by Bill and Mary Weaver
Researchers at Washington State University and the ARS are currently evaluating the impact of tiny soil-growing organisms with the jaw-breaker name of arbuscular mycorrhizal fungi (AM fungi) for enhancing onion growth and suppressing soil borne diseases caused by soil pathogens such as Rhizoctonia solani, which causes stunting and pink root, caused by Phoma terrestris.
Work done at WSU and by ARS researchers in Pullman, WA has shown that in soil inoculated with Rhizoctonia solani, but not AM fungi, onion plants were reduced in height by 56 percent and onion shoot and dry weight were reduced by 73 and 70 percent respectively, a dismal scenario.
However, on a more positive note, when onion plants inoculated with rhizoctonia solani were also inoculated with AM fungi, onion plant height and dry weight were 160 percent and 427 percent greater respectively. In addition, onion plant height was increased in soil inoculated with only AM fungi, to an average of 20 cm., vs. only 15.8 cm. in soil not inoculated with AM fungi, a significant increase.
Future work on this project will assess how the contribution of AM fungi to soil quality can help to reduce dependence on inputs such as soil fumigation and soil applications of fungicide, in addition to Phosphorus fertilizer rates. AM fungi are particularly beneficial to onion plants because of the onions’ smaller root systems. Since the Columbia Basin produce about 27 percent of the nation’s onion supply, these are very significant findings.
AM fungi form mutually beneficial relationship with most crop plants (except brassicas,) helping to enhance growth, defend plants against some soil-borne disease pathogens and improve uptake of immobile nutrients in the soil, such as Phosphorus, zinc and copper.
But I’m getting ahead of myself. Soil contains a multitude of bacteria, fungi, and other organisms that can promote the health of crop plants. Soil is not just “inert dirt.” Healthy soil is teeming with beneficial life forms that can help you produce more food with fewer inputs.
Dr. Mike Lehman, USDA/ARS researcher and microbiologist, states that one gram of soil (about the size of the tip of your thumb) holds 100 billion bacteria, in roughly thousands of species; millions of fungi; dozens of nematodes; thousands of millions of algae; and up to a million protozoans.
Lehman has spent many years peering through his microscope at these tiny life forms. “Together,” he said, “They are the ecosystem service-providers. They are the unseen majority, the below-the-surface microbial masses, roughly equal to, and sometimes greater than, the mass of life forms above the soil surface. They’re just more difficult to see, and certainly more difficult to count, and our knowledge of them borders on the insignificant,” he added.
“We do know, however, that if growers manage their underground “herd” of microorganisms so that they grow and prosper, we can benefit our crops in many ways, as well as reducing our need for costly inputs.
“The interesting thing is that the methods of successfully managing this microbial “herd” in your soil are actually common practices, some of which you may already be using.” Lehman’s work has been in evaluating soil microbial responses to agricultural management practices, determining if they make a difference that is economically significant in reducing input requirements. His conclusion: certain agricultural practices CAN make an economically significant difference.
The focus of his work is on using the “3 Cs” – Cover Crops, Conservation Tillage and Crop Rotation. Together these three practices manage your unseen “herd” of microbial life in positive ways that benefit your crop plants and can reduce your inputs. With vegetable crops, Lehman would add mulching and green manures to the 3 Cs. “Together, these form the basis of sustainable, profitable production.”
“Your crop plants and the soil microorganisms that inhabit their roots have evolved together for over half a million years. They depend on each other. By contrast, we have only been managing crops with fumigation and soil-applied fungicides for a short period of time. When we use these chemicals, we’re killing off a significant part the support system our plants depend on.” When we use growing practices that encourage the unseen life in the soil, we benefit our crop plants’ ability to resist soil-borne disease without chemicals.
Cover crops are an important part of managing your ”herd” of soil microbes because keeping something growing in the soil for more of the year gives continuing food for beneficial soil dwellers. Mulches, compost and green manures accomplish the same thing.
Inorganic nitrogen from a fertilizer bag can be quickly lost to leaching below the root zone in a rainy season. Organic nitrogen from decaying plant material, on the other hand, is released more slowly and adds stable fertility to the soil.
In addition, the organic matter from cover cropping, mulching and green manures has other very valuable benefits. The bacteria feeding on the organic matter improve the soil structure and the aeration of the soil. Higher levels of organic matter enrich the diversity of the microbial community.
On the other hand, in a field that is left bare after the crop is harvested, our microbial friends can be killed by UV light; by the heat absorbed by the dark soil; or because the soil crusted and was unable to absorb rainfall, leaving it too dry to support the life of soil microbes.
The beneficial bacteria in the soil are the nitrogen-fixers, the weed suppressors and the disease suppressors, competing with the disease-causing bacteria for food and space. The AM fungi mentioned earlier are of exceptional benefit to plants. These fungi harmlessly penetrate plant roots, forming a beneficial relationship in which the AM fungi feed on the carbon (sugars) produced by the plant, and their many long, thin hyphae scavenge the soil for minerals over a much larger area than could be reached by plant roots alone. The AM fungi may also have chemical defenses against pathogens.
In addition, the AM fungi help to make plants more drought resistant, reduce erosion by making small filaments that help bind soil particles together, and exude a proteinaceous glue, glomalin, that helps to hold soil aggregates together.
Good soil aggregation increases soil water infiltration and storage and reduces the need for irrigation. The soil seems more sponge-like in its ability to hold water. Tillage destroys many of these effects. Because the AM fungi are filamentous, tillage chops them up so they can no longer have this soil binding ability.
Crop rotations, which result in plant diversity, are also correlated with greater shoot biomass, root biomass, soil and plant tissue phosphorus and diversity of AM fungi.
“It can be done,” Lehman continued. “An organic system incorporating the “3 Cs” uses 20 to 56 percent less energy per pound of dry matter produced compared to conventional methods”. A 21-year study showed that an organic system combined with these agricultural practices produces a lot of positive, economically profitable results.
Great crops with less inputs by farming to favor beneficial soil organisms
by Bill and Mary Weaver