Drop-type spreaders provide fairly uniform distribution of granules across the swath width. The distribution is not affected much by product physical characteristics, weather conditions, etc. However, rotary spreaders are very sensitive to these variables, and patterns can be severely skewed if these variables are neglected. Therefore, always check the distribution pattern of granules when using rotary spreaders.
The best method to check the distribution pattern is to lay out a row of shallow cardboard boxes on a line perpendicular to the direction of travel (Fig. 19). Boxes with partitions will prevent chemical particles from rebounding. All boxes should have the same area and height (1 sq ft area, 1 or 2 inches tall). Space the boxes 1 foot apart, and make sure the row is 1.5 to 2 times the width of the anticipated effective swath.
To conduct the test, fill the spreader at least half full, set it at the label setting for rate and pattern, and make three passes in the same direction over the boxes. Be sure to turn the spreader on and obtain proper walking speed before crossing the boxes. Put the material from each box into a test tube, vial or small narrow bottle. When the bottles are placed side by side in order, a plot of the pattern is visible. The effective swath width is twice the distance out from the center of the spreader to the point where the rate is one-half the average rate at the center of the pattern. For example, as illustrated in Fig. 20, if the first three to four bottles from the center have material 4 inches deep and the bottles from the 6-foot positions (6 feet left of the spreader centerline or 6 feet right of the spreader centerline) have material 2 inches deep, the effective swath width is 12 feet. Therefore, you should be making passes 12 feet apart to provide a uniform coverage throughout the area covered.