Why artificial grass smells, in plain terms

Urine is mostly water, salts and urea, and urea itself has very little smell. The process that creates the odor happens after the urine has already soaked into the infill: bacteria that live naturally in soil, sand and organic debris consume urea as a food source, and ammonia is a byproduct of that metabolic process. So the smell is not urine at all; it is the output of a bacterial colony that has set up shop wherever urine residue has accumulated over time.

Urine passes through turf fibers in seconds, which is why hosing the surface does almost nothing about a urine smell: by the time you are spraying the visible grass, the residue has already moved on to the infill and the backing underneath it. That is also where the bacteria responsible for the smell are living and where any effective treatment has to reach, rather than the fiber tips a casual rinse actually touches.

The bacteria that produce ammonia need water to stay metabolically active, the same way most bacteria do. In a dry stretch, they go dormant and ammonia production slows or stops, which is why a yard can seem to have no smell at all for a week and then smell strongly the morning after rain, or on a humid morning with no rain in sight. The residue never left; the bacteria simply went quiet and then woke back up once moisture returned, ready to keep working on the same residue as before, until the next dry stretch quiets them down again.

Hosing adds the moisture bacteria need without removing the urea they are feeding on. Fragrance sprays add a stronger smell on top without touching either the residue or the bacteria. Vinegar neutralizes a small amount of surface ammonia but does not reach what is in the infill. Once the actual chain, urea to bacteria to ammonia, is clear, it is obvious why each of those common fixes only ever works for a day or two before the smell returns.

A yard with no pets, or a light-use yard where waste is picked up immediately and the area gets rinsed regularly, simply never builds up enough urea for a meaningful bacterial colony to establish in the first place. The chemistry is the same everywhere; what differs is whether there is enough raw material and enough undisturbed time for bacteria to multiply. That is why the fix for most yards is not a stronger chemical, it is interrupting the buildup before a colony has the chance to get established.

A treatment that only removes existing residue leaves the bacteria already established in the infill free to work on whatever residue arrives next. A treatment that only kills bacteria without removing the residue they were feeding on leaves food behind for the next colony that moves in. Effective treatment removes the fuel and reduces the population producing ammonia from it, which is why a proper visit pairs an enzyme step with an antimicrobial step rather than relying on either one by itself.