The Soil Food Web Tuning in to the World Beneath Our Feet by Mary-Howell R. Martens Elaine Ingham, Ph.D., has been at nseen beneath our feet, there dwells the forefront of Ua teeming microscopic universe of educating farmers complex living organisms that few humans about the value of ever consider. In one teaspoon of soil alone, understanding there may be over 600 million bacterial soil life. cells, and if that soil comes from the im- mediate root zone of a healthy plant, the number can exceed a million bacteria of many different species. These bacterial cells exist in complex predator-prey relation- ships with countless other diverse organ- isms. This topsoil food web forms the foun- dation for healthy soil, for healthy plants, and ultimately for a healthy planet. It is an essential but exceedingly delicate founda- The exchange of minerals and nutrients between plants tion that even the brightest scientists know and fungi represent a critical link to plant health and very little about. productivity. Above, ectomycorrhzial fungi, commonly Dr. Elaine Ingham has been research- associated with trees and grapes, form a protective ing this tiny universe for nearly 20 years. sheath around plant roots, making soil nutrients She has sought to understand the impor- available to the root cells and protecting the plant from tance of these organisms and the relation- pathogenic attack. ships that exist between them, and to elu- cidate the effects that various agricultural Again she found great differences in spe- web? This is not a simple question. The practices have on this vast network of life. cies present and numbers of the typical or- food web has a basic set of expected or- For part of her Ph.D. dissertation in 1981 ganisms in response to other important fac- ganism groups, but the numbers of organ- at Colorado State University, Dr. Ingham tors in the soil. isms and different species in each group researched the soil food web structures in Over the course of Ingham’s education can vary significantly by soil type, climate, Colorado soils that were farmed with and and subsequent career as a professor at plants present and management. Plants and without irrigation. She compared these re- Oregon State University and most recently plant structures are a major component in sults to native grassland soils. Not surpris- with Soil Foodweb Inc., a research and determining the food resources in soil that ingly, she discovered that the introduction consulting firm in Corvallis, Oregon, she will be available to bacteria and fungi. Pho- of agricultural systems altered the species has developed methods to quantify and tosynthesizing living plant material pro- of organisms present, particularly the fun- identify the microbial populations of soils. vides the initial energy to the soil food sys- gi, which are easily destroyed by agricul- She has learned that most traditional tech- tem through their roots. Living plant roots tural pesticides. For her postdoctoral work, niques of petri plate counts grossly under- exude many types of complex high-energy she compared grassland soils to high moun- estimate both number and diversity of spe- nutrient molecules into the surrounding tain meadow and pine forest soils, work- cies present in soil, since the artificial soil. Dead plant material is decomposed by ing across a typical successional gradient. conditions are not suitable for the growth bacteria and fungi, building up even great- of 99.99 percent of bacte- er numbers of these or- Reprinted from “It is important to view the rial species and of most ganisms and their meta- other organisms. To cir- bolic products. The more soil food web as a complex, cumvent this problem, In- diverse the initial plant whole system. When any group gham has developed effec- population, the greater the tive alternative techniques April 2000 - Vol. 30, No. 4 - Cover Story diversity of plant products of organisms in the system is based on direct enumera- that will be released, eliminated or damaged, the tion methods. She uses this information to thereby sustaining an increased variety of assist farmers and researchers by offering microbial organisms. delicate balance of a service that assesses the health and pro- For a healthy soil, unaltered by the appli- interrelationships can ductivity of their soil by measuring the di- cation of lethal agricultural chemicals, these versity and vitality of the soil food web. “microherds” of microbes colonize the root be shifted.” Which organisms compose this soil food zone, the rhizosphere, of the plant. Most are 75 have been studied to determine their ef- fects on soil organisms. The remaining in- gredients have never been studied for their effects on the whole system or on any non- target group. Scientists don’t fully under- stand the effect of any individual ingredi- ent on soil life, much less the synergistic effects of the ingredients, or combination- al effects with inert or soil materials. It is hardly surprising that a soil treated with numerous agricultural chemicals lacks a healthy food web. Plants growing in un- healthy soil require additional fertilizers and pesticides, furthering the deadly spiral. As a plant grows, photosynthesis sup- plies much more than the individual plant’s carbohydrate requirements. It has been documented that plant roots can exude over 50 percent of the carbon fixed through pho- tosynthesis in the form of simple sugars, proteins, amino acids, vitamins, and other complex carbohydrates. The types of mol- ecules released are specific for a variety of plants grown under certain conditions, forming in effect a unique chemical signa- The soil food web is the community of organisms living all or part of their lives in the ture. As these molecules are released into soil. A food web diagram shows a series of conversions (represented by arrows) of the rhizosphere, they serve as food and energy and nutrients as one organism eats another. growth stimulants for a certain mix of mi- crobes. Dr. Joyce Loper, of the USDA Ag- beneficial bacteria and fungi; they do not interrelationships can be shifted. Soil ecol- ricultural Research Service, and other sci- damage living plant tissue and are critical to ogists are just beginning to understand how entists have shown that for each plant making essential minerals available to the plant production can be affected when this species, this characteristic chemical soup plant. These microbes retain large amounts balance is altered. Many species of benefi- stimulates the development of a select, ben- of nitrogen, phosphorous, potassium, sulfur, cial bacteria and fungi die as food supplies eficial company of root-dwelling microbes. calcium, iron and many micronutrients in dwindle. Reduction in natural predators and This microbial population colonizes the their bodies, preventing these nutrients from decreased competition for certain food sup- root zone, producing certain chemicals that being leached or removed by water runoff. plies may allow other species to grow ram- inhibit the growth of pathogenic species. Ideally, they out-compete pathogenic species pantly. Plant nutrient availability often can These organisms also are instrumental in and form a protective layer on the surface of decline, and populations of pathogens can supplying the plant’s unique nutritional living plant roots. It is usually only when the rise. Much research is currently being done needs. beneficial species of bacteria and fungi are on this subject, attempting to comprehend The residual effects of this unique mi- killed by continuous soil disturbance and tox- how such changes occur. crobial population in subsequent years may ic chemicals that pathogenic species have an also help explain why certain crop rotations advantage. HERBICIDES, PESTICIDES work better than others. It is possible that As in more familiar aboveground eco- & FERTILIZERS the microbial population nurtured by one systems, there are other organisms present As part of her research, Dr. Ingham has crop species creates a nutritional or micro- that prey on these herbivores. The preda- shown that herbicides, pesticides and fer- bial environment that is well suited to a par- tors are primarily beneficial nematodes, tilizers have many non- ticular subsequent crop predatory nematodes, protozoa, mites and target effects. The most Reprinted from species but not perhaps other tiny animals which serve to recycle common pesticides are for others. For example, it nutrients in the system and to keep other fairly broad spectrum; that is likely that crops like populations in balance. These predators, in is, they kill much more broccoli, which inhibit the turn, are eaten by other animals, primarily than the target species. April 2000 - Vol. 30, No. 4 - Cover Story growth of mycorrhizal those that spend some portion of their life Residual pesticides that fungi, reduce the produc- aboveground, such as insects, birds and accumulate in soil over many years may tivity of a following crop such as corn, small animals. recombine and form new, unintentional which requires mycorrhizal fungi. While It is important to view the soil food web chemicals that have additional and often this has not been conclusively proven, it as a complex, whole system. When any synergistic negative effects. Out of the 650 could form part of the basis for a better un- group of organisms in the system is elimi- active ingredients used to formulate most derstanding of observable crop rotational nated or damaged, the delicate balance of common agricultural pesticides, only about effects. Certain soil amendments favor the system of nitrogen cycling that has development of a diverse microbial pop- worked brilliantly for the past million ulation. Compost in particular can im- years. prove soil nutritional availability and Compare this system to another famil- soil tilth because of its complex micro- iar situation; when inorganic ammonium bial population. Composts bring with nitrate fertilizer is applied to agricultural them a wide array of bacteria, fungi, pro- soil, ammonium and nitrate ions are rap- tozoa, nematodes and microarthropods, idly released into the soil solution.