PRINT Cultivating beneficial soil fungi to increase yields

RESEARCH UPDATE: New Farm Research
Cultivating beneficial soil fungi to increase yields
Low-cost, on-farm system for producing mycorrhizal fungi inoculant takes another step forward

By Laura Sayre

Soil fungi, soil carbon, and a substance called glomalin

One of the reasons mycorrhizal fungi (MF) are so important for healthy soils and high-performing agroecosystems is that MF produce a soil substance called glomalin. Glomalin is a glycoprotein (or sugar protein) that helps form stable soil aggregates and may be responsible for much of a soil's ability to store carbon.

Glomalin is difficult to extract from soils in the laboratory—which is probably why it wasn't even discovered and described until 1996, by USDA Agricultural Research Service soil scientist Sara Wright. Since then, research on the structure and functions of glomalin has been advancing rapidly, driven in part by scientists' desire to understand the role of soil carbon in the global carbon cycle.

In the coming weeks, we'll be featuring more stories on indigenous soil microorganisms, soil quality and the potential of organic farming systems to fight global warming by building soil carbon.—NF

For more on soil microorganisms and David Douds' research, see

Cultivating diversity underground for better yields above

MF fact sheet

Using the ordinary to cultivate beneficial indigenous microorganisms

Also, look for the Rodale Institute's Building Healthy Soil field day July 16. For more information contact Maria Pop at maria.pop@rodaleinst.org.

April 2, 2004: Results are in from the 2003 field tests of a low-tech system for cultivating mycorrhizal fungi (MF) to improve plant health and boost yields.

In preliminary trials at The Rodale Institute Experimental Farm, substantial differences were found in the response of MF populations to different soil environments. Test plots designed to demonstrate how farmers can produce their own MF inoculant—to use in a greenhouse mix, for instance—showed that two key MF species performed better when grown with a dairy manure-leaf compost or a yard clippings compost than when grown with a controlled microbial compost.

A mycorrhizae factory: To produce a MF inoculant, construct a simple enclosure out of landscape fabric, fill it with a mixture of compost and vermiculite, and then transplant pre-colonized bahiagrass seedlings into the mixture. 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 the following spring.

The different MF response may be attributable to the higher phosphorous levels in the controlled microbial compost, since MF populations are believed to be inhibited by high phosphorous conditions. The 2003 data will help researchers refine the MF inoculant production system so that it can be passed on to farmers.

The inoculant production research is part of a multi-year project led by Agricultural Research Service scientist Dr. David Douds. A soil microbiologist who has dedicated his career to the study of mycorrhizal fungi, Douds has been collaborating with Rodale Institute researchers since 1989, studying the effects of conventional and organic farming practices on MF populations and the effects of MF populations on crop yields.

Previous years' field studies found higher and more diverse MF levels in organically-farmed than in conventionally-farmed soils, in part because of the use of over-wintering cover crops in organic systems. Field experiments have also demonstrated dramatic yield increases when crops were inoculated with MF: up to a 34 percent yield gain in sweet peppers, and up to a 45 percent gain in potatoes.

The on-farm inoculant production system should enable farmers to realize similar yield increases with minimal costs. "These systems could become as common as a compost pile" for organic and sustainable farms, notes Rodale research technician Matt Ryan. Constructed out of landscape fabric, the yard-square planting enclosures are simple to build and require little maintenance other than watering.

This season, Douds and the Rodale researchers will repeat the experiment with a few adjustments based on the 2003 results. The yard clippings compost and the dairy manure-leaf compost—which produced the best results at higher concentrations—will be tested at compost:vermiculite mixtures of 1:1, 1:2, 1:4, and 1:9. The controlled microbial compost—which produced the best results at lower concentrations—will be tested at dilutions of 1:9, 1:19, 1:49, and 1:99.

In addition, Douds plans to evaluate the feasibility of using native soils, instead of host plants pre-inoculated with MF, in the production system. Reasoning that undisturbed soils from a given farm—say from a hedgerow or native prairie remnant—should contain the MF species best suited to the local conditions of that farm, the researchers will mix controlled amounts of native field soils into sterilized compost:vermiculite mixtures, and then plant non-inoculated host plants into the enclosures.

A final, more elementary possible refinement is the use of large bags—either of woven polyethylene or of burlap—for the propagation system, rather than having farmers construct the raised beds out of sheets of landscape fabric and wooden stakes. In addition to being simpler, using bags would enable farmers and growers to start the MF propagation beds in the greenhouse in early spring, and then move them outside when conditions permit. Last year's results indicate that inoculant enclosures started earlier in the season produced significantly more viable MF propagules.