The spacing interval between trays can be changed, depending on swath width. The spreader is adjusted for a given rate of application and operated at normal field speed in a direction perpendicular to the line of trays while straddling the middle tray. Material collected in each tray is then poured into the corresponding test tube in the rack. If the spread pattern is uniform across the swath, as on the left side of Figure 3, and the proper swath width is used, the material will be deposited uniformly on the land. The effective swath width can be found by locating the point on the right and left side of the swath where the tubes are filled to about one-half the height found in the center tubes.
Fertilizers vary in moisture content and in size and particle density.
These variations will cause slight errors in the indicated rates of application, but this method provides a close approximation. Where application rates are more than 1,100 lbs per acre, larger test tubes are used with the same collecting trays. The larger test tubes are suggested for evaluating spread patterns for rates of application between 1,000 and 6,000 lbs per acre, such as in the application of lime. The spread pattern is determined by collecting the material in trays and pouring it into test tubes. Material overlap, as shown, results when swath spacing equals the effective swath width.