Greener Thumb
Posts: 970
Joined: Thu Sep 09, 2010 4:50 pm
Location: MO

Results rolling in from the first batch of samples, and it's not too impressive. I'll be posting more detail later, but I'm sure you're all dying to know, so, the 5 inexpensive retail products looked like this:

Organic matter (loss on ignition) 3.2 to 15.7%, average 8.4%
Water 28 to 52%, average 38%
Everything else (?????), by subtraction, average 54%.

Now, really good garden soil is about 10% OM. I would have thought compost would be much higher than that, although the mineral fraction will be higher than in uncomposted vegetation after being concentrated in the compost.

I found results online for our city landfill compost. It's 15-19% OM, which still seems low. They do use drywall scrap, so there is going to be significant mineral content there, but the gypsum is actually beneficial for the clay soil here, so they don't get dinged for high mineral content. At least you know what it is, and it's a waste reduction approach.

I am sending another batch of samples including some high dollar products - I got a bag of cotton burr compost that seems very fluffy, and some more expensive mushroom and cow manure from nurseries (egad, 10 bucks a bag! Still pays to make your own). And, my own backyard stuff from last winter's batch. This way there will be something to compare the low quality products with.

Thinking about a 1-pg fact sheet to put out at our city compost demo trailer at the Earth Day celebration.
Last edited by toxcrusadr on Fri Apr 01, 2011 2:40 pm, edited 1 time in total.

Super Green Thumb
Posts: 7500
Joined: Tue May 06, 2008 7:02 pm
Location: El Cerrito, CA

Thanks for the update; will be looking forward to more info as it becomes available!

Great idea on the handout for Earth Day. Will you have results that soon?

Cynthia H.
Sunset Zone 17, USDA Zone 9

Greener Thumb
Posts: 970
Joined: Thu Sep 09, 2010 4:50 pm
Location: MO

It only takes a couple days for the lab to email me the results, so the hardest part will be deciding what to write and what it should look like.

Greener Thumb
Posts: 728
Joined: Wed Sep 24, 2008 1:15 am
Location: Ventura County, CA, Sunset 23

10 percent rule ???

toxcrusadr wrote:
Now, really good garden soil is about 10% OM.
I've heard there's no point to more than 5%.

Anyone want to explain how much and what for and maybe where they got that number from would be helpful to the edumacation of this dweeb.

thanks in advance

User avatar
Full Member
Posts: 33
Joined: Tue Apr 26, 2011 11:41 am
Location: Texas


This is a great project! I can't wait to hear what the results of your homegrown compost will be.

By the way, when you say that the "Everything Else" category averages out to be 54%, what do you think falls into that category? I'm not sure I would even have a guess at what could make that up. :?:

Greener Thumb
Posts: 970
Joined: Thu Sep 09, 2010 4:50 pm
Location: MO

Just got the comprehensive analysis on 12 samples this morning - not only moisture and OM but also NPK, Ca/Mg/Fe etc., and CN ratio. I'll post some stuff when I have digested it.

swickstrum: the 'other material' category is all mineral matter or 'ash' content, including all minerals incorporated in the original vegetation and any soil or other mineral matter mixed into the ingredients or the compost. They say manure is 'salty' which I assume means that salts are concentrated in cattle poo. Of course urine is salty and that is probably mixed with the manure. Our city landfill mixes shredded scrap drywall into their ingredients, which boosts up the mineral content considerably. But they disclose that information freely, and the other guys do not.

I was surprised at the mineral content of my own compost, since I don't add any soil. But it was lower than all the commercial products.

More coming soon!

Return to “Composting Forum”