Mycorrhizal Fungi and pH of Soil or Water
In natural settings, the vast majority of plants associate with mycorrhizal fungi. Mycorrhizal plants occur in all terrestrial environments where their host plants grow. This includes soils with varying pH from as low as 3 to as high as 9.5. However, at the extremes of this range, the occurrence of plants is much more limited. Most plants grow in soils of pH 4.0 to 8.0.
pH Preferences in Plants and Mycorrhizal Fungi
Typical soils encountered in landscape planting range from pH 4.5 to about 8.0. Mycorrhizal fungi occur routinely across this entire range. However, there appear to be some pH preferences among both plants and mycorrhizal fungi. In general, oaks, pines, spruce, and many other ectomycorrhizal trees prefer acid soils. However, these trees may sometimes be found growing in more extremely acidic soils as low as pH 3. There is published research with small pH electrodes showing that plants and mycorrhizal fungi can modify the pH of their root environment. Therefore, the bulk soil may be outside the preferred pH range, but the mycorrhizal fungi can surround the roots with a more tolerable pH environment. This is particularly true with ectomycorrhizal fungi, which characteristically produce a tightly woven fungal sheath wrapped around the mycorrhizal root. The pH inside the sheath can be at or near the optimum pH for root growth compared to the pH of the bulk soil. This protection of the root afforded by the fungal sheath can allow the trees to grow in extremely acidic soil, such as abandoned coal mine sites.
Plants that associate with VAM (endo) fungi occur across a broad pH range, both acid and alkaline, with more species occurring in the alkaline range compared to ectomycorrhizal plant species.
While plants and mycorrhizal fungi may have preferences regarding soil pH, these are not absolute. And there are various strategies to accommodate any such preference. For example, ROOTS mycorrhizal inoculants contain a mixture of several species of mycorrhizal fungi. These species can have different preferences regarding soil types, climate, pH and other parameters. By offering a mixture of different preferences, you broaden the effective range of the inoculum across several parameters at once (not only pH).
Adjusting Soil pH
When planting oaks or other acid-loving plants into neutral or alkaline soils, it is often a good practice to incorporate a soil-acidifying additive (like sulfur) along with your mycorrhizal inoculum. While this will only reduce the pH temporarily (a few weeks), it will allow the mycorrhizal fungi to colonize the roots while the pH is low. Once the fungi set up residence within the root tissue, they can take refuge from the higher soil pH and work to increase acidity inside the fungal sheath. Subsequently, as soil pH in the planting hole gradually rises, the mycorrhizal fungi are already well-established and capable of controlling the pH of the roots’ micro-environment.
In summary, while soil pH affects plant growth and establishment, it is not a defining factor regarding mycorrhizal inoculum success. While it can play a part in plant growth and establishment, both plants and their fungal partners are capable of actively adjusting this important parameter to the benefit of the host plant and their fungal partners.