Alkaline compost

glib(5.5)April 17, 2012

I thought I would pick this group's brains regarding 8.3 pH municipal compost. If you google "ann arbor compost analysis" the first link is a PDF with the results. It is not a typo, as in past years the results were 8.2 and 8.3 (on page 2).

I get one c.y. at a time (the first one is free) and use the compost for seed starting, mostly with success although IMHO tomatoes do not really thrive in it (I assume it is the pH and add some peat moss and Miracid in the mix). My questions:

1) what causes the pH to be so high? AFAIK the City uses only city waste, that is leaves, grass clippings, and chipped branches. Visually, it looks to be just that. Let me mention that I have an idea, but I will wait to see a few replies.

2) what do you think of the cadmium content (page 1)?

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toxcrusadr(Zone 6a - MO)

I will post the link below to save everyone else having to follow your Googling instructions.

That's the carbonate buffer pH they may be using limestone somewhere in the process. If there was manure involved, I'd say ag lime was added to keep down odors, but you didn't list manure as an ingredient.

The Cd level does not bother me, I'm looking at 17 as a safe level for residential soil so that's well below. If it's above local background however, there could nevertheless be a source of it getting in somewhere.

Here is a link that might be useful: Ann Arbor Results

    Bookmark   April 17, 2012 at 2:56PM
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Huh, sorry, but lately I have had trouble sending web sites to people, I figured I would do it this way. I am fairly sure there is no manure.

So you say the Ca content, which I notice is very high (2.93%!) must come from additives. I was wondering if it is due to the local soils (I had the soil analyzed at two sites, and got pH of 7.7 and 7.6, Ca 0.25% and 0.45%, and Mg 160-240 ppm), so that trees leave such minerals in the leaves in the Fall, reabsorbing only those that are scarce (one of the two sites had 7ppm P).

    Bookmark   April 17, 2012 at 3:27PM
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tsugajunkie z5 SE WI

Is the city composting on a limy soil (or crushed limestone, for that matter) and every time they turn the pile the front-end loader just scoops some more up and adds it in?


    Bookmark   April 17, 2012 at 6:24PM
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toxcrusadr(Zone 6a - MO)

Any chance they are including shredded drywall scrap in the mix? Our city composting operation does that. They accept clean (not painted, but new) drywall scrap and unpainted lumber scrap at the compost facility and it gets shredded and mixed in. Drywall is gypsum, calcium sulfate, which should not raise the pH but will profoundly affect Ca content.

I have results in front of me for a dozen bagged compost products, plus my home made, and our city product. Ca varies from a low of 0.6% in a commercial manure compost to a high of 11% in our city compost with drywall included. Mean was 4%. My home made stuff with yard waste and kitchen scraps was 9%. So one doesn't have to add anything to get Ca in the percent range.

Curious about the cadmium you mentioned, or was that a typo and you were talking about calcium all along?

    Bookmark   April 18, 2012 at 11:07AM
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Thanks for the replies. I feel I can discount the soil scraping hypothesis, the local soils are 7.6-7.7, and clay, which would show up in lumps in the compost. Also, the compost is 8.3, and adding even large amounts of soil to neutral compost will not change its pH to above 7.6-7.7.

About cadmium, no, I really was concerned about the cadmium content, which is larger than that of many micronutrients.

I called them, and they confirmed that their compost is only leaves and twigs. No drywall, no lime (I asked explicitly). Assuming it is the truth, the hypothesis that local organic matter is alkaline is the best explanation. Leaves may simply leave all Ca in the leaves, and just pull back P and other rarer micronutrients in the Fall. This seems to be at odds with the notion that one can use compost to moderate the pH.

    Bookmark   April 18, 2012 at 3:37PM
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Compost finishes out with a pH of very near neutral (7.0) unless something is added (CaCo3) to raise it Somewhere in that waste stream something is being added that is raising the pH, unless what they are selling is not finished compost. Even adding an alkaline soil would not raise the pH of finished compost into the pH 8 range and what I have seen in the past is that even when that is done the finished compost would test at near neutral.

1 Like    Bookmark   April 19, 2012 at 6:55AM
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toxcrusadr(Zone 6a - MO)

+1 on what kimmsr said.

Regarding cadmium, micronutrients vary all over the place depending on the compost inputs, local background, and they also vary by chemical element. Cd is actually a toxic metal, one of the 'RCRA 8' that is commonly monitored in waste streams, so that's probably why they are monitoring it. I don't think it has any nutritional value for plants. There will be a natural background in most soils and local plants. As long as it's not at toxic levels, the actual number is not that important in comparison to other nutrients.

    Bookmark   April 19, 2012 at 11:41AM
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Lower your pH in your compost naturally with sawdust or human urine, for real, national geographic did a study on fertilizing with wood ash and human urine, study said no greater results with anything else including, chemical fertilizer

    Bookmark   February 13, 2015 at 5:03PM
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Do you have a link to that study, plowboy101? I'd be interested in reading it.

My soil tested around 8, from memory, for pH. Maybe 8.5

I was a little aghast and hoped it was the cheap kit, but secretly I suspected that it was correct because my acid-lovers had been complaining for years. Yet, over the decades I've added grass clippings; leaves from deciduous trees; sheep, horse, rabbit and guineapig manure; the occasional branch. Disliked neighbours. So why isn't it slightly acidic?

The soil is a lovely clay-based one.

Two years ago I collected a huge pile of oak leaves and they have rotted nicely (finally). But I bet if I tested the soil it would still be the same pH as before. I tested a few sites, back then. I suppose it's time to do it again and see.

    Bookmark   March 9, 2015 at 4:11PM
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toxcrusadr(Zone 6a - MO)

Soil has a natural pH based on its mineral base, and it can be hard to change because it always wants to go back. Having said that, a real laboratory test would be better than a test kit. Much more reliable. Where are you located? pH 8-8.5 is right in the range of the bicarbonate buffer pH (about 8.3) which suggests a dry arid place with plenty of limestone.

    Bookmark   March 16, 2015 at 3:23PM
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Wow, I love your answer, toxcrusadr. And apparently you are spot on.

I just read my question and you response out loud to my husband and he said, "We are the limestone plain and you will not be able to change the pH without disolving the entire bedrock of Canberra."

I wish he had told me this 20 years ago.

    Bookmark   March 16, 2015 at 4:36PM
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toxcrusadr(Zone 6a - MO)


Don't despair though. Lots of compost will help things grow in spite of the pH. Focus on what you can do and don't worry about the pH.

    Bookmark   March 17, 2015 at 8:05AM
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just don't plant blueberries.

    Bookmark   March 17, 2015 at 9:10PM
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Ha ha. I tried a blueberry. Tried to get the pH down even further, but the soil wasn't having it. Tried to get the climate to change too, to a more mountainous, humid, serene one, and that didn't work either.

    Bookmark   March 19, 2015 at 11:43PM
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toxcrusadr(Zone 6a - MO)

You just need to find another spot on the earth's surface. Once you identify it, it's fairly easy to bring it to you. Just face in the direction of the desired climate, and run in place while the planet rotates beneath you. When you get to a nice spot, stop.


1 Like    Bookmark   March 20, 2015 at 8:16AM
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