The short answer
A high-nitrogen feed in spring, and a different autumn feed high in potassium and phosphate. Using the spring one in autumn produces soft growth that disease attacks.
- Spring, roughly April: a nitrogen-rich feed to drive leaf growth and colour.
- Summer: only if the lawn is watered. Feeding a dry lawn scorches it.
- Autumn, roughly September or October: an autumn-specific feed, low nitrogen, higher potassium and phosphate, to harden the plant and build roots for winter.
- Apply to damp grass and water it in if rain is not forecast within a day or two.
- Measure it. Over-application scorches in visible stripes that last months.
What will ruin it
Never use spring or summer feed in autumn. The soft leafy growth it produces has no time to harden and is exactly what fusarium and snow mould attack over winter.
Why it works
Nitrogen drives leaf growth, potassium builds cell wall strength and cold tolerance, and phosphate goes into roots. The seasonal products differ because the plant needs different things going into growth and going into dormancy.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.