Considerations for spraying microbial applications
To produce the most effective response from applications that contain living microbes, we need to consider the path through the sprayer nozzle and the environment on the leaf surface or within the soil once the product is applied.
It is best to have spray pressures below 55 psi to reduce or avoid sheer at the nozzle. Higher pressures can produce a sheer force with a markedly negative effect on living organisms in the solution.
When applying products that are suspended in solution and with larger particle size, such as mycorrhizal fungi which can have a spore size up to 50 microns, use larger nozzle and screen sizes. We generally recommend a 50 mesh screen or even no screen in some cases.
When applying products to the soil surface that will not be incorporated, add humic substances or dark-colored material such as molasses to the solution to protect organisms from UV. I suspect (but don’t know for certain) that this may be less necessary when applying to the leaf surface, since organisms which can survive on the leaf surface can likely handle UV exposure.
Combine the inoculant with a biostimulant to develop a ‘synergistic stack’ of products that produces a much greater performance than either product by itself. These could be materials such as humic substances, seaweeds, food sources, prebiotics, enzymes, etc.
And by all means, avoid adding antimicrobials into the spray solution, at any concentration. This means you don’t use water that contains chlorine, chloramines, or anything with a similar antimicrobial purpose. Don’t add ionic or salt forms of boron, zinc, manganese, or copper. After all, in the right concentrations, each of these minerals are very effective antimicrobials. We need to consider not only the solution in the spray tank but also the concentration of the droplet when it begins drying on the leaf surface or soil surface.
Field experience indicates it seems to be ok to add chelated or complexed forms of these minerals that aren’t immediately absorbed by the microbes in the solution, yet are quickly absorbed by the plant while the droplet is still liquid on the leaf surface.