Urea, Ammonium Sulfate, SCU or Biosolids? Nitrogen Carriers Compared

Salt index here is on the conventional material basis with sodium nitrate set at 100. Higher means more osmotic pull on leaf tissue at the same weight of product, which is what burn actually is. Cost is shown as a multiple of urea, because the ratio between carriers holds up far better over time than any dollar figure.

The single most useful column is the second-to-last. A methylene urea or biosolids product applied at 45 degree soil in November is not a slow feed. It is no feed. The microbes that break those carriers down are barely active, and the nitrogen sits in the thatch until spring. That is why the late-autumn application should be soluble nitrogen and the September one should not.

These are representative shapes at roughly 70 degree soil with adequate moisture, not laboratory values from any specific product. Treat them as the profile of the carrier class. Cold soil stretches every coated and microbial row substantially and flattens the early weeks close to zero.

The week 1 column explains burn. Urea delivers roughly 60 percent of its nitrogen in seven days, so a 1.5 lb N application behaves like nearly a pound hitting the plant at once. Polymer-coated urea at the same rate delivers about a tenth of a pound in that first week. Same nitrogen, entirely different risk profile, and it is the coating doing all the work.

Salt index measures how much a fertilizer raises the osmotic pressure of the soil solution relative to sodium nitrate at 100. High osmotic pressure outside the root means water moves out of the plant rather than in. The blade dehydrates and turns straw-coloured, usually within 48 to 72 hours, and usually in stripes that match your spreader passes.

Two adjustments make the raw number honest. First, compare per pound of nitrogen, not per pound of product: ammonium sulfate looks tamer than urea at 69 versus 75, but it is only 21 percent nitrogen, so delivering the same pound of N means putting down more than twice the weight of salty material. Second, potash matters. Potassium chloride runs higher than urea on the same scale, which is exactly why high-potassium winterizer bags carry real burn risk despite a small first number.

Practical mitigations, in order of how much they help: water in with about a quarter inch immediately after application; never apply to wet foliage, because granules stick to blades and dissolve directly on leaf tissue; halve the rate and make two passes; and do not apply to drought-stressed turf at all, because a plant already short of water has no margin.

Coated products defer the salt as well as the nitrogen. That is the whole reason they carry a burn rating several bands below the urea inside them.

Sulfur-coated urea is a urea prill with a sulfur shell and usually a thin wax sealer. Water works through microscopic imperfections in the shell, dissolves the urea inside, and it seeps out. Because the imperfections vary between prills, release happens as a population effect: some prills open in week one, some in week eight. Handle the bag roughly and you crack coatings, which converts slow-release product into fast-release product with no warning. Do not drop the bag off the tailgate.

Polymer-coated urea uses a thin plastic membrane instead. Water vapour diffuses in, dissolves urea, and solution diffuses back out at a rate governed mostly by temperature. That temperature dependence is elegant, because the plant's demand is also temperature-driven, so the fertilizer releases faster exactly when the grass is growing faster. It is also why PCU is nearly inert in cold soil.

You will also see products with both coatings, sulfur under polymer, sold as a middle option on price and duration. They behave broadly like SCU with a longer tail.

Methylene urea is not coated at all. It is urea chemically reacted with formaldehyde into chains of varying length, and soil microbes cleave the chains to release nitrogen. Short chains go quickly, long chains take months. So it is genuinely slow-release, but the release mechanism is biological, which is why it stalls when the soil gets cold.

Everything microbial has a floor. Soil microbial activity drops steeply below about 55 degrees and is negligible below 45. Methylene urea, biosolids and feather meal all depend on that activity, so in a late-autumn application they behave like an expensive mulch.

Coated ureas do not need microbes but they do need temperature to drive diffusion. PCU below 50 degree soil releases very slowly. SCU is slightly less temperature-sensitive because the mechanism is more physical than diffusive, but it still slows markedly.

Urea and ammonium sulfate work in cold soil, because they simply dissolve. The plant's ability to take the nitrogen up drops off with soil temperature too, but cool-season roots remain meaningfully active down to about 40 degrees, which is the window the late feed exploits.

So the carrier decision at the end of the season is not really a preference. September gets the slow-release. November gets the urea. Reversing that wastes most of a bag.

Ammonium-based nitrogen acidifies soil as it nitrifies. The effect is real and cumulative, and the amount of lime needed to neutralise it differs a lot between carriers. Ammonium sulfate is by a wide margin the most acidifying common source per pound of nitrogen; urea and ammonium nitrate are roughly a third as acidifying; calcium nitrate is mildly basic.

On alkaline soil, which covers a great deal of the arid West and plenty of limestone country elsewhere, that acidification is a feature. Ammonium sulfate is the standard tool for nudging pH down while feeding, and it brings sulfur along, which those soils are often short of.

On soil already sitting at 5.5, running ammonium sulfate for several seasons is how people end up with a pH problem they did not know they were creating. Nutrient availability tightens below about 5.5, and it is a slow, quiet decline rather than an obvious failure.

Get a soil test before you commit to a carrier for the long run. The test is an input to this decision, and pH is the single value on it that most changes which bag you should be buying.

For granular lawn fertilizer with no pesticide component, the standard instruction is to keep pets and children off until the product has been watered in and the turf has dried. In practice that is a few hours, and 24 hours is the conservative figure most labels support. The label on the bag in your hand governs; nothing here overrides it.

The meaningful hazard is not contact with treated turf. It is a dog eating granules off the surface, and two carriers deserve specific mention. Biosolids-based products are organic, faintly meaty-smelling, and some dogs will actively seek them out and eat them off the lawn, which is a genuine gastrointestinal upset risk and, because those products carry 2 to 4 percent iron, an iron ingestion concern at volume. Iron sulfate granules used for colour carry the same iron concern in a more concentrated form.

Watering in solves most of it, because it dissolves the granules and removes the thing that can be eaten. Apply, water with a quarter inch, let it dry, then let the dog out.

If a dog does eat a quantity of granular fertilizer, that is a call to a vet with the bag in hand, not a judgement call made from a web page. Keep the bag. The guaranteed analysis panel is what the vet will want to see.

The guaranteed analysis panel is legally required and it is where the real information lives. Total nitrogen is broken into forms, and those line items are the carrier declaration.

Ammoniacal nitrogen means ammonium, so ammonium sulfate or ammonium nitrate, and it is immediately available. Urea nitrogen is straight urea unless another line qualifies it. Water insoluble nitrogen, abbreviated WIN, is the slow-release fraction, and it is the number to look for. Slowly available or other water soluble nitrogen usually indicates methylene ureas or a coated product.

Divide the slow-release nitrogen figure by total nitrogen and you have the fraction that will not hit the plant at once. Above about 50 percent, you can run a full pound of N in a single pass with reasonable confidence. Below about 25 percent, treat it as a soluble product and halve the rate.

That one division does more for your lawn than any brand comparison. It is the same move as the rate calculation, applied one layer deeper: the front of the bag tells you how much nitrogen; the panel tells you when you are going to get it.