What the Three Numbers on a Lawn Fertilizer Bag Actually Buy You

Nitrogen is reported as elemental N. Phosphorus and potassium are not. They are reported as oxides, phosphate (P2O5) and potash (K2O), which is a nineteenth-century fertilizer-trade convention that never went away. So the middle number overstates the actual phosphorus in the bag, and the third overstates the actual potassium.

This matters exactly once: when a soil test report comes back in parts per million of elemental P and K and you need to work out how much of a bag to put down. Then you convert.

Find your bag. The pounds-per-pound-of-N column is the one you carry to the spreader; the burn band tells you whether you can run a full rate in one pass.

The 5-10-31 row is the one that catches people. It looks like a heavyweight because the third number is enormous, but it is a 5 percent nitrogen product. You would need 100 lbs of it to feed a 5,000 sq ft lawn a pound of N per thousand, which nobody does, which means most people applying winterizer are applying almost no nitrogen at all and thinking they have fed the lawn.

Turfgrass takes up more nitrogen than every other nutrient combined, and it loses it constantly. Clippings carry it off if you bag them. Leaching moves nitrate down past the root zone on sandy soil. Surface-applied urea volatilizes as ammonia if it sits warm and dry, and documented losses in that scenario commonly land in the 10 to 30 percent range.

So nitrogen is the recurring purchase, and the reason feeding is a schedule rather than a one-time event.

Watering in with about a quarter inch immediately after a granular application solves most of the volatilization problem, because it dissolves the granule and carries the nitrogen into the top inch of soil where the ammonia cannot escape. If rain is not coming inside 48 hours, do it yourself.

Ratio-wise, 4-1-2 and 3-1-2 are the two ratios worth knowing. They roughly track how turf removes nutrients over a season. Anything close to those proportions is a real turf product. A 1-1-1 balanced blend is a vegetable-garden ratio wearing a lawn label.

Phosphate on established turf is heavily regulated. A long list of states, including much of the Great Lakes basin and the Chesapeake watershed, restrict phosphorus in lawn fertilizer except for new seeding or where a soil test documents a deficiency. That is why so many mainstream bags now read something-zero-something.

The regulation exists because phosphorus runs off into surface water and feeds algae. It is also, agronomically, the easiest nutrient to already have enough of. Established lawns on soils with any history of fertilization are usually adequate or high in phosphorus, and adding more does nothing visible.

So the honest reading of a middle number above zero on a maintenance bag is: this is a general-purpose blend, not a turf product. Buy the zero.

The exception is real. New seeding and new sod genuinely benefit from phosphate in the root zone, because phosphorus barely moves in soil and young roots need it close. Most states carve out exactly that exception in the statute.

Potassium regulates water movement in and out of plant cells, and it shows up as improved cold hardiness, better drought tolerance, and stronger cell walls that resist disease. That is a genuine and well-supported set of effects, and it is why autumn bags lean on it.

What potassium does not do is make grass green. It produces no visible colour response, which makes it a hard sell and an easy one to overstate.

A sensible autumn approach: keep nitrogen as the main event, at the rate your species and soil temperature call for, and choose a carrier that happens to bring potassium along, something in the 24-0-8 or 21-7-14 shape. Buying a 5-10-31 as your entire autumn programme means you fed the lawn potassium and essentially skipped nitrogen.

Potash also carries salt. Potassium chloride, the cheapest and most common source, has a higher salt index per unit than urea does. High-potash winterizers earn their high burn band honestly.

A ratio is the analysis reduced to lowest terms. 16-4-8 and 24-6-12 are both 4-1-2. Extension recommendations are written as ratios because they are shopping instructions; bags are printed as analyses because they are legal guarantees.

To translate: divide all three numbers by the smallest non-zero one. 21-7-14 divided by 7 gives 3-1-2. 30-10-10 divided by 10 gives 3-1-1, which is a lot of phosphate for turf.

Once you can do that in your head at the shelf, the entire wall of bags collapses into about five real products with different price tags.

• ▸4-1-2 and 3-1-2: proper turf maintenance ratios.
• ▸Anything-0-anything: phosphate-free maintenance, the correct default for an established lawn.
• ▸1-1-1: garden blend, wrong ratio for turf, usually low analysis on top of it.
• ▸High third number with a small first number: winterizer or potassium supplement, not a feed.

Scores in the table above are not a verdict on brand quality. They rate how well the analysis serves as an autumn turf feed on four axes: release fit against the feeding window (35 points), burn headroom from salt index and safe single rate (25 points), delivered cost per actual pound of nitrogen (20 points), and how predictably the granule meters through a spreader (20 points).

That is why 24-0-4 outscores 30-10-10 despite carrying less nitrogen. The 30 percent product is more concentrated, but the phosphate is regulated in much of the country, unnecessary on established turf, and comes with a higher burn band from a soluble nitrogen load you have to halve anyway.

It is also why 10-10-10 sits at 42. It is not a bad fertilizer. It is a fertilizer for something other than a lawn.