Compost too Alkaline from Fly Ash

by Tom Blake
(Beechmont, Australia)

I bought some compost which I found out had a pH of 8.4! I rang the business and they said it was probably due to fly ash being added to the compost. The compost felt and smelt alright.

What are the ramifications of this? I do have acidic soil with a pH of around 5.5 which I need to raise to 6.5. What can I do?

What if the vegie bed already has a pH of 6.4?

I live at 500m altitude in the sub-tropics.

Fly Ash = Alkaline Compost.

Fly Ash is one of the by products of combustion, usually of coal. It includes the fine particles that rise with the gases. The ash is usually collected with a kind of static electricity. All fly ash includes silicon dioxide and calcium oxide. It may also include trace amounts of other elements some of which could be toxic.

The studies I've read have found that compost mixed with fly ash is a decent fertilizer and soil conditioner. One set of tests showed that fly ash did not have a negative effect on the soil microbes - very good news. Other tests on corn crops showed that using a fly ash compost blend such as what you have was beneficial to the corn crop grown.

What About the pH?

Your question about the pH is a good one. Adjusting the soil pH is one of the major management tools for conventional growers using the chemical fertilizers. This is because the availability of nutrients tends to depend on the pH of your soil unless you have a healthy soil microorganism community.

Keep in mind that compost is a community of microorganisms rather than a bunch of chemical elements. As you use compost rather than regular fertilizers your soil community becomes more and more active and balanced. This community buffers the soil pH. The availability of nutrients start to be more a function of the types of microbes in your soil rather than the pH.

Applying the Compost

Apply the compost to the top of the soil like a mulch and let the worms till it into the soil. This will help balance the soil over the season. You can put up to 3 or 4 inches of compost on your soil. Just pull the compost back and plant seed and plants into the soil. Leave the compost a ways back from the plants and just let your soil microbes do the work of mixing everything together.

You have acidic soil anyways so this compost will modify it in a good way. The community of microbes you are introducing to your garden will find ways to work with the plants you decide to grow.

Good Luck Tom.
Regards Leslie