Linda Benedict, Parish, Richard L.
Homeowners have two types of fertilizer spreaders available for use on their lawns: rotary spreaders and drop spreaders.
And an LSU AgCenter engineer says they use entirely different principles to distribute granular materials.
Dr. Richard Parish of the LSU AgCenter’s Hammond Research Station says drop spreaders have a row of holes – or metering ports – across the full width of the bottom of the hopper with a shut-off-bar to open and close the ports to control the delivery rate. Rotary spreaders have one or more metering ports that drop the fertilizer granules onto a rotating impeller that slings the material in a pattern wider than the spreader.
Parish says a good drop spreader will put down a uniform rate of granules across the width of the hopper. A rotary spreader pattern from a single pass, on the other hand is not uniform – the pattern will be heavier in the center and taper toward the edges.
When operating a spreader, you make multiple parallel passes across your lawn. "With a good drop spreader, the pattern will be uniform if you space the patterns exactly with no skip or overlap," Parish says. "Normally, you do this by running the tire just inside the tire track from the previous pass."
The engineer points out that with a drop spreader, an error of only an inch or two will result in a stripe with either a double fertilizer rate or no fertilizer at all.
Because individual patterns taper off with a rotary spreader, you must overlap the patterns somewhat. But because of the feathering at the edges, rotary spreader patterns are more forgiving of small errors in swath width.
Nevertheless, Parish says, even properly overlapped rotary spreader patterns are seldom as uniform as good drop spreader patterns.
"Drop spreaders have a narrow, fixed swath width," Parish says. "The swath width is the width of the hopper no matter what you’re spreading."
But swath width for rotary spreaders varies with the type of material being spread. Not only is the pattern normally wider for larger or heavier granules, but some products will produce such poor patterns that a double overlap or half swath width is required.
Parish says the effective swath width for a homeowner-style rotary spreader can vary from as little as 1½ to 2 feet up to 10 to 12 feet.
"You have to know what width is correct for each product," he warns.
Parish says drop spreaders provide an abrupt cutoff at the edge of the pattern, so you can apply products right up to the edges of beds, sidewalks or driveways without placing any granules outside of the grass area. Since rotary spreaders throw the granules, however, you have two choices when working near the grass edge. You can get a full rate right up to the edge and throw fertilizer off of the lawn, or you can keep the spreader well away from the edge and end up with a light rate on the adjacent lawn.
Parish says one model of homeowner rotary allows the right side of the pattern to be closed off for applying along non-target areas.
The engineer says homeowners can apply granular pesticides with any spreader, but the risks of pesticide error are greater with rotary spreaders.
"Throwing the granules beyond the spreader rather than just dropping them a few inches increases the risk of pesticide drift onto non-target areas," Parish says. "Also, the typically less uniform patterns from rotary spreaders increase the risk of over- or under-application. These problems are most serious with herbicides."
In the end, Parish says drop spreaders are better suited to small lawns where you want very uniform patterns or have many obstacles to trim around while rotary spreaders are generally better for larger, open areas.
For information on related engineering, landscape and gardening topics, visit the LSU AgCenter Web site at www.lsuagcenter.lsu.edu.