Compost pH and content

5 years ago

last modified: 5 years ago

I can buy some compost in bulk, but I'm concerned that the pH may be too high for my soil. The compost pH is just under 8. My soil's pH was too low a year ago, so I added the amount of lime that the soil report recommended—some last summer and some this past winter. The soil here in central NC is mostly clay, and needs some lime from time to time. If I spread 1/2" of the compost over my lawn, will I throw the soil pH off too much?

Also, what percentage of organic matter should I look for in a compost product?

Featured Answer

toxcrusadr

5 years ago

About 10 years ago (how time flies!) I worked on a little project sampling commercial (mostly bagged) compost products available locally, plus the city's compost facility product, and my own homemade compost. Organic matter ranged from 7 to over 30% (dry weight basis). I was surprised it was so low, but you have to consider that the decomposition and shrinkage of the vegetative matter that goes in does not really remove much of the mineral content, but it does give off a lot of carbon dioxide and water as the organic matter decays. So ash content is much higher than in the input ingredients.

The most important factor is to compare different composts (if you can). I learned a couple basic rules of thumb:

1. The cheaper composts tend to have lower organic matter and nutrients, but most expensive is not necessarily the best.

2. Homemade compost using a variety of ingredients was the best in nearly every category. Mine was top of the pack at 33% OM. There's no one around to intentionally or sloppily put soil into the product.

3. A pound of organic matter cost anywhere from 7 cents (city compost purchased in bulk $12/ton) to $2.99 (fancy mushroom compost). The cheapest OM in a bagged product was Earthgro from Wal-Mart (other than homemade, which was free of course).

4. The best NPK content in a bagged product was Cotton Burr compost which you may not be able to get everywhere. It's a bit more expensive (middle of the pack) but it's what we use at my house when we need more than we produce.

5. Keep in mind all these products vary regionally so the best I can do is give general guidelines.

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Comments (7)

gardengal48 (PNW Z8/9)
5 years ago
A mature or finished compost will have a pH ranging somewhere between 6.0 to 8.0 - essentially neutral. So your reading is completely acceptable. You would need to spread feet - not inches - to have it make any impact on soil pH.

We might be having some confusion in terms........compost is ALL organic matter, 100%. Were you asking something different?
User
Original Author
5 years ago
That makes sense. Thank you! I tend to overthink things...

I'm probably just confused about the organic content percentage. I've seen a compost spec sheet that compared the percentages of organic matter and ash in the product. Maybe there's more in there than just compost. I'll ask the seller about it.
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The Lawn Medic
5 years ago
If your looking at a soil Test: Organic matter is the amount of decayed or humified organic material (crop residues, compost, manures, etc) in the soil. The higher the number on your soil test. What is Soil Organic Matter? Plant residues and living microbial biomass, Active soil organic matter also referred to as detritus, Stable soil organic matter, often referred to as humus.

Physical Benefits: Enhances aggregate stability, improving water infiltration and soil aeration, reducing runoff, Improves water holding capacity. Chemical Benefits: Increases the soil’s CEC or its ability to hold onto and supply over time essential nutrients such as calcium, magnesium and potassium, Improves the ability of a soil to resist pH change. Biological Benefits: Provides food for the living organisms in the soil, Enhances soil microbial biodiversity and activity which can help in the suppression of diseases and pests.

I also have a new YouTube channel check it out: https://www.youtube.com/channel/UCZWRXoXKb_2eE2mRYT95d6g

Maybe you will find a video that would help you with your lawn care
krnuttle
5 years ago
last modified: 5 years ago
Ash is caused by the inorganic material that remains in the organic (Compost). There are very few organic materials that have NO ash. All living things absorb nutrients from the soil ie Iron, Potassium, Sodium, and many trace elements. All of these things contribute to the amount of ash found. As the compost decomposes, the inorganic material will be reused by the plants growing in it.

https://www.astm.org/Standards/D2974

"1.3.2 Method B—The ash content and organic material of peat or organic soils is determined by igniting the air-dried then oven-dried specimen obtained from the water content determination in a furnace at 750 ± 38°C. This method is used when the peat or organic soil is being used as or evaluated for use as a fuel." Note Methods are the same C and F degrees.

There is another source of ash. Because of the nature of the way the raw material for compost is created, it can pick up organic materials such as sand gravel etc.

If there is a lot they could cause problems in your yard.

In most cases the ash from the first source will be small less that a percent. Ash from the second source could vary significantly. What level of ash are you finding?
User
Original Author
5 years ago
Thanks, everyone! I found a spec sheet for the compost: https://atlantic-mulch.com/wp-content/uploads/2015/02/01-01-21-Compost-Sample-STA-2.pdf. I ended up purchasing several cubic yards to spread over my lawn. The spec sheet said the organic matter was 33.4%, and the ash content was 66.6%. That's pretty close to what @toxcrusadr had in the homemade compost.
toxcrusadr
5 years ago
Interesting!

Glad you got the info you needed to get the job done. Looks like you got a good product there.