'Fraid there isn't one. The usual soil test kits are unreliable about nutrient status mostly because they are so one-dimensional, and the alternatives are extremely expensive. In my opinion it's best to fork out for a soil sc...

Probably a pressure wash will sort it out for the meantime and in the future if repeated once or twice a year. Don't put ridges in it, it makes things worse (in my opinion) as gubbins collects in the troughs and the ridges become wette...