Most professional applicators use rotary spreaders. Rotary spreaders are not only faster than drop spreaders, but are more forgiving of small errors in swath width.
 
Many spreader models on the market claim to be professional quality, but many are basically homeowner models with larger wheels, frames, and hoppers. If you want to do professional-quality work, whether as a commercial applicator or on your own lawn, you need a true professional-quality spreader.

Multiple Ports
Most homeowner spreaders have only one metering port. One port is adequate for a low-cost homeowner spreader, but it will not give adequate uniformity over a wide swath with multiple materials. It is unlikely that any single-port spreader will ever give professional results over a wide swath with multiple materials. One manufacturer does make a good "semiprofessional" spreader that has one port and a pattern slide that changes the drop point of granules on the impeller (Figure 1). It is capable of good uniformity with intermediate pattern widths.
 
On the other hand, just having multiple ports is no guarantee of professional quality. Some very low-quality spreaders do have multiple ports. You can dependably reject most single-port spreaders (except the one with a pattern slide), but you must then look further before selecting a multiple-port spreader. A single port will normally result in a triangular pattern that often is skewed to one side or the other. A three-port design (Figure 2) will normally yield a pattern with three peaks that approximates a trapezoid.

Pattern Adjustment
A spreader must be able to apply a wide range of products from low-density pesticides on ground corn cob carrier granules up to high-density, large-granule fertilizer materials. One distribution setup just won’t do it! Any true professional-quality spreader will have some means of adjusting the distribution pattern to accommodate different products. There are two common means of pattern adjustment on professional walk-behind rotary spreaders. The simplest method is to partially close off a metering port (Figure 2), reducing the amount of material thrown to the corresponding portion of the pattern. This can work well, but will affect delivery rate since the port opening is changed. Note that not all spreaders with a provision for shutting off ports are precise.

The second and more modern method uses a spiral cone-shaped deflector between the ports and the impeller to both determine the drop area of granules onto the impeller by rotating the cone and to spread the flow from three ports into a fairly uniform arc of material (Figure 3). Spreading the material into a wide arc eliminates the three peaks and can provide a more uniform trapezoidal pattern.

Calibration
Professional-quality spreaders should provide a means of recalibration by the operator. It is unrealistic to expect a spreader to remain calibrated through months or years of hard professional use (or even through shipping and setup), so the better manufacturers provide some means of checking the calibration relationship between port openings and the rate scale and a simple means of correcting that calibration relationship as needed.

Other Features
Professional-quality spreaders may offer other features such as stainless steel construction and remote shut-off of one side (for work along sidewalks). While desirable, these features by themselves are not indicative of spreader pattern quality.

There is more to spreader quality than shiny stainless steel, a big hopper or gadgets. The most critical factors are consistent metering and uniform application. Knowing what to look for can help you select a true professional-quality spreader.