Pesticide and Environmental Update
Organic Fertilizers and Mycorrhizae
Mycorrhizal fungi—naturally occurring, beneficial soil organisms—have
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.
Agricultural Research 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
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 treatments
Linderman first looked at dozens of fertilizers on various nursery
crops—particularly marigolds, because they are very responsive to
mycorrhizae—to 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 with
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 coir—like organic fertilizers—does not
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.