Coffee grounds (UCG), clay and fertility

idaho_gardenerMay 18, 2009

I have some experiences with coffee grounds and clay that should be shared.

For over a year, I stopped at the local Starbucks and picked up about 3 gallons of used coffee grounds (UCG). Having read the Sunset Magazine analysis of the nutritional values and the recommendations there and on this board, I assumed that it would be good to dump lots of coffee grounds in my vegetable garden beds.

I estimate that I had applied about 5-8 cubic yards (yes, really) to various garden beds, including berry beds and vegetable beds, as well as to the lawn. Two of the vegetable beds got covered with at least two inches.

I subsequently found research that showed that caffeine inhibits root growth in a dose dependent rate.

Here are my observations so far. My soil is alkaline clay, pH of 7.8. I use lots of homemade compost, some agricultural sulfur pellets, and bone meal as soil supplements.

Legumes don't germinate well in soil with lots of raw coffee grounds in them, despite being pre-soaked with vitamin C and inoculated. Peas and beans did not do well despite. Pumpkin, watermelon, and muskmelon likewise did not grow at all.

Corn, tomatoes, peppers, radish and carrot, potatoes all seemed to tolerate the coffee grounds better than the more tender plants. Some sunflower plants were allowed to grow and they reached a height of 12' or so, so they liked the coffee grounds.

This spring, while turning some compost into the vegetable beds, I found that the clay soil had been stained nearly black to a depth of about 6-8". In some places I found many young worms living in the stained soil. The clay soil in my yard gets hard as rock when it is dry, but the stained soil was friable and porous.

I have not added raw UCG to my beds this year. Again, the peas are not germinating well. Corn and potatoes are sprouting well, and the tomatoes and peppers are growing well so far.

I'll have to remember to update this later in the year.

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I've seen a lot of people post here that you should not put UCG down in a layer more than 1/4 to 1/2 inch thick. If they dry out (and in the intermountain west, we know they will) they will cake over and cause problems.

I've used lots of UCG in the garden and on the lawn and have never seen a detrimental effect. I don't put them on too thickly, though, and for the garden, I usually compost them first, which seems to get rid of the caking effect. The only time I've put them out in a layer thicker than 1/2 inch was when I was making a lasagna bed, alternating layers of UCG and leaves. That was done in the fall and they stayed wet through the fall winter and early spring due to rain and snow, so by planting time, it was mostly compost anyway.

    Bookmark   May 18, 2009 at 9:39PM
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Coffee grounds can be a very good addition to soil as part of the total organic matter mix, but they should not be the only organic matter mixed in or used as mulch.

    Bookmark   May 19, 2009 at 8:26AM
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dchall_san_antonio(8 San Antonio)

5-8 cubic yards is two dump trucks full. That is enough to cover 1,300 square feet to depth of 2 inches. How big an area do you have?

Coffee grounds are a food source for the microbes living in your soil. They produce plant food when they are well fed. When the plants do not need all the plant food being produced, the microbes create an extraordinarily stable super food molecule called humic acid. The stain that you noticed is likely to be humic acid. That material is what causes healthy soil to look dark brown. Humic acid is a storage type of molecule like fat in animals.

    Bookmark   May 19, 2009 at 1:38PM
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Karchita(WA Z8)

I am not at all surprised that your vegetables didn't do well with a two inch mulch of UCGs. I haven't heard of anyone having success using them the way that you did. And I am also not surprised that your soil improved, which should help the veggie crop this year.

I do use a lot of UCGs in my garden. I have found that the best ways to use them are:
*as a light mulch (1/4 -1/2" deep) around nitrogen loving plants like hostas, heucheras and ornamental grasses especially when applied early in the season,
*as a top-dressing on lawns (also 1/4 -1/2" deep),
*as a green in the compost bin. Wet grounds are also good in a worm bin, in limited amounts.

I consider them more of a fertilizer than an amendment and they work extremely well for me in the ways that I use them. Plus it's really hard to beat the price. ;-)

    Bookmark   May 20, 2009 at 2:43AM
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the_virginian(Zone 7 NoVA)

I have only had excellent results with using UCG from Starbucks. Worms love them and my palm trees like a top dressing of before I mulch them in the Spring and it seems to green them up after winter when soil temps are low.

    Bookmark   May 20, 2009 at 10:12AM
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terran(zone10/Sunset20 CA)

Sure it wasn't the Sulphur pellets causing part of the problem? Elemental Sulphur is an antibiotic. How much is "some"?

If your soil needs additional sulphur it should be added in the sulfate form.


    Bookmark   May 20, 2009 at 2:37PM
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The sulfur is probably is help her alkaline soil which she said was 7.8. That combined with too many UCGs explains it to me. Who knew too much of a good thing could be bad?

    Bookmark   May 20, 2009 at 3:05PM
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