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EQUIPMENT: Granular Spreader Calibration for Lawn Care

September 12, 2001

A fall application of fertilizer can improve a lawn's appearance and health in the coming year. If the lawn has problem winter annuals or certain perennials, the right granular herbicide can control them while they are susceptible seedlings and reduce the weeds next spring. Whatever the reason for fall spreading, or whenever you are spreading, your money goes farther if you get an accurate, uniform application.

Accurate spreader rates

Sometimes, a lawn-care product has directions for the more common brands of lawn spreaders. Instructions might include what setting to use for a certain rate of product. However, the setting should be verified. Your spreader may not be listed; then you must determine what setting is needed.

The easiest way to check the application rate of a spreader is to apply a weighed amount of product to a known area. For drop spreaders, use 1,000 square feet; for rotary spreaders, use a larger area, about 5,000 square feet. Spread the product on the area and weigh what's left. The amount you applied is the difference. Some drop spreaders may come with a pan to collect the product while calibrating the spreader. This is convenient because then you won't spread the wrong amount of product on the lawn while you're calibrating the equipment.

If you want to avoid spreading anything until your spreader is calibrated properly, the spreader can be raised on blocks, and the wheels turned. Of course, if you have a rotary spreader, disconnect the spinner drive mechanism so you don't scatter the product. It would be messy but, worse than that, it could be unsafe. If the wheels are turned at the right speed (the speed they would turn when really spreading) for the correct number of turns, you can collect the granules in a container, weigh them, and reuse them when spreading on the lawn. The formula for the number of turns for the wheels (to simulate 1,000 square feet) is

45,860 ÷ (wheel diameter in inches x swath width in inches) = number of wheel turns

So the process is to collect the product while turning the wheels the right number of turns; then weigh the product to see if the output is right. For adjustment, open or close the metering slide as needed and try it again until the spreader output is set right.

Uniform spreader patterns

A drop spreader is usually more precise and has a more uniform application pattern than a rotary spreader. Rotary spreaders cover a wide swath and thus cover a given area more quickly, but they can be less precise in uniformity and distribution. The first two steps to a good application are simple: (1) read and follow the spreader instructions, and (2) read and follow the product label.

Drop spreaders drop the product straight down. The pattern ends abruptly at the end of the spreader; so for a uniform application, be careful not to leave a gap between spreader swaths. Likewise, be careful not to overlap swaths when applying the full rate, or the overlapped strips get a double rate. Simple maintenance helps keep a drop-spreader pattern uniform. Keep all metering holes clean and unplugged, and keep rust or flaked paint from choking down the metering holes.

Patterns for a rotary spreader are more difficult to assess. One method is to lay out a row of shallow boxes (1 to 2 inches deep, like a pop or beer flat) at regular intervals, every 1 or 2 feet. Spread a pattern three times going the same direction, perpendicular over the line of boxes. Put the product caught in each box in a clear test tube, vial, or bottle; and keep the containers in the order the boxes were laid on the ground, left to right. The pattern should smoothly taper from nothing at the far left to maximum in the center and to nothing on the far right. If the pattern isn't smoothly tapered, follow the spreader manual to adjust the pattern if possible. The appropriate swath width should be to the point where the pattern is half what it is in the center. For example, if the center three or four bottles have material 2 inches deep, and the bottles at the 6-foot positions (6 feet to the left and right of the spreader centerline) have material 1 inch deep, the effective swath width is 12 feet.

Never leave a lawn-care product in an unlabeled container. Empty any container used for the pattern testing. Also, never reuse a container for anything else after it contains pesticide. Either clearly label all the boxes and jars you used during the tests and keep them locked in a safe place, or discard them in the trash.

With a little extra care, the performance of your spreader can be greatly improved. That means your lawn-care products can be applied more efficiently and therefore work better. The ultimate result is more response for your dollar and less wasted product, which is good for the lawn and the environment.

Author: Mark Mohr


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