Soil temp has little to do with the effectiveness of kelp on the leaves. Anything that runs off to the soil will help the soil. I would do it if I used kelp.

You do realize you are feeding the microbes on the grass blades and not the plant itself, right? Recent research has found up to 20 different layers of microbes living on the outside of plant tissues formerly thought to be too exposed to UV from the sun to support life. It is these creatures that do the good for the plant. Chiefly they seem to promote the plant's "immune" system and ability to ward off insects and disease.