitic nematodes are root feeders and require plant roots to thrive and reproduce. Few effective nematicides are available, so control is difficult. The best option is to fumigate prior to establishment, if possible. Biological control may hold some potential for nematode control. Perhaps in the future, such products will provide practical effective control.
Viruses are intracellular parasites that do not respire or metabolize and must have a biological host to reproduce. Soils may harbor many viruses, but it is believed that they do not directly affect soil characteristics. Little is known about pathogenic turfgrass viruses. Augustinegrass decline and centipedegrass mosaic have been attributed to panicum mosaic virus.
The virus is spread through sap on mower blades. Mowing when the turfgrass is dry has proven to reduce the spread of the virus. To best control this pathogen, use resistant St. Fortunately, many of the improved varieties currently on the market are resistant. These organisms mostly affect turf by churning the soil while feeding, which aids aggregation, water movement, aeration and thatch degradation. These serve vital functions in the rootzone — most are beneficial but some are detrimental.
Our understanding of soil micorbiology is in the early stages. As we learn more, there's little doubt we'll be able to put this knowledge to use to grow better turf. Clint Waltz is a graduate assistant, H.