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Alkaline compost

Posted by glib 5.5 (My Page) on
Tue, Apr 17, 12 at 14:28

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)?

Follow-Up Postings:

RE: Alkaline compost

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

RE: Alkaline compost

  • Posted by glib 5.5 (My Page) on
    Tue, Apr 17, 12 at 15:27

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).

RE: Alkaline compost

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?


RE: Alkaline compost

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?

RE: Alkaline compost

  • Posted by glib 5.5 (My Page) on
    Wed, Apr 18, 12 at 15:37

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.

RE: Alkaline compost

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.

RE: Alkaline compost

+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.

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