Wetting Agents
Fix dry patches fast. If water beads on the surface, races down cracks, or the lawn goes patchy while the sprinklers are clearly running, the problem is usually the soil repelling water rather than a shortage of it. A wetting agent breaks that repellency down so moisture spreads sideways and soaks into the root zone instead of running off.
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When a wetting agent is the right answer
There is a ten-minute test that settles it. Push a bottomless tin into the surface, pour in 25 mm of water and time how long it takes to disappear. Minutes is healthy. Half an hour with water still sitting there means it is not getting into the profile at all.
Water-repellent soil is most common on sandy profiles, on the strips that dry out first, and on any lawn going into a hot summer. It is also frequently confused with two other problems that produce the same complaint: compaction, where the pore space has been crushed out of the soil, and thatch, where a mat of dead runners holds water above the soil and evaporates it. A wetting agent fixes none of the other two, and coring a lawn that needed a wetting agent is a wasted weekend. Our guides to hydrophobic soils and compacted soil set out how to tell them apart.
The wider point is that a wetting agent makes the rest of your program work. Fertiliser you cannot water in is fertiliser the roots never reach, so infiltration is upstream of everything else you apply — there is more on getting watering right in The Water Bible.