I have done some research and discovered some interesting facts on soil; the following is what I have found. Some of the information is elementary other information makes good sense. Our soil systems are a complex interrelated living, breathing system. I don’t profess to be a soil scientist, I just interpreted these simple facts to be true.
The Living Soil
If you look out at an untouched landscape you might wonder how native prairies and forests function in the complete absence of fertilizers and other man made intervention. These soils are tilled by soil organisms, not by fancy Toro aerifiers. They are fertilized too, but the fertility is used again and again and never leaves the site. Native soils are covered with a layer of plant litter and/or growing plants throughout the year. Beneath the surface litter layer, a rich complexity of soil organisms decompose plant residue and dead roots, then release their stored nutrients slowly over time. In fact, topsoil is the most biologically diverse part of the earth. Soil-dwelling organisms release bound-up minerals converting them into plant-available forms that are then taken up by the plants growing on the site. The organisms recycle nutrients again and again from the death and decay of each new generation of plants growing on the site.
There are many different types of creatures that live on or in the soil. Each has a role to play. These organisms will work for the superintendents’ benefit if we simply manage for their survival. Consequently we may refer to them as soil livestock. While there is a great variety of organisms that contribute to soil fertility, earthworms, arthropods, and the various microorganisms merit particular attention.
Earthworms: Although earthworm cast on the surface of a tightly mown fairway turf are a bit of an inconvenience to players their role in soil management cannot be disputed. Earthworm burrows enhance water infiltration and soil aeration. Earthworm tunneling can increase the rate of water entry into the ground 4 to 10 times higher than soils that lack worm tunnels. This reduces water runoff, recharges groundwater, and helps store more soil water for drought spells. Vertical earthworm burrows allow gas exchange deeper into the soil, stimulating microbial nutrient cycling at those deeper levels. Tillage done by earthworms can replace some expensive aerification work done by machinery.
Worms eat dead plant material left on top of the soil and redistribute the organic matter and nutrients throughout the topsoil layer. Nutrient-rich organic compounds line the tunnels that may remain in place for years if not disturbed. During droughts these tunnels allow for deep plant root penetration into subsoil regions of higher moisture content. In addition to organic matter, worms also consume soil and soil microbes as they move through the soil. The soluble nutrient content of worm casts is considerably higher than those of the original soil. A good population of earthworms can process 20,000 pounds of soil per year, with turnover rates as high as 200 tons per acre.
Earthworms also secrete a plant growth stimulant. Reported increases in plant growth due to earthworm activity may be attributed to this substance - not just improved soil physical qualities.
Earthworms prefer a near neutral soil pH, moist soil conditions, and plenty of plant residue on the soil surface. They are sensitive to certain pesticides and some fertilizers. Carbamate (SEVIN) insecticides are harmful to earthworms, while synthetic pyrethroids are harmless to them. Most herbicides have little effect on worms except for the triazines, such as Atrazine, which are moderately toxic.
Arthropods: In addition to earthworms, there are many other 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 a small insect, which eat mostly fungi. Their waste is rich in plant nutrients that are released after other fungi and bacteria decompose it.
Bacteria: Most numerous among soil organisms are the bacteria; every gram of soil contains at least a million of these tiny one-celled organisms. There are many different species of bacteria, each with its own role in the soil environment. One of the major benefits bacteria provide for plants is in helping them take up nutrients. Some species release nitrogen, sulfur, phosphorus, and trace elements from organic matter. Others break down soil minerals and release potassium, phosphorus, magnesium, calcium and iron. Still other species make and release natural plant growth hormones, which stimulate root growth.
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.
Fungi: Fungi come in many different species, sizes and shapes in soil. Some species appear as thread-like 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 early to colonize larger pieces of organic matter and begin the decomposition process. Some fungi produce plant hormones, while others produce antibiotics compounds including penicillin. There are even species of fungi that trap harmful plant-parasitic nematodes.
Mycorrhizae: (my-cor-ry-'zee) group of fungi lives 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 in less fertile soils. Roots colonized by mycorrhizae are less likely to be penetrated by root-feeding nematodes since the pest cannot pierce the thick fungal network. Mycorrhizae also produce hormones and antibiotics, which enhance root growth and provide disease suppression. The fungi benefit from plant association by taking nutrients and carbohydrates from the plant roots they live in.
Actinomycetes: (ac"-ti-no-my'-cetes) are thread-like bacteria that look like fungi. While not as numerous as single celled bacteria, they also perform vital roles in the soil. Like the bacteria, they help decompose organic matter into humus, releasing nutrients. Actinomycetes are responsible for the sweet, earthy smell of biologically active soil noticed whenever soil is cultivated or aerified.
Actinomycetes are one of the most poorly understood groups of soil microorganisms. Although their populations in some soils can be high, their growth rates are far slower than other soil microorganisms.
Actinomycetes typically are more abundant in dryer soils high in organic matter or in high-temperature soils. As a group, they are not tolerant of low soil pH (less than 5.0). They grow best at temperatures that range from 80°F to 100°F. The major genera of soil Actinomycetes include Streptomyces, Nocardia, Micromonospora and Actinoplanes.
These organisms are best known for their ability to produce several industrially and medically important compounds. Many antibiotics important to human and animal medicine come from soil Actinomycetes. Like the fungi, Actinomycetes rely on organic matter for their nutrition. Actinomycetes are well-adapted to the decomposition of the more resistant plant polymers such as cellulose, hemicellulose and lignin, as well as the fungal and insect polymer chitin. Because of this, Actinomycetes play a major role in the formation of humus in soils largely from the decomposition of the turf thatch layer.
Like some bacteria, Actinomycetes help suppress soil-borne turfgrass diseases. Many of the antibiotic compounds of Actinomycetes affect the growth and development of pathogenic fungi. Composts are particularly rich in pathogen-suppressing Actinomycetes. The beneficial effect of amending soils with composts is partly due to the disease-suppression properties of Actinomycetes.
For best Actinomycetes activity the superintendent should:
• Maintain soil moisture at constant levels, never allowing the soil to become overly dry. If other agronomic parameters are correct and microbial activity is high, disease should not be a less serious problem.
• Maintain balanced pH and consistent fertility. Avoid "Yo-Yo" nutrient cycling. Also, research has shown that organic nitrogen sources or ammonium-based (NH
4) fertilizers result in greater microbial populations than synthetic nitrate (NO
3) sources.
• Maintain good soil porosity. Use physical amendments if necessary. Adequate soil-oxygen levels are extremely important for soil all beneficial micro-organisms.
• Any practice that enhances the volume of the turfgrass root system; for example, aeration or raising the height of cut enhances microbial activity in the rhizosphere.
• If practical, limit the use of pesticides and growth regulators. Many of these have anti-microbial properties and may negatively impact soil and rhizosphere microbial communities.
Algae (Cyanobacteria): Algae and Cyanobacteria are often mistaken for each other in the soil matrix. An assortment or different species of algae also live in the upper half-inch of the soil. Unlike most other soil organisms, algae actually produce their own food through photosynthesis. They appear as a greenish film on the soil surface following a good rain. Algae may improve soil structure by producing slimy substances that glue soil together into water-stable aggregates in small populations. Some species of algae (Cyanobacteria) can fix their own nitrogen, some of which is later released to plant roots. High populations of algae and / or Cyanobacteria are not beneficial to soils and turf populations. The blue – green algae (Cyanobacteria) can easily overpopulate and is generally due to poor soil drainage, air movement and poor light exposure to turf areas. If left unchecked an overpopulation of this organism can seal – off soils and cause anaerobic conditions and eventual turf death.
Protozoa: Protozoa are free-living microorganisms that crawl or swim in the water between soil particles. Many soil protozoa are predatory, eating other microbes. One of the most common is an amoeba that eats bacteria. By eating and digesting bacteria, protozoa speed up the cycling of nitrogen from the bacteria, making it more available to plants.
Nematodes: While nematodes are abundant in most soils, 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.
More on Mycorrhizae
Most grass species in their undisturbed natural environments form a beneficial association with mycorrhizal fungi. The resulting structure is called a mycorrhiza, or literally "fungus-root" (from myco meaning fungal and rhiza meaning root). Although several types of mycorrhizal fungi form mycorrhizae with plants, the largest group, -endomycorrhiza or also called arbuscular mycorrhizae form with most grass species. Mycorrhizal fungi are present in soil as spores, as hyphae in soil (filaments) or as colonized bodies. Hyphae of mycorrhizae penetrate into and between the outer cells of the root. Inside the root the fungus forms special coiled hyphae (arbuscules) that provide increased surface area for exchanges of food to the fungus and nutrients for the grass.
The mycorrhizal fungi once established on the turf root system radiate out from the roots to form a dense network of filaments. These filaments form an extensive system of hyphae that grow into the surrounding soil and provide a variety of benefits for the grass plant. This network of filaments obtains 15 major macro and micro nutrients and water and transport these materials back to the turf root system. Mycorrhizae are especially important for uptake of nutrients that do not readily move through the soil such as phosphorous and many of the micro-nutrients. The elaborate network of hyphae beneath the soil surface greatly increases the potential of the root system to absorb nutrients and water. The network also binds soil particles together, improves soil porosity and the movement of air and water within the soil.
New scientific advancements in the cost effective growing of certain mycorrhizal species beneficial to turf grass are rapidly bringing mycorrhizal products to the golf management marketplace. Healthy living soil and turf will retain nutrients, build soil structure, reduce stress and suppress disease, thus reducing the frequency and level of certain maintenance activities. Choosing to incorporate mycorrhizal fungi into aerification programs will not only benefit the environment but improves turf cover, rooting, fertilizer utilization, disease and drought resistance.
All these organisms–from the tiny bacteria up to the large earthworms and insects–interact with one another in a multitude of ways in a whole soil ecosystem. Organisms not directly involved in decomposing plant wastes may feed on each other or each other's waste products or the other substances they release. Among the other substances released by the various microbes are vitamins, amino acids, sugars, antibiotics, gums, and waxes.
Roots can also release various substances into the soil that stimulate soil microbes. These substances serve as food for select organisms. Some scientists and practitioners theorize that plants use this means to stimulate the specific population of microorganisms capable of releasing or otherwise producing the kind of nutrition needed by the plants.
Organic Matter, Humus and the Soil System
Critical to any model for sustainable soil management, is the understanding the role that soil organisms play, golf course superintendents should focus on strategies that build both their numbers and their diversity. That food for these soil organisms comes in the form of soil organic matter.
Organic matter and humus are terms that describe somewhat different but related things. Organic matter refers to the organic fraction of the soil that is composed of both living organisms and once-living residues in various stages of decomposition. Humus is only a small portion of the organic matter. It is the end product of organic matter decomposition and is relatively stable. Further decomposition of humus occurs very slowly in both cultured turf and natural settings. In natural systems, a balance is reached between the amount of humus formation and the amount of humus decay. In most turf grass soils, this balance also occurs, but often at a much lower level of soil humus. Humus contributes to well-structured soil that, in turn, produces high quality plants. It is clear that management of organic matter and humus is essential to sustain a vibrant interactive soil ecosystem.
The benefits of a soil rich in organic matter and humus are many. They include:
• Rapid decomposition of clipping and root residues,
• Granulation of soil into water stable aggregates,
• Decreased crusting and clodding,
• Improved internal drainage,
• Better water infiltration, and
• Increased water and nutrient holding capacity.
Improvements in the soil's physical structure facilitate easier aerification, increased soil water storage capacity, and deeper, more prolific plant root systems. Improvements in nutrient cycling also reduce the fertilizer bill.
Soil organic matter can be compared to a bank account for plant nutrients. Soil containing 4% organic matter in the top 7 inches has 80,000 pounds of organic matter per acre. Those 80,000 pounds of organic matter will contain about 5.25% nitrogen, amounting to 4,200 pounds of nitrogen per acre. Assuming a 5% release rate during the growing season, the organic matter could supply 210 pounds of nitrogen to the turf grass plants. If the organic matter is allowed to degrade, purchased fertilizer will be necessary to prop up turf growth due to lost organic-matter nitrogen.
Ultimately, building organic matter and humus levels in the soil is a matter of managing the living organisms in the soil–something similar to wildlife management or animal husbandry. This entails working to maintain favorable conditions of moisture, temperature, nutrient status, pH, and aeration. It also involves providing a steady food source.
Organics and the Soil System
Soil management involves stewardship of the living soil systems. The primary factors affecting organic matter content, build-up, and decomposition rate in soils are: oxygen content, nitrogen content, moisture content, temperature, and the addition and removal of organic materials. All these factors work together at any one time. Any one can limit the others. These are the factors that affect the health and reproductive rate of organic matter decomposer organisms.
Superintendents need to be aware of these factors when making decisions about their soils. Let's take them one at the time.
Increasing oxygen speeds decomposition of organic matter. Aerification is the primary way extra oxygen enters the soil. Texture also plays a role, with sandy soils having more aeration than heavy clay soils. Nitrogen content is influenced by fertilizer additions. Excess nitrogen without the addition of carbon speeds the decomposition of organic matter. Moisture content affects decomposition rates. Soil microbial populations are most active over cycles of wetting and drying. Their populations increase following wetting as the soil dries out. After the soil becomes dry, their activity diminishes. Just like humans, soil organisms are profoundly affected by temperature. Their activity is highest within a band of optimum temperature. Above and below optimum temperature their activity is diminished. Adding organic matter provides more food for microbes.
To achieve an increase of soil organic matter, additions must be higher than removals. Over a given year, under average conditions, 60 to 70 percent of the carbon contained in organic residues added to soil is lost as carbon dioxide. Five to ten percent is assimilated into the organisms that decomposed the organic residues and the rest becomes 'new' humus. It takes decades for new humus to develop into stable humus which imparts the nutrient holding characteristics humus is known for. The end result of adding a ton of residue would be 400 to 700 pounds of new humus. With a 7-inch depth of topsoil over an acre weighing 2 million pounds, you can see that building organic matter is a slow process. One percent organic matter weighs 20,000 pounds.
Building stable humus is a slow and long-term process. It is more feasible to stabilize and maintain the humus present before it is lost than to try to increase it. The value of humus is not fully realized until it is severely depleted. If your soils are high in humus now, work hard to preserve what you have. The formation of new humus is essential to maintaining old humus and the decomposition of raw organic matter has many benefits of its own. Increased aeration caused by aerification coupled with the absence of organic carbon in fertilizer materials can caused greater than 50% decline in native humus levels on many US golf courses.
Appropriate mineral nutrition needs to be present for soil organisms and plants to prosper. Adequate levels of calcium, magnesium, potassium, phosphorus, sodium and the trace elements should be present but not in excess. Several books have been written on balancing soil mineral levels and several consulting firms provide soil analysis and fertility recommendation services based on that theory.
Nitrogen Applications
Excess nitrogen applications stimulate increased microbial activity that speeds organic matter decomposition. The extra nitrogen narrows the ratio of carbon to nitrogen in the soil. In well balanced native soils the carbon to nitrogen ratio (C : N ratio) is around 12:1. At this ratio, populations of organic matter decay and bacteria are kept at a stable level. When large amounts of inorganic nitrogen are added, the C : N ratio is reduced, which increases the populations of decay organisms and allows them to decompose more organic matter. While soil bacteria can efficiently use moderate applications of inorganic nitrogen accompanied by organic amendments (carbon), excess nitrogen causes bacteria populations to explode, decomposing existing organic matter at a rapid rate.
Eventually, soil carbon content may be reduced to a level where the bacterial populations are on a starvation diet. With little carbon available, bacterial populations shrink and less free soil nitrogen is absorbed. Thereafter, applied nitrogen, rather than being cycled through microbial organisms and re-released to plants slowly over time, becomes subject to leaching. This can greatly reduce the efficiency of fertilization and lead to environmental problems.
To compensate for the fast decomposition of native soil organic matter, carbon should be added with nitrogen. Typical sources–such as activated sewage sludge, animal manure and compost–serve this purpose well. Amendments containing too high a carbon to nitrogen ratio (25:1 or more) can tip the balance the other way, resulting in nitrogen tied up in an unavailable form. The soil organisms consume all the nitrogen in an effort to decompose the abundant carbon. The nitrogen is unavailable because it is tied up in the soil organisms themselves. As soon as one dies and decomposes, its nitrogen is consumed by another soil organism until the balance between carbon and nitrogen is achieved again.
Rich humus and soils high in organic compounds is the golf course’s capital. Sustaining turf management means sustaining the soil resource.
Humates
Humates and humic acid derivatives are a diverse family of products, generally obtained from various forms of oxidized coal. Coal-derived humus is essentially the same as humus extracts from soil but there has been reluctance in some circles to accept it as a worthwhile soil additive. In part, this stems from a belief that only humus derived from recently decayed organic matter is beneficial. It is also true that the production and recycling of organic matter in the soil cannot be replaced by coal-derived humus. However, while sugars, gums, waxes and similar materials derived from fresh organic-matter decay play a vital role in both soil microbiology and structure, they are not humus. Only a small portion of the organic matter added to the soil will ever be converted to humus. Most will return to the atmosphere as carbon dioxide as it decays.
Many studies have shown positive effects of humates, while other studies have shown no effects. Generally, the consensus is that they work well in low organic matter soils. In low amounts they do not produce positive results on soils high in organic matter. At high rates they may tie up soil nutrients.
There are many humus products on the market. They are not all the same. Humate products should be evaluated in a small test plot for cost effectiveness before using. Sales people sometimes make exaggerated claims for their products.
Sum it Up
Humates, many forms of bacteria, fungi and Mycorrhizae play a significant role on a well balanced soil strategy. The import take-away of this message is that application of any chemical and/or fertilizer product will have and effect on organic content in the soil. Choose your weapons carefully!
