Sciency Answers: Mycrorrhizae

Liz, of Ginkgo Grass, sent a question:

Does adding mycorrhizae to a garden help? I have heard that there is plenty in the soil, and the only good use is for sterile potting mix.

Thanks,

Liz

The short answer is that you are right. But there is a longer answer, and it is much more fun.

Soil biology is incredibly complex, with every handful of healthy soil containing many species of bacteria, fungi, protists and nematodes, many still unknown to science. We know that many of these soil organisms for beneficial, mutualistic or even symbiotic relationships with plants, but the sheer complexity of these interactions is just beginning to be unraveled. Several species of fungi which form close, beneficial, association with plant roots (mycorrhizae) have been identified, and adding these fungi to soil has shown benefits, in a few specific situations.

The first situation is in sterilized soil or soilless media in pots. This isn't particularly surprising -- when you start with sterile soil, adding a beneficial fungi can be helpful. But frequently the results aren't dramatic, or significant in normal conditions. Which isn't surprising either -- mycorrhizae help plants primarily by acting like extensions to the root system, scavanging up scare nutrients (especially phosphorus) and water. In very poor, acidic soil, this can be the difference between a plant living or dying. In a carefully watered, fertilized container, it doesn't have much effect.

The other situation where adding mycorrhizae can be beneficial is best illustrated by a story (taken from Soils in Our Environment by Duane Gardiner and Raymond Miller): People tried transplanting pine trees grown in the US in Puerto Rico, but they only grew a few inches and died. The problem was solved when some soil from a pine-growing part of the US was taken to Puerto Rico and used to inoculate the soil there. The mycorrhyzae from the US soil hooked up with the pine roots, and hey presto, they grew 2.4 meters in a year rather than 30 centimeters. This case worked because the mycorrhizae used were species specific. These pines needed this specific fungi in order to thrive in particularly harsh conditions, namely, extremely nutrient poor tropical soils. Adding some generic "helps everything grow better!" commercial mycorrhizea product to those pines wouldn't have helped because it wouldn't have been the specific species those pines needed. And adding even the right species of mycorrhizae to the soils in the US wouldn't have done any good either -- because it is already naturally in those soils. I should add here: This story makes a good point, but you should follow their example. Moving soil around to get mycorrhizae VERY BAD IDEA! Soil from where a plant grows naturally may have beneficial mycorrhizae. It also probably has all sorts of soil born diseases which you do NOT want to be helping spread around. You don't want to be the person who introduced the soil equivalent of kudzu to a new area.

So the take home message is: mycorrhizae in potting soil might be beneficial, but I wouldn't expect to see a huge effect. If you are curious, it might be fun to give it a try, but be sure to keep an untreated pot for comparison. Also, check the label to see if the mycorrhizae treatment also includes fertlizers, which of course will result in added growth, but not because of the inoculation. 

Adding mycorrhizae to good garden soil will probably do nothing unless it is a specific mycorrhizae for some specific plant that isn't native to your area. Any product claiming to be a generic helpful mycorrhizae that will make all your plants grow better is going to be (almost certainly) a waste of money.

If you do want your plants in the ground to grow better, your best bet is to keep your already existing soil organisms happy with lots of organic matter and mulch.

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