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Added by: MarvinM92
The Soil Food Web
Contributed by: paraniodpete
THE SOIL FOOD WEB
Unseen beneath our feet, there dwells a teeming microscopicuniverse of complex living organisms that few humans ever consider. Inone teaspoon of soil alone, there may he over 600 million bacterialcells. These bacterial cells exist in complex predator-preyrelationships with countless other diverse organisms. This topsoil foodweb forms the foundation for healthy soil, healthy plants, andultimately, a healthy planet.
The soil food web is the community of organisms living all or partof their lives in the soil. The food web has a basic set of expectedorganisms groups, but the numbers of organisms and different species ineach group can vary significantly by plant and soil type.Photosynthesizing living plant material provides the initial energy tothe soil food system through their roots. Living plant roots exude manytypes of complex high-energy nutrient molecules into the surroundingsoil. Dead plant material is decomposed by bacteria and fungi, buildingup even greater numbers of these organisms and their metabolicproducts. As a plant grows, photosynthesis supplies much more than theindividual plant’s carbohydrate requirements. It has been documentedthat plant roots can exude over 50 percent of the carbon fixed throughphotosynthesis in the form of simple sugars, proteins, amino acids,vitamins, and other complex carbohydrates.
photosynthesis
pho•to•syn•the•size - synthesis of chemical compounds with theaid of radiant energy and especially light; : formation ofcarbohydrates from carbon dioxide and a source of hydrogen (as water)in the chlorophyll-containing tissues of plants exposed to light
ORGANIC SOIL STRUCTURE
Around plant roots, bacteria form a slimy layer. They produce wasteproducts that glue soil particles and organic matter together in small,loose clumps called aggregates. Threading between these aggregates andbinding them together are fine ribbon-like strands of fungal hyphae,which further define and stabilize the soil into macro aggregates. Itis this aggregated soil structure, which looks a bit like spongychocolate cake that effectively resists compaction and erosion andpromotes optimal plant and microbial growth. Water and air are alsostored in the aggregate pores until needed.
MYCORRHIZAL FUNG
Mycorrhizal fungi are especially effective in providing nutrientsto plant roots. These are certain types of fungi that actually colonizethe outer cells of plant roots, but also extend long fungal threads, orhyphae, far out into the rhizosphere, forming a critical link betweenthe plant roots and the soil. Mycorrhizae produce enzymes thatdecompose organic matter, solubilize phosphorus and other nutrientsfrom inorganic rock, and convert nitrogen into plant available forms.They also greatly expand the soil area from which the plant can absorbwater. In return for this activity, mycorrhizae obtain valuable carbonand other nutrients from the plant roots. This is a win-win mutualismbetween both partners, with the plant providing food for the fungus andthe fungus providing both nutrients and water to the plant. Theimportance of mycorrhizae in plant productivity and health has oftenbeen overlooked. EXAMPLE Pines are not native to Puerto Rico andtherefore the appropriate mycorrhizal fungi were absent in the soil.For years, people unsuccessfully tried to establish pines on theisland. The pine seeds would germinate well and grow to heights of 8 to10 cm but then would rapidly decline. In 1955, soil was taken fromNorth Carolina pine forests, and the Puerto Rico plantings wereinoculated. Within one year, all inoculated seedlings were thriving,while the un-inoculated control plants were dead. Microscopic analysisshowed that the healthy seedlings were well colonized by a vigorousmycorrhizal population. While the benefits of mycorrhizae is not alwaysas dramatic, it has been well documented that mycorrhizal plants areoften more competitive and better able to tolerate environmentalstress.
mycorrhizal
my•cor•rhi•zal
The symbiotic association of the mycelium of a fungus with the roots of a seed plant.
hyphae (plural of hypha)
hy•ph•ae
The microscopic, non-photosynthetic branching filaments thatcollectively form the feeding structure of a fungus called themycelium.
rhizosphere
rhi•zo•sphere
The soil surrounding and directly influenced by plant roots and micro-organisms.
COMPOST ADVANTAGES
Compost in particular can improve soil nutritional availability andsoil tilth because of its complex microbial population. Composts bringwith them a wide array of bacteria, fungi, protozoa, nematodes andmicro arthropods, along with the food resources needed to feed theseorganisms. However, not all composts have the same beneficial effects.There are many different types of composts, as determined by theiroriginal ingredients and their degree of maturity. The greater thediversity of food resources in the original composted material, thegreater the diversity of microorganisms that can grow in that compost.Soil from potted plants may be composted in the fall and used again thefollowing year. It is advantageous to leave the roots in the soilrather than removing them, fostering the presence of beneficialrhizoshperic organisms.
SOIL DISTURBANCE
In general, the largest soil organisms are the first damaged bysoil compaction and disturbance. These include earthworms and smallinsects, which are at the top of the soil food web and are essential tokeeping microbial populations in balance. When these organisms arelost, an otherwise undisturbed soil will have the tendency to shiftfrom being fungal dominated to being more bacterially dominated. Thiswill alter nutrient availability and soil structure, effectivelylimiting the types of plants that can grow. Some species of anaerobicbacteria thrive in a soil deprived of oxygen and can produce chemicalmetabolites, such as alcohols, aldehydes, phenols and ethylene, thatare toxic to plant roots and to other microorganisms. As compactioncontinues to eliminate pore space, plant roots have difficultyobtaining sufficient water, air and nutrients, placing them underconsiderable stress. This stress, added to the shift in beneficialorganisms, will create a situation where plant pathogens may increaserapidly and cause serious problems. No-till gardening methods can bevery useful in minimizing soil disturbance. When re-potting plants ofany kind, minimal disturbance to the root structure and soil isessential.
DISEASE SUPPRESSION
Dr. Ingham and others in her field have found that plant roots,well colonized by a mixture of different bacterial and fungal species,are far more resistant to pathogenic attack. Mycorrhizal fungi form animpenetrable physical barrier on the surface of plant roots, varying inthickness, density and fungal species, according to the plant species,plant health and soil conditions.
This layer of beneficial fungi plays a powerful role in diseasesuppression, both through simple physical interference as well asthrough the production of inhibitory products. Some species of fungithat parasitize other fungi, such as Trichoderma, have been observedphysically attacking and destroying pathogenic fungi. Dr. WilliamAlbrecht reported that Fusarium, a fungal species often maligned in itsrole in many plant diseases, could actually be one of the most commonbeneficial saprophytes in a healthy soil. He stated that the dividingline between beneficial symbiosis and parasitism could be very narrow.When Fusarium encounters a root that is poorly nourished or is understress, it can become rapidly pathogenic.
In healthy soil, unaltered by the application of lethalagricultural chemicals, “microherds” groups of microbes colonize theroot zone or the rhizosphere of the plant. Most are beneficial bacteriaand fungi; they do not damage living plant tissue and are critical tomaking essential minerals available to the plant. These microbes retainlarge amounts of nitrogen, phosphorous, potassium, sulfur, calcium,iron and many micronutrients in their bodies, preventing thesenutrients from being leached or removed by water runoff. Ideally, theyout-compete pathogenic species and form a protective layer on thesurface of living plant roots. It is usually only when the beneficialspecies of bacteria and fungi are killed by continuous soil disturbanceand toxic chemicals that pathogenic species have an advantage.
HERBICIDES, PESTICIDES
& FERTILIZERS
As part of her research, Dr. Ingham has shown that herbicides,pesticides and fertilizers have many non-target effects. The mostcommon pesticides are fairly broad spectrum; that is, they kill muchmore than the target species. Residual pesticides that accumulate insoil over many years may recombine and form new, unintentionalchemicals that have additional and often synergistic negative effects.Out of the 650 active ingredients used to formulate most commonagricultural pesticides, only about 75 have been studied to deter minetheir effects on soil organisms. The remaining ingredients have neverbeen studied for their effects on the whole system or on any non-targetgroup.
Scientists don’t fully understand the effect of any in individualingredient on soil life, much less the synergistic effects of theingredients, or combinational effects with inert or soil materials. Itis hardly surprising that a soil treated with numerous agriculturalchemicals lacks a healthy food web. When inorganic ammonium nitratefertilizer is applied to agricultural soil, ammonium and nitrate ionsare rapidly released into the soil solution. Nitrate ions arenegatively charged and can be quite mobile. The result is that a largepercentage of these nitrogen-containing ions may move rapidly out ofthe plant root zone (rhizosphere) and into the groundwater. Thisproduces not only reduced plant growth but also environmentalpollution. Plants growing in unhealthy soil require additionalfertilizers and pesticides, furthering the deadly spiral.
PLANT GROWTH REGULATOR COMPOUNDS
In return for the release of nutritional substances from plantroots, microbes themselves produce chemicals that stimulate plantgrowth or protect the plant from attack. These substances includeauxins, enzymes, vitamins, amino acids, indoles and antibiotics. Thesecomplex molecules are able to pass from the soil into plant cells andbe transported to other parts of the plant, with minimal change tochemical structure, where they can stimulate plant growth and enhanceplant reproduction. They may also play a role in enhancing thenutritional composition of the plant. The types of molecules releasedare specific for a variety of plants grown under certain conditions,forming in effect a unique chemical signature. As these molecules arereleased into the rhizosphere, they serve as food and growth stimulantsfor a certain mix of microbes. Dr. Joyce Loper, of the USDAAgricultural Research Service, and other scientists have shown that foreach plant species, this characteristic chemical soup stimulates thedevelopment of a select, beneficial company of root-dwelling microbes.This microbial population colonizes the root zone, producing certainchemicals that inhibit the growth of pathogenic species. Theseorganisms are also instrumental in supplying the plant’s uniquenutritional needs.
NUTRIENT CYCLING & RETENTION
Plants require many different mineral ions for optimal growth.These must be obtained from the soil. Many nutrient ions aresolubilized from the parent rock material in a process known asmineralization. Bacteria and fungi produce enzymes and acids necessaryto break down inorganic minerals and to convert them into stableorganic forms. Other nutrients are released through the decompositionof organic matter. In all cases, a healthy, diverse microbialpopulation will develop with rapid decomposition of organic materialand will facilitate the recycling of nutrients. Organic matter is alsoelectrically charged and therefore critical to its ability to attractand hold many different nutrient ions. The higher the organic matter inthe soil, the greater the ion holding capacity, resulting in reducedleaching of either an ions or cations from the soil.
There is much competition for nitrogen among soil organisms. Thoseorganisms that have the best enzymes for grabbing nitrogen are usuallythe winners. Bacteria possess the most effective nitrogen-grabbingenzyme system, closely followed by many species of fungi. Plant enzymesystems do not produce enzymes that operate outside the plant andcannot compete well when there is strong competition for limitednitrogen resources. In a healthy soil, this does not mean that theplant will be deprived of adequate nitrogen. Bacteria require onenitrogen atom to balance every five carbon atoms, and fungi require 10carbons for each nitrogen. Therefore, the predator organisms that eatbacteria and fungi get too much nitrogen for the carbon they require.Since excess nitrogen is toxic, is excreted as a body waste productback into the soil in a form that can be absorbed by plant roots.Nitrogen is not the only nutrient effectively stored and recycled bysoil microbes. Carbon is the major constituent of all cells. When soilsare depleted of organic matter and healthy microbial populations, theability of a soil to hold carbon is destroyed and it enters theatmosphere as carbon dioxide, now recognized as one of the greenhousegases that are responsible for breaking down the ozone layer.
There is little scientific evidence that bacteria and fungi simplydie and decompose. If another bacteria or fungus uses the dead cellsfor a food source, there is no release of nitrogen. It is only when apredator consumes excessive amounts of nitrogen in the dead cells thatit is released into the soil solution. It is this system of nitrogencycling that has worked brilliantly for the past million years.
SUMMARY
HOW IS THIS INFORMATION USEFULL TO YOU AS A GROWER?
Mycorrhizal fungi will colonize the rhizosphere
of any plant, given the right conditions. These fungi are asdiverse as the stars in the sky, and many fungi are plant specific,some are not. We have had great success with MJ inoculated with SC-27,developed by Dr. Frank McKenna of Australia. We have also witnessedmycelium from fungi on MJ roots, visible to the naked eye, develop overtime with no inoculation.
The problem with MJ, and so many plants, is that they are beinggrown outside of their native soil environment, much like the southernpines in Puerto Rico. Some plants adapt more readily to foreignenvironments than others and are less dependent on the symbioticrelationship that exists between plant and fungi. In nature, plantsgrow in the same soil season after season, developing a "relationship"so to speak with soil and its microscopic inhabitants.
It should be noted, that the regeneration of soil is beneficial tothe cultivation of fungi and bacteria. (Composting old soil from pots)The benefits of organic cultivation simply cannot be measured. Try aswe might, there is no improving on Mother Nature.
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