Soil Fungi Plants' Natural Friend
To determine the effects
of compost on mycorrhizae in
greenhouse production of
nursery crops, plant
pathologist Robert Linderman
and technician Anne Davis
grow plants (marigolds shown
here) in a potting mix with
various levels of compost.
Some plants are inoculated
with mycorrhizal fungi and
others are not. Compost was
added to the larger plant
that Linderman is holding
(left) while none was added
to the smaller plant.
Mycorrhizal funginaturally occurring, beneficial
soil organismshave been helping farmers for thousands of years
by improving water and nutrient use efficiency and suppressing diseases
in the plants they colonize. Applying certain chemicals to the soil
during the last half century-while increasing crop yields and fighting
diseases-has likely inhibited these important fungi.
Service plant pathologist Robert G. Linderman is one of only a few
scientists studying how mycorrhizae affect the nutrition and health
of nursery crops. Other ARS scientists look at the fungi in food crops.
At the Horticultural Crops Research Laboratory in Corvallis,
Oregon, Linderman is investigating various factors that affect mycorrhizal
relationships. He measures the level of mycorrhizal colonization of
roots and compares it to control groups to see how effective various
Linderman first looked at dozens of fertilizers on various
nursery cropsparticularly marigolds, because they are very responsive
to mycorrhizaeto see whether they help or inhibit fungal growth.
He found that organic fertilizers are generally compatible with mycorrhizae,
whereas phosphorus-rich inorganic fertilizers inhibit the fungi.
"It's good that organic fertilizers don't inhibit
mycorrhizae, but the plants do not grow as large or as fast as the ones
treated with inorganic fertilizers," Linderman explains. Manufacturers
of organic fertilizers are now advising users to apply more than they
were previously. This allows plants to grow normally without interfering
Linderman is looking at other things growers add to their
potting mixes. Peat moss has traditionally been a popular component
in potting mixes. Linderman observed that some peat types suppress mycorrhizal
associations, while others do not.
Instead of peat, some growers are starting to use coir
(fibers from coconut) as a potting mix component. Coir has a more uniform
texture than peat, and it has a better water-absorbing and nutrient-holding
capacity. Linderman's studies show that coirlike organic fertilizersdoes
not inhibit mycorrhizae, although it may reduce growth of some plants.
Linderman is researching composts that might be added
to potting mixes. Composts differ in the types of materials they contain.
Nurseries in different parts of the country also use different amounts
of compost in their media. Even the way compost is made and stored makes
a big difference. "Overall, presumably because of the high levels
of phosphorus, fresh composts appear to suppress mycorrhizae,"
Linderman says. But some very mature composts are not inhibitory.
Linderman admits he has not settled on the one perfect
ingredient to add to potting mixes that will establish mycorrhizae in
nursery crops and produce healthy plants. "Growers just need to
think ahead of time what will happen when a particular product is used,
since they wouldn't want to add an ingredient that would suppress the
beneficial fungi."By David Elstein, Agricultural
Research Service Information Staff.
This research is part of Methyl Bromide Alternatives
(#308) and Plant Diseases (#303), two ARS National Programs described
on the World Wide Web at www.nps.ars.usda.gov.
"Soil Fungi Plants' Natural Friend" was published in the May 2004 issue of Agricultural Research magazine.