Is a fungicide such as neem oil going to kill of the mycorrhizae in the soil? I know that hydrogen peroxide kill off pests and fungus but also wipes out the mycorrhizae. I assume it is the same for most other fungicides whether it be neem or a bacteria-based product.
You'll be fine with neem but don't spray the neem oil on your soil - trust me; it makes a yucky mess. Instead, use neem cake which is the ground up leftover neem seeds which are left over once the neem oil has been extracted. Oil for foliage, neem cake for soil applications. Neem cake is both an excellent soil amendment, as it falls into the class of Dynamic Accumulators, which utilize deep tap roots to mine and store the nutrients in the surrounding soil, as well as its excellent pesticide properties. Neem cake can also be soaked in water to make nutrient and pesticide teas. The very best neem is cold-pressed and comes from India. You will get an excellent product from neemresource.com (Ahimsa neem products) Here is a really nice thread by one of the old organic gurus - and is totally worth reading. http://forum.grasscity.com/organic-growing/982164-whats-neem.html J