I receive a lot of questions regarding Round Up and in particular its effects on mycorrhizae. I often answer it would be preferable not to use it but the tests show that sometimes it does not have much of an effect and in other cases it may be slightly detrimental. Therefore, it does not appear to completely destroy the mycorrhizae. It may however harm the colony but they are very resilient. Moreover, it seems to have an effect on the other beneficial organisms, mostly the beneficial bacteria, in the soil. Again, we obviously encourage other means of weed control. In fact, some presence of weeds can be beneficial to crops as they can often serve as conductors for mycorrhizae, where the mycorrhizal fungi will colonize and multiply through the roots of weeds.
The interaction of Round Up and microbiology brings up the important topic of what other inputs can have detrimental effects on the soil biology. Mycorrhizal fungi is generally quite durable. However, there are a few things that can harm or even destroy it.
(1) It should never be fully exposed to direct sunlight. The fungus should be in the ground at the roots. Sunlight can destroy the fungal spores.
(2) Avoid fast-acting synthetic fertilizers with high N and/or high P. The high N is less of a problem and the degree of what is high will always vary (a rule of thumb should be 10 – although some conditions can handle a bit higher). High P is more of an issue for mycorrhizae because one of its main functions is to help plants regulate and intake phosphorous. If it is being overloaded with P, it will harm the mycorrhizae. The rule of thumb we suggest for P is no higher than 10 as well. This number can vary based on many conditions. If fertilizers are used, we recommend slow release fertilizers (synthetic fertilizers will work as long as they are slow release). Ideally a good compost or compost tea is available to provide the extra nutrients to the soil.
(3) Systemic fungicides should be avoided. Since mycorrhizal fungi are a fungus any systemic fungicide will destroy them. A localized fungicide not directed at the roots should not have an effect.
We do encourage growers to test using as much organic methodology as they can. Mycorrhizae is a key component, however, it needs a healthy environment with beneficial bacteria, earthworms, etc. that will help to create the necessary ecosystem at the roots. Building up the soil via cover cropping, reduced tillage and compost or other soil amendments will encourage the mycorrhizae to begin regulating a complex biological ecosystem leading to healthier and more abundant crops.