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Rotary fertilizer spreaders are a popular way to apply fertilizers and granular pesticides to your lawn. Some rotary spreaders can deliver a reasonably uniform distribution pattern with some products but not with others, and some homeowner rotary spreaders don’t deliver a good pattern with any product.
How Rotary Spreaders Work
The metering and distribution functions are separate on a rotary spreader. One port (occasionally 2 or 3 ports) is used to meter the granules, and the opening of the port(s) controls the rate. Distribution is accomplished when the granules fall onto and are then thrown from the spinning impeller (Figure 1).
On many small rotary spreaders, the metering port is at the center of the rear of the hopper. This might imply that the pattern from the spreader will be centered and be the same on both sides of the spreader, but this is seldom true. After the granules fall onto the impeller, they are carried around the impeller while simultaneously sliding out along the impeller fins until they are finally thrown off. The granules do not leave the impeller in a radial direction, but rather in a direction that is closer to tangential. This means that a spreader pattern is inherently skewed; the left side will not be a mirror image of the right side.
Since all small rotary spreader patterns are inherently skewed, it is desirable to be able to adjust the pattern to compensate for skewing. Professional rotary spreaders used by grounds maintenance companies generally have such an adjustment. The adjustment usually consists of some way of moving the point(s) where the granules drop onto the impeller (Figure 2). Small rotary spreaders used by homeowners do not have such an adjustment. With a small homeowner rotary, you must accept the patterns your spreader puts out.
Different Products -- Different Patterns
Because different granular products have different granule characteristics (size, density, shape, frictional properties, etc.), you can expect each product to perform differently in your rotary spreader (Figure 3). Some may result in a pattern that is heavy to the left, and some may be heavy to the right.
Obtaining Uniform Patterns
Since you have no means of adjusting the pattern on your spreader, the only way to obtain acceptable overall uniformity is to use a double or even triple overlap to average the skewing. The spreader settings on the product label should provide a recommendation that reflects this reduced swath width, but many don’t. For most products in most home lawn spreaders, the recommended width should be about 2-3 feet rather than the 8-12 feet some manufacturers recommend.
Some homeowner spreaders will deliver a decent pattern with a few products at widths of 6-10 feet, but they are rare. Just because you observe material flying out from the spreader several feet in each direction doesn’t mean you can obtain a decent pattern from a wide swath width.
Remember that most granular materials in homeowner rotary spreaders need to be spread at a very narrow swath width to obtain a uniform overlapped pattern. You should be suspicious of product labels or spreader instructions that say you can use a wide pattern with a homeowner rotary spreader.