NS4Life is our core plant probiotic.   It can be used for soil amendment (added to soil in the same way as a fertiliser) or as a foliar spray.
 
Probiotics relay on the principle of dominance to be effective.  In other words the good bugs need to be the dominant species and the bad bugs need to be significantly outnumbered by the good bugs.  This generally requires regualr application of the probiotics to ensure that the good bugs remain the dominant species.   Typically an application once/month will achieve this.
 
While this might  seem like additional effort, it is important to remember that regular use of probiotics acts to stregthen plants and reduce the impact of stressor (such as drought, pests, excessive heat, etc).