SOIL ECOLOGY & THE SOIL FOOD WEB Copyright May 1975 © Revised in 2003 Originally written by Michael Martin Melendrez in 1974 Updated by Michael M. and Dr. Michael Karr, Ph.D. ARCPACS Cert. Prof. Soil Scientist Introduction With Water becoming more and more of an issue in the Western United States, Landscapes are increasingly being targeted as one of the main culprits of water waste. The problem lies with the absolute necessity of shade and greenscapes in our Southwestern High Desert Cites. Gravel is simply not a viable answer for converting or creating low water landscapes as the main criteria of function is lost. Who wants to sit in a gravel yard on a typical hot, sunny summer day? The construction of shade structures cannot replace the cooling benefit of trees over the same square footage. The second problem lies with our historical use of fertilizers to help prop up the health of our Trees and other landscape materials. You will learn by reading this paper that fertilizers result in growing all plants Hydroponically or with water. In other words, the only way to keep fertilizers available to plants is to keep the nutrients in a soil solution of water with the soil always well hydrated. This contradicts what happens in nature and how plants acquire nutrients in a natural Soil Food Web Cycle. You cannot convert to lower water usage in a landscape and continue to use Fertilizers at the same time. Please read on and learn how the Natural Process of Nature works and how we can help you build a truly 'Ecologically Sound Landscape.' Why We Need a Soil Food Web Plants depend on beneficial soil organisms to help them obtain nutrients and water from the soil, to prevent nutrient losses, to protect them from pathogens, and to degrade compounds that could inhibit growth. Each class or type of microorganism plays unique roles in these processes. Soil organisms create a living, dynamic system that can do all these things, but must be managed properly for best plant growth. A spoonful of healthy soil contains millions of beneficial microscopic organisms of various kinds that perform vital "functions" in the root zone that can bring plants to health, if soil conditions are managed in ways that allow the microbes to live and work. These organisms include beneficial species of bacteria, fungi, protozoa, microarthopods and nematodes that never cause disease or become pests. Why We Need a Soil Food Web continued In healthy soil ecosystems, while nutrient cycling and productivity increases, nutrient loss is minimized. What makes this possible is the complexity of the soil foodweb. The greater the interaction of decomposers, their predators, and the predators of those predators the more tightly nutrients cycle from stable forms in soils to plants, and back again (Coleman et al, 1985; 1992). Effects of Pesticides, Herbicides and Fertilizers Pesticides, which include plant killers (herbicides), bug killers (insecticides), fungi killers (fungicides) and bacteria killers (bactericides) also kill related and often beneficial organisms. While each application may impact only a few species, the cumulative effect of multiple and repeated pesticide applications is a reduction in the numbers and diversity of soil organisms. Fertilizers do not kill many soil organisms directly. The effect of high levels of nutrients from fertilizers is that the plant is less likely to form beneficial associations with soil organisms like mycorrhizae and rhizobium. Since the plant is less able to obtain water and nutrients without their help, it becomes dependent on continuously high levels of nutrients and water. The plants are less able to tolerate water stress or low nutrient levels. Thus the landscape suffers unless both are continually applied. Organic fertilizers, like composts and manure, have nutrients stored in the organic matter. The beneficial microorganisms are fed as the nutrients are slowly released to plants. Adding inorganic fertilizers supplies nutrients but does not feed microorganisms. The microorganisms decline in numbers and, since they aren't there in sufficient numbers to release nutrients stored in organic matter, the plants become even more dependent on repeated applications of fertilizer nutrients. And if you don't have many beneficials then you are left with relatively high numbers of pathogens, because only the plants are present for them to feed on! To correct this problem you need to inoculate the soil with the beneficials to make sure they are there, and then put back the food in those systems to grow and maintain these organisms. What Happens to Soil Nutrients Without a Functioning Food Web When we add fertilizers containing N, some of the fertilizer will dissolve and diffuse directly to the roots and be taken up, helping the plant to grow. Much of the excess N is in danger of being leached away. Without large numbers of soil organisms that can "capture" excess N, retention in the soil doesn't happen and the nutrients can be leached into the groundwater. Other nutrients, like phosphorus, iron, manganese, zinc & copper, are rapidly converted into insoluble (less available) forms. The solution? Protect the nutrients and cycle them! Apply the microbes that make up the food web and feed them. In return they will retain and cycle plant nutrients. And they will also do a great deal more. What Happens to Plants Without a Functioning Soil Food Web Disease organisms are not suppressed; and therefore multiply and threaten plants. The loss of symbiosis with soil microorganisms results in reduced ability to take up water and nutrients. Not only are plant growth adversely affected but resistance to temperature and moisture stress is reduced as well. The lasting solution Restore the health of the soil ecosystem, the soil foodweb. A highly populated and balanced soil foodweb will: 1. Create humus by decomposing organic matter 2. Improve soil structure by binding particles together and creating microaggregates 3. Protect roots from diseases and parasites 4. Retain nitrogen and other plant nutrients 5. Slowly release retained nutrients to the plant 6. Produce enzymes and hormones that help plants grow and resist stress 7. Decompose pollutants that enter your soil To do all this, the following types of organisms need to be present in sufficient numbers: First Level Soil organisms which capture nutrients before they are lost. The soil bacteria and fungi form the first level of organisms - the consumers of organic matter and leftover nutrients. Nitrate nitrogen and some other nutrients can leach out and be lost unless they can be held in soils until the plant needs them. When bacteria and fungi multiply they gather up free nitrogen from the soil and convert it to protein in their bodies. Nitrogen in this form cannot be leached away or be lost as a gas. Bacteria - the "Cows" of the soil. Total bacterial numbers range between 5 million and 500 million per teaspoon of soil in agricultural soils, and between 20 million and 2 billion in forest soils and highly productive garden soils. Soil bacteria are like cows in soil. They tackle the easy to decompose materials, like green yard waste and manure. These materials contain most of the nutrients. These organisms need a lot of nitrogen, and they grab it quickly (more quickly then plants!) so they often go after some of the residual nitrogen from fertilizers, if present. They retain nutrients like N, P and S. in the soil as bacterial biomass. Productive garden soil should contain more bacteria than any other kind of organism. Bacterial waste products that cannot be broken down any further become soil humus (humic substances) Fungi - The Goats of the Soil A single teaspoon of healthy soil can contain up to 40 miles of fungal hyphae! The soil fungi consume the tougher, hard to decompose materials, like straw, pine needles, bark and wood. The nutrients from these types of organic matter are not lost by leaching or other processes, but are incorporated into the fungal biomass. Just like bacteria, fungal waste products and materials that cannot be broken down any further become soil humus.