Avoid Right-angle Patterns with Rotary Spreaders

Figure 1. Normal back-and-forth spreader operating mode.

Figure 2. Right-angle operating mode.

Figure 3. Half-width operating mode.

Some spreader experts and some spreader and granular material operating instructions advise you to go over your lawn twice when spreading fertilizer or granular products, with the two trips over the lawn made at right angles to each other.
Don’t do it! The experts are wrong; two passes at right angles will not result in uniform application; it will just change a striped pattern into a checkerboard pattern.

The Problem
In some cases, a rotary fertilizer spreader will deliver a good pattern that is fairly symmetrical from side to side. In that situation, a reasonably wide swath width can usually be selected that will give acceptable pattern uniformity. Unfortunately, most homeowner rotary spreaders will not provide a symmetrical pattern with most granular products.
In most cases, a spreader will throw more material to one side than to the other (Figure 1). When you push the spreader back and forth over the lawn, you will be overlapping the right side of one pattern onto the right side of the next pattern, and the left side of one pattern onto the left side of the next pattern. Thus, if the left side is heavier than the right, normal back-and-forth overlapping will tend to exaggerate the problem. The result will be alternate heavy and light stripes on your turf.

The Wrong Solution
Many so-called experts recommend that you use the normal width, set your spreader to apply a half rate, and then go over the lawn twice, once running back-and-forth north-south and then a second time running back-and-forth east-west (at right angles) (Figure 2).

Research at the LSU AgCenter has proven that this right-angle method of pattern correction is not effective. Using this method will often give slightly better average uniformity, but it will not have any effect on the extreme high and low rates in an uneven pattern. All the right-angle method will do is change high and low stripes into a diagonal checkerboard pattern of high and low areas. Instead of a striped lawn, this method will give you a plaid lawn.

The Correct Solution
A much better approach that requires no more effort than the right-angle method is to set your spreader to apply a half rate and then reduce your swath width in half (Figure 3). This method is sometimes incorporated in the setting and width recommendations on granular product labels – especially those developed at the spreader test facility run by the LSU AgCenter. Using a half swath width results in a double overlap that moves the heavy stripes over onto the light stripes and vice versa, resulting in significantly better pattern uniformity in nearly all cases.

When Do You Need This?
You don’t need to use the half-width, half-rate method routinely – only when there is a problem. As noted, some label recommendations will incorporate this method so you may not even be aware you are doing it. If, however, the label recommendation is for a wide swath, you might consider using the method under two circumstances: (1) if you have had problems with uniformity with that product or spreader in the past, or (2) if it is important to you that this application be uniform and provide uniform greening and/or control. If you want to ensure uniformity, consider using this half-width, half-rate method.

There is one caveat to using the half-width, half-rate method, and the same caveat occurs with the right-angle method: you have to know the setting for a half-rate application. Using half the numerical setting on the spreader won’t do it! The rate scale on a spreader is never linear! Sometimes the product label will give a half-rate setting. If not, you have no way of knowing what setting to use. You might try using a setting that is numerically about ¾ of the normal setting, but that is only a guess. There is no way to be sure without testing the delivery rate.

12/6/2004 11:41:32 PM
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