7/2/2013 - By Dr. Travis Shaddox

Common granular fertilizer spread rates vary from 150-350 pounds of fertilizer per acre.  In general, these rates provide sufficient coverage to produce a uniform turf.  However, little thought is given to how many fertilizers granules are spread per a given area.  This becomes increasingly important when fertilizer is sprayed with pre-emergent herbicides.  When sparged, each particle is coated with a small amount of herbicide and, in order to effectively control weeds, the herbicide must be placed near emerging weeds.  So the question arises, “What is the lowest spread rate I can use that will still provide sufficient weed control?”  Research on this topic is limited or non-existent.  However, Harrell’s recently conducted a fertilizer particle study which should provide you with more information on this topic and, hopefully, allow you to understand the risk associated with reducing your spread rates.

The information gathered from the study is included in the PDF presentation below.  To summarize the presentation, particle distribution may vary by as much as 20X within the same size-guide-number range (ie. micro, mini, regular).  Secondly, as spread rates lower, the space between fertilizer particles increases and pre-emergent control may become an issue.  Lastly, minor elements are blended in low amounts and, thus, the particle distribution is very low and may not be as effective as sprayable minor sources.

Additional Resources

• Particle Size (PDF)