Spreader Settings Conversion: Why the Number on the Bag Is Wrong for Your Hopper

Gate geometry. A rotary spreader meters through an adjustable opening above a spinning plate. Two hoppers with the same nominal setting can have openings that differ by 30 percent in area, because the linkage and the plate size differ.

Granule size and density. Fertilizer particle size is described by SGN, the Size Guide Number, which is the average particle diameter in millimetres times 100. Homeowner granular products commonly land around SGN 145 to 240. A coarse SGN 240 product falls through a given opening far more slowly than a fine SGN 145 one, and dusty organics bridge and stall.

You. Output per 1,000 sq ft is a function of pounds per minute divided by square feet per minute. Walk faster and you apply less. A tall person at a brisk pace can put down 25 percent less than a shorter person strolling, on the same setting, same day, same bag.

• ▸Bag settings are typically published for two or three spreader families and extrapolated for the rest.
• ▸Humidity changes flow. A bag that has sat in a damp shed meters slower and clumps at the gate.
• ▸A spreader with a worn gate cable will read one number and open to another. Check that the gate closes fully before you trust anything.

This table does one job: it gets you close enough that your first weighed pass lands in a usable range instead of dumping the bag. These are starting points, not application settings. Nobody should fertilize a lawn off this table without the catch test that follows it.

If your spreader is not on this list, find the closest match by scale span. A hopper with a 1 to 10 scale behaves broadly like other 1 to 10 scales; a 1 to 30 scale is finer-grained and needs a proportionally larger number. Everything past that is what the test pass is for.

Twelve minutes, a bathroom scale, and a tape measure. Do it once per product, write the number on the hopper in permanent marker, and never think about it again for that bag.

Mark a strip that is easy to multiply. A 20 ft by 50 ft rectangle is 1,000 sq ft and takes very little marking. Four landscape flags will do it.

Find your measured output, read across to the factor, apply it to whatever dial number you tested at. Target here is 4.8 lb of product per 1,000 sq ft, which is a 21-7-14 at one pound of nitrogen. Swap in your own target if you are running a different analysis; the factor column works the same way.

Output tracks the dial roughly linearly through the middle two-thirds of most scales, and stops behaving at the ends. If a correction pushes you below 2 or above 80 percent of the scale, the product is wrong for that spreader rather than the setting being wrong.

Once you have a verified setting, halve it and go over the lawn twice, the second pass at ninety degrees to the first. This is the single highest-value habit in home lawn feeding and it is free.

Rotary spreaders do not lay a flat rectangle of product. They lay a rough triangle, heavy in the middle and feathering to nothing at the edges. That is by design, and it is why proper overlap matters: the thin edge of one pass fills in under the thin edge of the next. Two half-rate passes at right angles average out both the pattern and any wobble in your walking line.

The alternative is the classic striped lawn, dark green ribbons alternating with pale ones, occasionally with scorched lines where two heavy centres overlapped. Once you see it you are stuck with it until it grows out.

Drop spreaders meter straight down through a full-width gate. Pattern accuracy is excellent, edges are razor-sharp, and there is zero throw onto the driveway. They also require exact wheel-track overlap or you get a pale stripe between every pass, and they are slow, because coverage width is the hopper width and nothing more.

Broadcast rotary spreaders throw wide and are forgiving of line wobble, which is why almost everyone owns one. The cost is control at edges. Product lands in beds, on paths, and into gutters unless you use the side shield or pull the gate closed early.

The practical answer for most lawns: one drop-spreader border pass around the perimeter, then broadcast the interior. Ten extra minutes, and it keeps nitrogen out of the storm drain, which in several states is a legal matter and not a courtesy.

Sweep or blow granules off hard surfaces back onto the turf before you water. Product on concrete is product going into a drain.

SGN is genuinely useful once you know it exists. It is the average particle diameter in millimetres multiplied by 100, so SGN 150 means 1.5 mm particles. Golf greens grade runs near SGN 100. Fairway grade sits around 150. Bagged homeowner product is usually coarser, often 200 to 250, because coarse granules throw further and produce fewer complaints about dust.

Coarse granules also mean fewer particles per square inch. At a low rate on short turf, a very coarse product can leave visible green polka dots where each granule landed. That is a particle count problem, not a rate problem, and the fix is a finer product or a higher rate, not more nitrogen.

Blended bags are the other trap. If a bag contains three different granule sizes, the spreader sorts them by throw distance and you get one nutrient concentrated in the middle of the pass and another at the edges. Uniform-particle blends spread uniformly. Ugly mixed blends never will, regardless of your dial number.

• ▸Masking tape on the hopper lid: product name, analysis, verified dial number, date.
• ▸One line per product. Most people run two or three bags a year, so the tape stays readable.
• ▸Recheck after any spreader repair, and after a season in a damp shed.
• ▸Rinse the hopper after every use and dry it. Fertilizer salts eat gate cables and steel frames faster than anything else in the garage.
• ▸If the gate no longer closes fully with the lever released, replace the cable before the next application. A gate that leaks at zero will empty the hopper on the driveway while you are turning around.