By Jim McCausland, Sunset senior garden writer

When I learned that Starbucks was giving away coffee grounds as free garden mulch, I was skeptical: after coffee had the essence steamed or soaked out of it, what possible nutritive value could it have for garden soil? I was sure it would be zilch. So, with shameful disregard for my expense account, I sent a pound of used grounds to Soil & Plant Lab in Bellevue, WA, for a $250 analysis.

To my surprise, they found that coffee grounds make good mulch: its pH was 6.2, and to quote the report, "the availabilities of phosphorus, potassium, magnesium and copper are each sufficiently high that there will be a very positive impact on improving availabilities of these elements where the coffee grounds are used as a mineral soil amendment."

Further, though available nitrogen was low (about a tenth of a percent), total actual nitrogen was high, so that "over the long term the coffee grounds will act like a slow release fertilizer providing long-term nitrogen input which can then be utilized by plants."

Regarding coffee's affect on soil structure, Soil & Plant Lab reported that in "amending mineral soils up to 35% by volume, coffee grounds will improve soil structure over the short-term and over the long-term."

So bring on the coffee grounds. It may keep the worms awake at night, but your plants will love it.