| | Figure 1. Rotary broadcast spreader. |
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Two types of fertilizer spreaders are available for use on home lawns: rotary (Figure 1) and drop (Figure 2). They use entirely different principles to distribute granular materials.
Definitions
Drop spreaders have a row of holes or metering ports across the full width of the bottom of the hopper. A shut-off bar is used to close the ports and to control the delivery rate by incrementally opening the ports. Rotary spreaders have one (or sometimes more) metering ports that drop the granules onto a rotating impeller that slings the granules in a pattern wider than the spreader.
Basic Patterns
A good drop spreader will deposit a uniform rate of granules across the width of the hopper, with no feathering of the edges of the pattern. A rotary spreader pattern (from a single pass) is not uniform; the pattern will be heavier in the center and taper toward the edges.
Overlapped Patterns
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. Normally, you do this by running the tire just inside the tire track from the previous pass. Note that with a drop spreader, an error of only an inch or two will result in a stripe with either a double rate or no fertilizer.
With a rotary spreader, the individual patterns taper off, and you must overlap the patterns somewhat. Because of the feathering at the edges, rotary spreader patterns are more forgiving of small errors in swath width. Nevertheless, even properly overlapped rotary spreader patterns are seldom as uniform as good drop spreader patterns.
Swath Width
Drop spreaders have a narrow, fixed swath width. The swath width is the width of the hopper for all products. Swath width varies with product for rotary spreaders. Not only is the pattern normally wider for larger or heavier granules, but some products will yield such poor patterns that a double overlap (or half swath width) is required. The effective swath width for a homeowner rotary can vary from as little as 1½-2 feet up to 10-12 feet. You have to know what width is correct for each product!
Ground Clearance
Many homeowner drop spreaders have only 2-4 inches of ground clearance. If the grass is a bit tall or if seedheads or weeds are sticking up, they will brush the bottom of the hopper. When there is dew on the grass, this rubbing will plug the metering ports. Most rotary spreaders have much more ground clearance.
Pattern Consistency
Drop spreaders provide the same pattern with every product. Each granular product will yield a different pattern in a given rotary spreader. Professional rotary spreaders provide a means of pattern adjustment to compensate; homeowner spreaders do not.
Each model of homeowner rotary spreader is designed to provide optimum pattern uniformity with only one type of product; the pattern will be skewed with all other types of products.
Push Force
In general, drop spreaders require more push effort than rotary spreaders because of the full-length agitator used by drop spreaders.
Turning
As you make passes back and forth across your lawn, you will need to turn at the ends. With a drop spreader, you can generally achieve a smooth edge at the end of the lawn area. With a rotary, the arc-shaped pattern guarantees non-uniformity at the ends of the lawn area.
Trimming Around Beds, Etc.
Drop spreaders provide an abrupt cutoff at the edge of the pattern; you can trim right up to beds, sidewalks, etc. without placing any granules outside of the turf area. Since rotary spreaders throw the granules, you will have two choices when working near beds, sidewalks, etc.: you can get a full rate right up to the bed or sidewalk and then throw a tapered pattern onto the bed or sidewalk; or you can keep the spreader well away from the bed or sidewalk and not throw material onto the bed or sidewalk, but end up with a very light rate on the adjacent turf.
Granule Size
Rotary spreaders can handle larger granules than drop spreaders because rotary spreaders typically have one large metering port instead of 15-20 small metering ports.
Pesticide Application
You can apply granular pesticides with any spreader, but the risks of pesticide misuse 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. 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 summary, drop spreaders are best suited to small lawns where you want uniform patterns or have many obstacles to trim around. Rotary spreaders are generally best for large, open areas.