Fertilizer Spreaders - Metal or Plastic

Richard L. Parish  |  12/7/2004 9:05:06 PM

Figure 1. Drop spreader with steel hopper.

Figure 2. Drop spreader with plastic hopper.

Figure 3. Note thickness of hopper bottom on plastic drop spreader.

A few years ago, all high-quality, precision home lawn spreaders had steel hoppers (Figure 1); the models available with plastic hoppers had inconsistent metering and uneven distribution patterns.
This situation has changed. Now, most home lawn spreaders have plastic hoppers, and some perform well (Figure 2).

Metal Hoppers
Steel components can be manufactured with closer tolerances than can plastic components, thus it is easier to obtain uniform metering with steel components. Probably the most important advantage of steel hoppers is that a steel hopper bottom can be thin. A typical steel hopper may be about 0.03 inch thick, whereas some plastic hoppers are as thick as 0.25 inch (Figure 3).
The two major disadvantages of metal hoppers are higher cost and corrosion. Unless the hopper is made of expensive stainless steel, it will rust. Some manufacturers use an excellent powder coating process to protect steel hoppers, but eventually the paint gets chipped somewhere and rust starts. Fertilizer is very corrosive, so a steel hopper will eventually rust.

Plastic Hoppers
Plastic spreaders are more corrosion resistant, although even plastic spreaders have some steel components and are subject to rust. Plastic spreaders can be as strong as steel spreaders; the plastic components just have to be thicker to obtain equal strength. Plastic is easy to form into complex shapes, so designers have more freedom to be innovative with plastic. Many different plastic resins and processes can be used for hoppers, although injection-molded polyethylene and polystyrene, and polyethylene structural foam, are the most common.

Hopper Thickness
Why does hopper thickness make a difference? Isn’t a thicker hopper stronger? The problem comes when we consider the vertical component of the opening on drop-type spreaders. The size of the metering port opening is controlled by a shut-off-bar that moves forward and backward along the ports. With a thick plastic hopper, the actual vertical opening changes when the hopper is rotated about the axle.
The effect of this is that a tall operator will tend to hold the handle high, thus rotating the hopper forward. This will rotate the port and the vertical opening will become larger, increasing the delivery rate. A short operator will hold the handle lower, thus the hopper will be rotated back and the vertical opening will be smaller, resulting in a lower delivery rate.
This is not just a theory; testing of homeowner spreaders has demonstrated that spreaders with plastic hoppers can have a rate increase of more than 40 percent when the handle is raised only 3 inches and can have a rate decrease of more than 60 percent when the handle is lowered only 3 inches. With a plastic drop spreader, it is critical to hold the handle at a height that will keep the hopper level.

Precision Performance
A lawn spreader should be a precision tool. For granular fertilizers, herbicides, insecticides and fungicides to be effective, they must be applied (1) at the correct rate and (2) uniformly over the turf. To accomplish this, a drop-type spreader must have metering ports that are precisely sized and spaced.
Even more important, the shut-off-bar must be very, very straight in both directions; the port openings on the left side of the spreader must be the same as the openings on the right side, and the distance between the shut-off-bar and the hopper bottom must be the same all the way across. If the shut-off-bar is bowed or warped in either direction, some ports will deliver more than others. Metering is somewhat less of a problem with rotary spreaders since they typically have only one (sometimes two or three) metering ports.
Nevertheless, the size, shape and location of the metering ports must correlate correctly with the impeller to obtain a decent pattern from a rotary spreader.

To summarize, some high-quality plastic spreaders have precision equivalent to the better homeowner metal spreaders, but with plastic spreaders it is more critical that you hold the handle at a height that keeps the spreader level. It is also important to select a quality plastic spreader.

There is a tremendous difference in performance among brands and models. In the past, there were some excellent metal drop spreaders and some metal junk on the market; now there are some fairly good plastic drop spreaders and some very poor plastic junk on the market.

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