With the exception of carbon dioxide and sunlight, the other plant requirements are extracted from the soil substrate in which the turf grows. While the soil has long been thought to be an inert repository of water, nitrogen, phosphate, potash, and other micronutrients, in recent years more attention has been paid to the microbial activity in the soil and how plant – microbial communities have a profound impact on the health of the turf. It was also thought that little could be done to alter these microbial communities to enhance the benefits of these interactions, but work is being done with soil inoculants, and the tracking of the impact of the use of these products using genetic techniques like Real Time PCR (Polymer Chain Reaction) has shown that significant positive impacts can be realized using these soil products.
Inputs and Their Costs
With economics being a primary driver in turf maintenance, it is understandable how the application of any additional inputs will be scrutinized to determine the cost-benefit. In most cases, the primary costs incurred for turf maintenance are for watering, fertilizer, and pesticides, primarily fungicides. The pesticides, including fungicides and nematocides, are generally the most expensive of the inputs.
The demand and cost for watering will vary depending on the area of the country the course is located. Some areas are hotter and get less natural rainfall, so watering costs are higher. Many U.S. courses are setting a goal of sustainable development with minimal use of water, relying more on natural rainfall. Since rainfall can be inconsistent, this can result in stressed turf and brown conditions. While it appears that many of the people involved in the golf industry are willing to accept less than ideal conditions, it is less clear that the patrons at public courses, and members of private clubs are as willing to accept less than ideal conditions on the courses they choose to play.
Fertilizer use, while not as expensive as other inputs, has come under scrutiny for its potential to cause runoff pollution. Many watersheds areas are trying to minimize the damage to rivers, streams, and estuaries caused by runoff of fertilizers from agriculture and landscaped areas that can cause eutrophication. This has led to more careful and limited application of fertilizer. Properly managed, the limited use of fertilizer can be carried out with minimal impact on overall turf health and quality.
Soil Inoculants and How They Differ
Most soil inoculants fall primarily into two categories … microbial or fungal. Some combine both technologies. The most common fungal soil inoculants fall into the category of mycorrhizal fungi, although there has been some interesting work done with strains of Trichoderma. The mycorrhizal fungi inoculants have been pretty widely used for over 30 years and there is a fair amount of documentation regarding their benefits.
Many of the bacterial inoculants comprise primarily Bacillus strains. There are two reasons for this: first, there is a lot of documentation on the benefits of Bacillius species, e.g. Bacillus subtilis, on plant growth, function, and disease prevention; second, Bacillus are spore formers, which makes them easier to preserve. A spore is like a seed to a plant. When the vegetative Bacillus cell senses that conditions are not suitable to its growth and survival, e.g. inadequate carbon or energy source, extreme heat or dryness, it produces a spore. The spore is very resistant to adverse conditions and can survive for a long time without moisture, or nutrients. However, when placed back into an environment where there is adequate moisture, nutrients and the proper pH, temperature, etc. the spore germinates back into a vegetative cell, much like a seed germinates into a plant. From a production standpoint, this simplifies things greatly, as one of the greatest challenges in the production and distribution of bacterial products is keeping the bacteria viable until applied.
The key disadvantage of using spore formers only, is that it limits you to the genus of bacteria that form spores … primarily Bacillus and Clostridia. This leaves out the potential use of other benefical bacteria that are non-spore formers, unless a suitable way can be developed to preserve them in the vegetative state.
New Generation of Soil Inoculants
Recently a new technology has become available that allows many vegetative cells, including certain strains of photosynthetic bacteria with proven benefits to plants, to be preserved in a liquid state. This not only allows for products with a much greater diversity of organisms, but also being in a liquid state makes it much easier to apply using boom sprayers or fertigation
The primary benefit of a product with photosynthetic bacteria, such as Rhodopseudomonas palustris, or Rhodospirillum rubrum, is in the way they can improve the energy availability to the plants grown in the soils populated with these bacteria. Photosynthetic bacteria are like “nature’s solar collectors”. Like photosynthetic plants, they can convert solar energy or radiant energy to chemical energy in the form of sugars or other chemicals. Most photosynthetic bacteria are up to 10 times more efficient at converting solar energy to chemical than plants are, so they normally produce more energy than they require themselves. They can share this energy with plants in their vicinity, improving the transport of nutrients into the plant, and improving certain plant characterisitics, like plant turgor.
Microbial Diversity and Reduction in Disease Pressure
Many studies have shown that plants grown in soils that lack healthy microbial populations and microbial diversity will often exhibit a reduced resistance to disease pressure. This includes studies recently performed at the University of Florida on huanglongbing, or citrus greening. There are also studies that point to total microbial populations as an indicator of the activity in the soil, but it is not only the total counts of bacteria, but the diversity of constituents that make up that populations. Essentially, it comes down to a function of the most robust ecosystem, which results from the most dynamic, resilient soil food web that leads to the best overall health of the soil and turf that grows in it.
How a Healthy Microbial Population in Turf Can Reduced Irrigation and Input Costs
In many cases, where this new generation of soil inoculants has been utilized, superintendents have often found that they can reduce watering by as much as 50% with no stress, or deterioration in turf quality. We have been able to come up with two technical explanations for this. The first is the production of biopolymers (biofilm) that have an affinity for water and nutrients, will absorb and hold them in the turf until needed, reducing evaporation and transpiration, much like the synthetic superabsorbant polymers that are sold and used for this purpose. However, these naturally generated polymers are much more readily degradable so they do not persist in the environment, and the microorganisms produce the polymers wherever they grow, allowing for easier introduction into the soil, and better distribution. The photosynthetic bacteria in this new technology also produce water as a byproduct of the photosynthetic reaction, thus adding a net gain of water to the system.
The presence and importance of nitrogen fixing bacteria in the soil is well understood and accepted, especially those that occur in conjunction with the root nodules of certain plants like soybeans. However, there are other bacteria that have demonstrated the ability to do this in the soil of the rhizosphere, and do it efficiently enough to reduce fertilizer requirements by as much as 80%. While the cost savings, as noted earlier, are not as great with fertilizers as they are with pesticides, the combination of reduced cost and a reduced potential for runoff pollution make this an attractive benefit.
Finally, many superintendents have found that when utilizing this new generation of inoculants, they can often provide the same level of protection against insects and plant diseases with lower doses of the pesticides used because the turf is healthier and not as prone to many of these diseases, just as any healthy organism is better able to ward off infections when its own defense mechanisms are functioning better.
As in many cases, addressing the root cause of a problem (in this case it is more like the root zone cause) often yields a more cost effective and successful long-term solution. More and more studies in agriculture and turf management have shown that the excessive use of chemical treatments has led to soils with very low levels of microbial activity, and the resultant struggle to maintain healthy plants in what has become an unhealthy soil environment. It would therefore seem logical that the way to address the root cause is to find some way to increase not only the level of microbial activity in the soil, but the level of diversity as well. With new tools, like Real Time PCR, making assay for microbial soil populations faster and more economical, it will be easier to track the changes and benefits of soil inoculants going forward.
At this time, soil inoculants have already been shown to be cost-effective ways of improving turf health and reducing overall input costs. We believe that going forward they will be a key part of most superintendents integrated turf management program.
by Mark J. Krupka, Environmental Microbiologist,
Ecological Laboratories, Inc.
And Karl Manges, Technical Adviser,
Doug Speed and Associates