Horticultural experts all agree that one of the best things that you can do to improve overall soil health and to help plants uptake nutrients is to add soil ammendments that have a high percentage of mycorrhizal fungus in them. We particularily like a variety of products like MycoApply and Nutricast that we blend in with our soils to help increase a plants root growth. For those of you who might have found themselves daydreaming in biology class I am going to roughly break down how mycorrhizae function.
"Mycor" - "rhiza" literally means "fungus" - "root" and defines the mutually beneficial relationship between the plant root and fungus. These specialized fungi colonize plant roots and extend far into the soil resource. Mycorrhizal fungal filaments in the soil are truly extensions of root systems and more effective in nutrient and water absorption than the roots themselves.
These fungi increase the surface absorbing area of roots 10 to 100x thereby greatly improving the ability of the plants to utilize the soil resource. Estimates of amounts of mycorrhizal filaments present in soil associated with plants are astonishing. Several miles of fungal filaments can be present in less than a thimbleful of soil! But mycorrhizal fungi increase nutrient uptake not only by increase the surface absorbing area of roots, they also release powerful chemicals into the soil that dissolve hard to capture nutrients such as phosphorous , iron and other "tightly bound" soil nutrients. This extraction process is particularly important in plant nutrition and explains why non-mycorrhizal plants require high levels of fertility to maintain their health. Mycorrhizal fungi form an intricate web that captures and assimilates nutrients, thus conserving the nutrient capital in soils. In non mycorrhizal conditions much of this fertility is wasted or lost from the system.
Mycorrhzial fungi are involved with a wide variety of other activities that benefit plant establishment and growth. The same extensive network of fungal filaments important to nutrient uptake is also important in water uptake and storage. In non-irrigated conditions, mycorrhizal plants are under far less drought stress compared to non-mycorrhizal plants.
Mycorrhizal fungi also improve soil structure. Mycorrhizal filaments produce humic compounds and organic "glues" (extracellular polysaccharides) that bind soils into aggregates and improves soil porosity. Soil porosity and soil structure positively influence the growth of plants by promoting aeration, water movement into soil, root growth, and distribution. In sandy or compacted soils the ability of mycorrhizal fungi to promote soil structure may be more important than the seeking out of nutrients.
Since, most of the soil that is available on the marketplace is relatively sterile and most designers only specify around 10% organic material the soils are relatively poor in both nutrient content and beneficial mycorrhizae. In the fall, after the plantings have been installed we apply a generous amount of "worms castings" on top of the soil. Circle Organics (http://www.circleorganics.com/) carries pelletized castings that can be easily spread the same way that you would apply a "slow release fertilizer". We like to use a hand spreader that will allow us to specify the appropriate application rate. The nutricast product they carry contains a lot of the beneficial mycorrhizae fungus and since plants will put a majority of their energy in growing their plant roots over winter as the castings slowly degrade they release the mycorrihizae into the soil allowing the plants to fix larger concentrations of potassium and phosporous which then increases the root growth. By the time spring rolls around the expansive root system then can support much more top growth and helps the plants fill in anywhere between 25-50% faster than if no soil ammendments were added.
Higher coverage rates thus helps to reduce the weed pressure and thereby reduces the amount of maintenance that is required. So, hopefully, on your next project you will consider using a product with mycorrhizae in it and for designers I would recommend adding that you your specifications to improve the general performance and health of the green roof system!
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