Be careful with coffee grounds, apparently

Hopefully this is helpful.  Being a garden newbie, I'm trying to learn as much as possible about these types of things, so I thought I'd share in case other newbies were looking for the same information on these forums.

Apparently spent coffee grounds aren't good to add directly to the garden:

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28884

https://www.sciencedirect.com/science/article/abs/pii/S1618866716300103?via%3Dihub

https://www.pubhort.org/ipps/60/120.htm

https://www.gardenmyths.com/coffee-grounds-in-garden/

https://www.gardenmyths.com/coffee-grounds-garden-safe/

I think that last link (gardenmyths.com) sums it up in a seemingly reasonable way:

"There is good evidence that amending soil with coffee grounds can negatively affect plants. So don’t do it.

Mulching with coffee grounds probably will not harm plants, but keep it away from seedlings and new small plants. To be on the safe side keep them out of the vegetable garden. Mulching landscapes should be OK, but don’t use more than an inch at a time.

The best option is to compost coffee grounds first. When mixed with other organic material they decompose in a couple of months and become harmless to plants. It can then be used anywhere in the garden."

And even adding spent coffee grounds to vermicompost can have negative effects on earthworms:
https://www.sciencedirect.com/science/article/abs/pii/S0960852411007589?via%3Dihub

Coffee grounds also have anti-bacterial properties, which appear to kill negative microbes, but might also kill beneficial microbes:
https://www.sciencedirect.com/science/article/pii/S2213422016300932

So it sounds like spent coffee grounds are not good to add directly to the garden, and only add them sparingly to compost (as a 'green', not a 'brown').

Is this a fair assessment, you SFG veterans out there?

goodtogrow wrote:Is this a fair assessment, you SFG veterans out there?

In a word, No.   Coffee grounds are an excellent addition to garden soil as well as compost.  There have been many tales about how coffee grounds will keep seeds from germinating, but I have never seen this in my beds. Sprinkling coffee grounds on top of the soil will attract nightcrawlers which will quickly pull the grounds down into the bed to consume them.  I have filled compost tumbler bins with nothing but Starbucks coffee grounds and pine pellets, and they have created some of the richest compost I have ever seen.

There is one downside to coffee grounds, however. If they are spread to thickly the will form a mat which is water repellent, and they will prevent the soil from being able absorb needed water. But, if they are lightly scattered on the soil, this will not be a problem.

Coffee grounds are an excellent source of nitrogen, potassium, and phosphorus, in addition to other micronutrients.

But, this is just my opinion, based on decades of use, not based on google searches. Your experience my be totally different.

Side Note: Mel's Mix does not require any additives or amendments, such as coffee grounds.

I have been using coffee grounds on my gardens and in my compost forever and I don’t have any issues as described!

Ah, well then maybe this is just one of those "you can't believe everything you read on the internet" types of things! 

They say using it in compost is basically safe, but has anyone done a side-by-side test in soil vs not using in soil (non-MM)?  Maybe it just delays or stunts the plants?  Or just an entirely made-up thing?

Thank you for your post and all those links! I made the same comment awhile back, relaying what I had heard on NPR, and was practically pilloried here for it! Apparently, when you're talking coffee, you're on sacred grounds.

It's a shame that the one article is behind a paywall. It seems to be the article upon which all the other blogs are based. If you grow plants in sand, then they may not grow well. If you grow plants in sand and add coffee grounds, they may still not grow well. If you do a study with only 5 plants, the results may be too small to be statistically important.

Oh. But this is interesting. The same white paper is cited by an article that says something rather different:

Conversion of spent coffee grounds into vermicompost

Apparently, when you're talking coffee, you're on sacred grounds.

Haha!

Yes, it seems there is a lot of conflicting info, doesn't it...  I wonder why that would be?  I'm assuming because it is either bad science or that it just isn't true?

goodtogrow wrote:  I'm assuming because it is either bad science or that it just isn't true?

In academia they say, "publish or perish". Which means that a lot of questionable stuff gets produced, but no one reads it so it doesn't matter. So some small school in Australia does a paper. Just by chance it gets picked up by media. It gets repeated over and over. Is it bad science? Hard to know without shelling out $25 to get a copy! Apparently none of the media were willing to do that either.

goodtogrow wrote:Yes, it seems there is a lot of conflicting info, doesn't it...  I wonder why that would be?

It is because of the explosion of social media. A person feels compelled to post something, whether they have any knowledge of it or not, and if a troll decides to add his/her words of wisdom, it is off and running. Meanwhile, there are very, very few scholarly articles on the subject, and certainly no peer reviewed ones.

Even those written by a university extension office most often just repeat what was posted in other unverified articles.  This one from the University of Arizona (https://extension.arizona.edu/using-coffee-grounds-garden), makes unverified and uncredited statements such as this: "In compost, limit coffee ground content to no more than 20% of the total compost volume – more than 30% has often been detrimental."   There is no reference to where that "fact" was obtained, and there is no data as to what "often been detrimental" means.

One of the best articles I have found (Best because it agrees with me?) is from the Oregon State University Extension Service:
https://extension.oregonstate.edu/gardening/soil-compost/coffee-grounds-composting

But, even that one from OSU did the same trick of mixing coffee grounds with soil and then planting lettuce leaves in it, which showed "poor rates of germination and stunted growth".  Of course it will!  The same will happen if you mix wood chips or sawdust in the potting soil and then plant in it, due to robbing the soil of nitrogen and other nutrients.  If, though, you use just potting soil, plant the lettuce leaves in it, and then sprinkle the coffee grounds on top of the soil you will find no reduction in germination and superior growth after germination.

Washington State University has a good PDF file on using coffee grounds which actually includes references to substantiate their statements.
http://pubs.cahnrs.wsu.edu/publications/wp-content/uploads/sites/2/publications/fs207e.pdf

As I leave my soap box, I'll post this simple chart of the analysis of Starbucks Coffee Grounds, with the page it came from below the picture.

https://davesgarden.com/guides/articles/printstory.php?rid=1501&bn=%2Farticles%2Fview%2F1501

Well, thank you all for indulging me and for those excellent explanations and links, OhioGardener.  I am genuinely just seeking to learn here, so I greatly value and appreciate the information and your input.

You've convinced me.  I'll add coffee grounds as a green to my compost along with those pine bedding pellets you mentioned, OG.  I need a lot more mass in my compost, and these two ingredients should help a lot.  Thanks again!

goodtogrow wrote: I need a lot more mass in my compost, and these two ingredients should help a lot.

A 40# bag of Starbucks coffee grounds gives a lot of mass! And, the coffee filters in the bag provide some of the browns.