Vineyard views – soil microbes – Column
Cliff Ohmart
The foundation of sustainable viticulture is a healthy soil, and a healthy soil contains lots of soil microorganisms. A few growers have begun adding microorganisms to vineyard soils because they believe yields will increase and grapevines will be better able to resist pests. Some soil scientists predict that the time will come when farmers will routinely add microbes to soil to increase yields and control soil-borne diseases. However, there is not much data clearly demonstrating their effectiveness. One thing for sure, the development of the technology to manufacture microbes to add to the soil is way ahead of our understanding of how to use them and their cost effectiveness. Considering the important things microbes do in the soil there is remarkably little known about them and even less known about how they interact with each other and with plants.
Before looking at what kinds of microbes growers are adding to vineyard soils and why they are doing it, let’s quickly look at the major players in the soil and what we know about them. It may help to put this topic into perspective. An easy way to classify soil organisms is by body size. Insects, millipedes, mollusks and earthworms are the largest and are primarily responsible for the breakdown of leaves, twigs, roots and any other plant material that hits the soil. Some members of this group feed on plant roots and can be pests, while others prey on other soil organisms. The next smaller group are the springtails (Collembola) and mites (yes, there are lots of mites that live in the soil). These creatures continue the breakdown of plant material started by the bigger soil organisms.
The smallest soil organisms, or “microorganisms”, are the ones we are most interested in since some of them are the ones you can buy and add to your vineyard soil. There are two main groups of soil microorganisms: nematodes and protozoans; and the fungi and bacteria. We are all familiar with nematodes and the problems that they cause by infesting grapevine roots. However, about 70% of the nematodes living in the soil are beneficial species feeding on other soil microbes. Protozoans are single-celled organisms that we know very little about.
Bacteria are the smallest organisms in the soil and they can occur in huge numbers, from [10.sup.6] to [10.sup.9] per gram of soil. Bacteria play a role in some of the most important processes that happen in soil. For example, they are important in nutrient cycling. Some bacteria, such as Rhizobium spp., form nitrogen-fixing nodules on the roots of legumes. Most importantly, they are able to break down virtually all natural and synthetic compounds entering the soil, including pesticides and industrial pollutants. An interesting side note is that some soil bacteria produce toxins that kill insects and they have been mass reared in laboratories and made into insecticides. Bacillus thuringiensis (Bt) is one species that is widely used by growers. Very recently it was discovered that one strain of Bacillus produces a toxin that kills powdery mildew and many other plant pathogens. A company is now developing a fungicide that contains this toxin for use against powdery mildew and bunch rot.
There are many species of fungi living in the soil. Most of us are familiar with ones causing plant diseases but there are many species that are beneficial and do things like break down dead organic matter or kill other microbes. One group of fungi of particular interest to us forms a symbiotic relationship with plant roots called mycorrhiza. A symbiotic relationship is one where both species benefit. In this case the fungus obtains carbohydrates from the plant while supplying nutrients, particularly phosphate, to the plant from the soil, they also may reduce water stress, and in some situations may help roots resist fungal or nematode infestations. Soil scientists believe that about 80% of the plants in both natural and cultivated soils form mycorrhiza.
So why are some growers adding microbes to vineyard soil? There are at least three different reasons: (1) To replace ones killed by soil fumigation;
(2) To promote resistance to root diseases, particularly nematodes; and
(3) To enhance vine health and yield. Most of the microbes for sale are either bacteria or species of fungi that form mycorrhiza. Grapes form a particular kind of mycorrhiza that goes by the unfortunate name of vescicular-arbuscular mycorrhiza, or VAM for short. There are not very many species of VAM fungi but they are widespread and infest a wide range of plants, so beware when a manufacturer says that their VAM fungi is unique.
What do we know about the effects of adding soil microbes to vineyard soil? Unfortunately, not much. Companies selling these organisms usually have data from on-farm research trials or grower testimonials that show some kind of response, such as an increase in yield or better root development. However, there is little information in the scientific literature that shows consistent, cost-effective results. Studies have shown that inoculating previously fumigated soils is usually successful because these soils are devoid of any competing microbes. On the other hand, some companies are selling microbes to add to soil to supplement ones that are already there. The success of this strategy is much less likely since these microbes must compete with microbes already in the soil.
The interactions between VAM fungi with different kinds of soil are not well understood, either. It appears that some mycorrhizal associations work best on infertile soils and less well in fertile soils, where conditions are more favorable to plant growth. For example, an increase in soil phosphate decreases the amount of VAM infection of plant roots. Many other factors affect mycorrhiza, such as pH, temperature and other nutrient levels. These relationships are not well understood.
Some growers are adding carbohydrates to soil, literally feeding the microbes that are already in the soil, in an attempt to increase their numbers and their activity. This may seem like a logical thing to try, but based on our current knowledge of soil microbes we can’t even say with certainty that an increase in specific microbes or their activity means that soil quality is improved.
With so little known about soil microbes how can we make a reasonable decision about whether to use them or not? At this point in time I am not sure if we can make a reasonable decision about the cost-effectiveness of adding microbes to vineyard soils. One thing we can do is to appreciate the complexity of the soil ecosystem we are trying to manipulate and act accordingly. For example, some soil scientists think of the interactions among soil organisms as a soil foodweb. This is the same idea as a foodweb that exists above the ground, say in an oak woodland. We are all familiar with this concept where you have plants which are fed upon by herbivores, such as insects, rabbits and mice, which are in turn fed on by predators like other insects, birds, foxes and mountain lions. These systems are complicated and manipulating them can be difficult and unpredictable. Anyone who has tried releasing organisms into a vineyard, such as lacewings or mite predators, knows that you can’t just throw them out there and expect them take care of all your pests problems. Conditions need to be just right for them to survive, and even then they may not do what you want them to do. We shouldn’t expect things below ground to be any easier to manipulate or work any more predictably than things above ground.
We are in the early stages of using soil microbes to help us grow winegrapes. We will see more and more companies selling them and undoubtedly more and more growers will use them. Information to help us decide whether to use them and how to use them is going to be slow in coming because of the difficulty in studying soil microbes. One thing we do know about the soil community is that it is complicated, which means there will be no simple answers when it comes to using soil microbes.
(Cliff Ohmart is director of research and integrated pest management at the Lodi-Woodbridge Winegrape Commission.)
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