You've read the amazing facts and figures: one teaspoon of healthy topsoil can contain millions of individual microorganisms, all playing a part in the functioning of the soil ecosystem. But how much do you really know about the action of those diverse species and how to maximize their presence in your own fields?
Ongoing research at The Rodale Institute® sheds light on one important component of the soil community--mycorrhizal fungi--and its impact on agricultural production. Under the leadership of Dr. David Douds, a soil microbiologist with the USDA's Agricultural Research Service, field trials have shown yield gains of as much as 50% in the presence of healthy mycorrhizae populations. Now Douds is developing a practical, low-cost method for on-farm production of mycorrhizal soil inoculant, promising higher yields with lower nutrient inputs.
A mycorrhizal primer
Mycorrhizae are soil-dwelling fungi that live in and around the roots of plants ('myco-rrhizae' means 'fungus-root'). The fungi and the plants form mutually beneficial associations in which the fungi receive carbohydrates from the plants and the plants receive nutrients and other benefits from the fungi. Since the first mycorrhizae species were described by a German botanist in the 1880s, researchers have discovered that approximately 80% of all land plants form mycorrhizal associations. The relationship is so widespread, in fact, that it is sometimes referred to as 'the Universal Symbiosis,' and is believed to have played a key role in the evolutionary transition from aquatic to terrestrial plant forms.
Today, scientists divide mycorrhizae into two major types: endomycorrhizae, which penetrate and colonize plant roots, and ectomycorrhizae, which form sheaths around plant roots. Whereas ectomycorrhizal relationships tend to be highly specialized--with some 6000 fungal species worldwide associated with tree species of the oak, beech, and pine families, among others--endomycorrhizal associations are more generalized as well as more widespread, with fewer than 150 fungal species opportunistically colonizing the roots of the vast majority of terrestrial plant families. Ectomycorrhizal inoculants are already widely used in commercial forestry, but the possibility of developing endomycorrhizal inoculants for production agriculture is a more recent idea.
"These are beneficial soil fungi that colonize the roots of plants and help them take up phosphorus" and other immobile soil nutrients, such as zinc and copper, Douds explains. "The fungus colonizes the root and it also grows out into the soil; the part of the fungus that's in the soil acts as an extension of the root system, to explore a greater volume of soil and take up nutrients and bring them back into the root."
In addition to facilitating nutrient uptake, some mycorrhizae secrete a gluey substance, called glomalin, which helps develop soil structure and soil aggregation; others may help plants fight disease. Yet because endomycorrhizae are 'obligate symbionts'--they must have living plant roots to colonize in order to complete their life cycle--their numbers will decline under conventional agricultural monocultures, which have living crop covers fewer months of the year than organic rotations. The drop in yields typically seen after the first year of cultivation on virgin prairie or forest soils is probably attributable in part to the loss of native mycorrhizae, Douds says.
Fifteen years of research prove benefits of fungi
Douds has been conducting research in collaboration with The Rodale Institute since 1989, his first year at the Agricultural Research Service's Eastern Regional Research Center in Wyndmoor, on the outskirts of Philadelphia. "Some employees of The Institute farm came down to our research center as part of a kind of an interagency show-and-tell about research programs and facilities and what all we could do to help each other," Douds recalls. "Rhonda Janke"--The Institute's research agronomist at the time--"gave a presentation about The Rodale Institute Farming Systems Trial," a side-by-side comparison of organic and conventional production systems. Douds recognized it as a great opportunity to study endomycorrhizal associations.
"Later that year I started sampling, and right off learned that [the soils under] the conventional farming systems had fewer mycorrhizal fungi than the soils under the low-input farming systems. So right away we all got excited and we branched out from there."
Since that first season, Douds' work at The Rodale Institute’s 333-acre experimental farm has progressed in three overlapping phases:
In the first phase, from 1989 to 1995, Douds and his team surveyed native mycorrhizal fungi (MF) populations at Rodale and examined the impact of different agricultural practices--including tillage regimes, crop rotations, and soil amendments--on those populations. The second phase, which is still ongoing, looks at the utilization of MF by crop plants, comparing yields in the presence and absence of different MF species. The third phase seeks to apply those findings by devising a simple, on-farm MF inoculum production system, so that farmers can harness the benefits of endomycorrhizae without spending lots of money on commercial mixes. (Commercial products already on the market include Bio/Organics Endomycorrhizal Inoculant [$79.95 for 3 lbs, labeled to treat 500 plants], Plant Success Mycorrhizae Tablets [$19.95 for 100 tablets, labeled to treat 50 plants up to 1 ft tall], and Earthroots VAM Fungi by First Fruits LLC [$15 for 3 lbs, labeled to treat 200 seedlings].)
Agronomic practices that boost—or depress—mycorrhizal levels
Although the first phase of Douds's research found larger and more diverse MF populations in organically-managed soils than in conventionally-managed ones, it also revealed how specific agronomic practices can boost or depress MF levels.
"Over-wintering cover crops. . . are very beneficial to mycorrhizal fungi," Douds notes, whereas "tillage disrupts the mycorrhizal fungi in the soil and serves to decrease the initial colonization of the plants." Based on these findings, Douds emphasizes that all farmers, organic or conventional, can take steps to nurture the MF already present in their fields: reduce tillage, he says, use fungicides sparingly, and--most important--maximize cover cropping. "Over-wintering cover crops give the MF a host plant to colonize when there's no cash crop growing on the soil," Douds explains.
In the coldest part of the year the MF go dormant, but during warm spells in early spring and late fall, the MF will try to grow, and can exhaust their reserves if they find no plant hosts. "During these periods. . . the fungus is still respiring, it's still burning up its carbohydrate storage in the spores, it's burning up the lipids that were stored," leaving it "less viable when the time comes finally for the crop plant to be present." A cover crop or even just a weedy fallow will maintain healthy MF populations, which can then benefit the cash crop coming on to the field.
Crop rotations are another factor to consider, since a handful of crop species belong to plant families that do not form mycorrhizal associations (said to be 'non-mycotrophic'), including the Brassicaceae (rape, broccoli, cabbage, turnips, etc), the Chenopodiaceae (beets, spinach), and the Polygonaceae (buckwheat). Not only will these crops not benefit from the presence of MF, but MF levels in the soil will be depressed after these crops are grown, potentially showing an effect on any mycotrophic crops which follow.
Potatoes and peppers inoculated with mycorrhizae get yield boosts of up to 50 percent!
In the second phase of his research, looking at the impact of MF on crop yields, Douds began inoculating plants in the greenhouse and then tracking their performance in the field.
"We had some plants that were inoculated with a control mix with no inoculum, another one inoculated with a mix of mycorrhizal fungi, and another inoculated with just one species commonly present in commercial inoculum," Douds explains. "We transplanted them into the Compost Utilization Trial,"--another ongoing experiment at The Rodale Institute--"and we found over the course of the 3-year experiment that the mixture of mycorrhizal fungi increased the yield of marketable-sized peppers up to a maximum of 34% over the control. Last year we tried inoculating potatoes, and we got up to a 50% increase over the controls."
This year they are repeating the potato trial, measuring yields under four different treatments: one with no added MF; one with a commercially available MF; one with a mixed MF inoculant grown in a leaf compost and vermiculite medium; and one with a mixed MF inoculant grown in a dairy manure compost and vermiculite medium. Overall, this work suggests that a small amount of mixed MF inoculant can be substituted for a large amount of fertilizer--with no loss of yield, greatly reduced environmental impact, and lower production costs.
One unexpected finding of Douds' work at Rodale "is that mycorrhizae can be used to increase the yield of crops even in soils that are very high in phosphorous." Some of the soils at the Rodale Farm which have been heavily composted, Douds notes, "have available P in excess of 300 parts/million"--well above the level at which mycorrhizal responses are typically seen, around 20-50 ppm available P. "The generalization would be that P as high as 300 would be a situation in which the plant can take up all the P that it needs by itself without relying on the mycorrhizal fungi." Douds believes that at high nutrient levels, some of the other benefits of MF--enhanced disease resistance, improved soil aggregation and better water relations--could be showing an effect.
The third phase of Douds' research at Rodale Farm focuses on developing an inexpensive, practicable system for on-farm production of mycorrhizae inoculant. As obligate symbionts, endomycorrhizae have so far resisted attempts to create what scientists call axenic (or isolated, single-species) cultures--they can only be grown in the presence of a host plant. Douds' system works within this constraint, using bahiagrass (Paspalum notatum), a tropical grass native to the southeastern US, as a host.
The basic procedure is for the farmer to construct a simple enclosure out of landscape fabric (75 cm square and 20 cm high), fill it with a mixture of compost and vermiculite, and then transplant pre-colonized bahiagrass seedlings into the mixture. Over the course of the growing season the bahiagrass spreads within the enclosure and the mycorrhizal fungi spread and reproduce along with it. When the grass dies back in the winter, the farmer is left with a concentrated mycorrhizal inoculant that can be incorporated into his or her potting mix when starting seedlings in the greenhouse the following spring.
This year, Douds gave inoculated bahiagrass seedlings and other materials to a few Pennsylvania farmers to see how the method fares in the real-life conditions of farming. Meanwhile, Douds has 12 soil enclosures growing at the Rodale Farm in an experimental grid designed to identify optimum growth media.
Douds chose three different kinds of compost--yard-clippings compost, controlled microbial compost, and dairy manure-leaf compost--and then diluted each kind with vermiculite at four different ratios, ranging from 1 part compost:2 parts vermiculite, down to 1 part compost:49 parts vermiculite. Each soil enclosure, finally, has nine separate sections, three with no inoculant and three each with two different mixtures of MF.
At the end of the season, says Douds, "we'll sample the mixtures from within each enclosure, quantify the inoculum production, and then hopefully develop a prediction formula, where the optimum ratio [of compost to vermiculite] is a function" of the nutrient analysis and other properties of the compost. All the farmer will need to do, then, is get the nutrient analysis of his or her compost, plug it in to the formula, and find the optimal ratio of compost to vermiculite to use for his or her farm.
"On-farm methods have several advantages over commercial inoculants," Douds explains. In the first place, whereas commercial formulae typically only contain a single MF species (frequently Glomus intraradices), Douds' method yields a diverse inoculum containing many MF species. This is crucial because MF show significant 'functional diversity'--"some are good at holding the soil together, some are good at gathering nutrients," others help fight disease.
A second, related advantage is that by mixing in some soil from a nearby woodland, prairie, or hedgerow, the farmer can use Douds's system "to produce the native or indigenous strains of mycorrhizal fungi. . . the ones that are already adapted to his [or her] particular soil conditions." This could be especially important on problem soils, such as those with high aluminum, say, or high or low pH, where commercially-produced fungi may not survive.
Last but not least, home-grown mycorrhizal inoculum can be produced at a fraction of the cost of purchasing commercial mixes. "I've done some preliminary calculations," says Douds. "The on-farm system produces 100 million propagules [in a single enclosure] for approximately $50, not counting the cost of the farmer's labor, which is fairly minimal. To purchase 100 million propagules as listed on the bag of some commercial mixes would cost anywhere from $8,000 to $40,000." Commercial inoculants are sold in a peat- or vermiculite-based medium, so purchasers have to buy (and pay to have shipped) a large volume of material to get a small number of viable MF propagules--another reason it makes more sense to grow your own.
At the moment, Douds' system (like commercial MF inoculant) is suitable for two types of farms: vegetable growers on any scale who produce their own seedlings and can mix the inoculum into their potting mix; and smaller, labor-intensive farms or urban gardens where "the inoculum can be incorporated by hand, directly into the planting furrow or planting hole." Farmers growing field crops on a large scale can only take advantage of MF inoculants if they want to try them out in a relatively small area. "Delivery of MF inoculum to the field is a problem," acknowledges Douds. "Commercial companies are working on this for their particular inocula." He smiles. We can only hope that he will be too.