Verdure: Mind your prills and granules

What’s the difference between a fertilizer prill and granule? Like many historical events, it starts with a pint of ale.

One night in 1782 in England, while a plumber was walking home from the local pub, he got an idea that he tested the next day. He poured molten lead through small openings and allowed the liquid to fall at a distance. This caused it to solidify into perfect spherical drops or pellets as it was falling.

Today, this prilling concept is used to manufacture fertilizer, but it has been much improved with technology and the use of tall prilling towers. In 1965, a fertilizer granulation process was patented by the Tennessee Valley Authority. In a large rotating drum, small particles are subjected to various processes that essentially cause them to bind together to create the granules.

Regarding these particles, what is this “SGN” listed on a fertilizer bag? SGN is the “Size Guide Number,” defined as the average particle diameter of those fertilizer particles in millimeters multiplied by 100. For example, an SGN of 80 means the average diameter of the particle is 0.8 millimeters (0.03 inch or 1⁄32 inch); thus 0.8 × 100 = 80. Typical fertilizer SGNs for greens are 75 to 80, fairways 100 to 150 (1- to 1.5-millimeter diameter, or ~1⁄20 inch), and lawns 200 or more (2-millimeter diameter, or ~1⁄10 inch).

When applying granular fertilizer to a green, how much of those particles are actually picked up the next day by the mower? It’s natural to wonder about a potential loss of nutrients, disposing of fertilizer with the grass clippings, and don’t forget about wear on mower reels. Turf research consultant Clark Throssell, Ph.D., coordinated an experiment at three locations to investigate. The tests were conducted on MiniVerde ultradwarf bermudagrass at the University of Florida (Citra, Fla.) at a mowing height of 3.2 millimeters (0.125 inch); Pennlinks creeping bentgrass at Purdue University (West Lafayette, Ind.) at 3.2 millimeters (0.125 inch); and annual bluegrass at Rutgers University (North Brunswick, N.J.) at 3.0 millimeters (0.118 inch).

At each site, researchers evaluated four commercially available fertilizers (nitrogen-phosphate-potash, or N-P₂O₅-K₂O), all greens-grade sizes: LebanonTurf Country Club (LCC) MD 22-0-16 (SGN = 80), LCC MD 18-0-18 (SGN = 80), LCC MD 12-0-24 (SGN = 80), and The Andersons DG 17-0-17 (SGN = 75). The MD represents “maximum dispersion,” and DG is “dispersible granule” technologies.

Editor’s note: Granular vs. foliar fertilizers — researchers tested them alone and in combination to determine their effects on turfgrass quality and soil nutrient levels. Read more.

All fertilizers were applied once at two rates of 0.5 or 1.0 pound nitrogen per 1,000 square feet (2.4 or 4.9 grams nitrogen per square meter). Plot sizes were 4 feet × 10 feet (1.2 meters × 3 meters) at Florida and New Jersey, and 3 feet × 10 feet at Indiana (0.9 meters × 3 meters), and all treatments were replicated four times, including an untreated check. In June 2013 at all three sites, plots were mowed first, then fertilizer treatments applied uniformly, then irrigated. Prior to mowing the next day, plots were allowed to dry, then each plot was mowed individually, clippings were collected and placed into a bag, and mower reels were cleaned before mowing the next plot. The bags were dried overnight, and the next day, any fertilizer particles present were meticulously removed and weighed for each plot.

At the 0.5-pound N rate, percent LCC MD fertilizer collected ranged from 0.5% to 1.7% in Indiana and Florida, and 2.9% to 8.0% in New Jersey. The Andersons DG was 0% in Indiana and Florida, 0.4% in New Jersey. The average across all treatments and locations was 1.8% fertilizer loss. At the 1-pound N rate, percent LCC MD fertilizer collected ranged from 0.6% to 2% in Indiana and Florida, and 4.2% to 6.8% in New Jersey. The Andersons DG was again 0% in Indiana and Florida, 0.4% in New Jersey. The average across all treatments and locations was 2.0% fertilizer loss.

This minimal loss of fertilizer at either N rate did not adversely affect putting green quality or performance, or money spent on the fertilizer.

So, a good practice when applying granular fertilizer is to thoroughly irrigate later that night, wait until after the turf is dry the next day to mow, and, if possible, avoid collecting clippings when mowing that next day.

Source: Throssell, C., J. Kruse, C. Bigelow and J.A. Murphy. 2017. Fertilizer granule collection and nutrient removal from putting greens following mowing. International Turfgrass Society Research Journal 13:275-279 (https://doi.org/10.2134/itsrj2016.05.0372).

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Mike Fidanza is a professor of plant and soil science in the Division of Science at the Penn State University Berks Campus in Reading, Pa. He is a 19-year member of GCSAA.

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