Is Compost Acidic or Alkaline? Understanding Compost pH
Gardeners know that monitoring soil pH is crucial for healthy plants But what about the pH of compost? Is compost acidic or alkaline? The reality is that compost pH is dynamic and goes through changes during the composting process Understanding the factors that affect compost pH can help create optimized blends.
Initial Compost pH Trends Acidic
When composting first begins, the pH trends acidic. As organic waste breaks down, microorganisms produce acids that lower pH levels. Fresh compost from typical household materials like food scraps, grass clippings and leaves often starts off in the mildly acidic range of pH 5.5 to 6.5.
The initial acidity helps create ideal conditions for fungi and bacteria to thrive and decompose the organic matter. This is an important phase, as these microbes start transforming lignins, cellulose and other complex compounds into simpler organic forms.
What Affects Starting Compost pH?
The exact starting pH of fresh compost depends on:
-
Types of ingredients used – Acidic materials like pine needles, coffee grounds, citrus peels or maple leaves promote lower starting pH. More neutral or alkaline ingredients can raise it.
-
Particle size – Smaller particles have more surface area exposed to microbes, accelerating decomposition and acid production.
-
Moisture content – Proper moisture levels allow the decomposing microorganisms to metabolize efficiently and generate acidic byproducts. Too little moisture slows the process.
-
Presence of air – Exposing piles to oxygen aids aerobic bacteria that generate organic acids and lower pH in the initial stages.
While individual compost piles vary, most follow a similar acidic trend at the start when conditions are optimized for biological activity. But this acid phase is only temporary.
Transition to Neutral pH During Composting
As the easily digestible compounds get consumed, the compost enters a transition phase. The pH starts to climb back toward neutral levels around pH 7.0.
Several factors cause this rise:
-
Bacteria multiply – As decomposition progresses, bacterial populations explode. Different bacteria thrive at less acidic pH levels.
-
Fungi decrease – With food sources dwindling, fungal communities decline. Fungi are more prolific in acidic conditions.
-
Acids get metabolized – Microbes consume the organic acids they produced earlier as food sources disappear.
-
Ammonia released – Bacteria convert nitrogen into ammonia, which has an alkaline effect on compost.
-
Oxygen exposure – Turning and aerating compost piles raises redox potential, favoring bacteria over fungi.
During this dynamic phase, the pH can fluctuate between acidic and alkaline levels as microbial communities shift. But overall, compost becomes less acidic over time.
Mature Compost Is Neutral to Mildly Alkaline
In finished, mature compost, the pH settles into the neutral range of 6.5 to 7.5. Well-aged compost is a rich, dark brown color and has an earthy odor. The organic matter has largely stabilized at this point.
The main drivers of the neutral pH are:
-
Bacterial dominance – With organics broken down, bacteria prevail over fungi, favoring more neutral pH levels.
-
Remaining alkalinity – Ammonia and other alkaline byproducts from decomposition persist.
-
Lack of new acids – With limited fresh organics, microbes no longer generate new organic acids.
-
Oxidation – Exposure to air oxidizes compounds, using up acids.
The neutral nature of finished compost makes it beneficial for most garden plants. Unlike peat or pine bark, compost rarely acidifies soil significantly when added. Its balanced pH has a moderating effect, nudging both acidic and alkaline soils closer to neutral.
Testing and Managing Compost pH
To monitor the dynamic pH changes during composting, periodic testing is advised. Use a compost pH test kit to track pH from start to finish. Testing at different locations also checks for consistency.
While compost naturally trends neutral, the pH can be managed by:
-
Adding more acidic or alkaline ingredients to influence starting pH.
-
Turning piles to oxidize acids or exclude air to stimulate fungi.
-
Regulating moisture to favor acid-producing microbes or alkaline-releasing bacteria.
-
Allowing proper curing time for the pH to stabilize.
-
Amending at end with lime or sulfur to reach desired finished pH.
Understanding the factors that impact pH at each composting stage helps fine-tune piles for specific gardening needs. While compost alone won’t dramatically alter soil pH, monitoring and controlling compost pH provides added benefits.
The Bottom Line
How to Lower Compost pH
The pH value of compost tells you how acidic or basic it is. But what if you want it to be more of one or the other to change the soil? That’s where compost comes in. It can balance pH values. After it’s been turned into compost, the pH level will naturally go up in acidic soil and down in alkaline soil.
That said, sometimes you want to lower the pH of the compost before it is ready for use. The best way to do this is to add more acidic things to the compost as it breaks down, like oak leaves or pine needles. This type of compost is called ericaceous compost, loosely translated it means suitable for acid loving plants. You may also lower the pH of the compost after it is ready to use. When you add it into the soil, also add an amendment such as aluminum sulfate.
How to Test Compost pH
During composting, you may have noticed that the temperature varies. The pH will change, and not just at certain times or places in the compost pile. This is similar to how the temperature changes. In other words, you should get a pH reading from different parts of the pile when you do it.
You can use a soil test kit and follow the directions on the box to find out the pH of compost. If the compost is damp but not muddy, you can just use a pH indicator strip. You can also use an electronic soil meter to read the compost pH range.
Garden Hack! Find out if your soil is Acidic or Alkaline.
FAQ
Does compost make soil more acidic?
How do I know if my compost is acidic or alkaline?
What compost is alkaline?
Is manure acidic or alkaline?
Is compost acidic or alkaline?
Most compost has a pH between 6.5 and 8, which is only slightly alkaline or acidic and safe for most garden plants. Note that the longer you let the compost process, the more the ingredients will naturally become neutral. There’s also no need to panic if compost is very acidic during the early processing stages, as pH levels fluctuate at this time.
Is lime acidic or alkaline?
Lime, the fruit, contains carbonic acid, but it also has an alkaline reserve that helps balance the overall pH. Nonetheless, lime (the fruit) would be classified as slightly acidic on the pH scale. The pH of limes is around 2, which can still be considered acidic in nature (1 is most acidic). Alternatively, lime the mineral is alkaline.
What happens if compost is too alkaline?
As a result, the compost heap may become a bit smelly and slow to decompose. What happens if compost is too alkaline (high pH level)? High pH levels often hinder plants that do best in acidic soil, so if your compost pile is too alkaline, you will need to add organic matter to increase its acidity.
What happens if compost is acidic or anaerobic?
The acidic conditions are favorable for growth of fungi and breakdown of lignin and cellulose. As composting proceeds, the organic acids become neutralized, and mature compost generally has a pH between 6 and 8. If anaerobic conditions develop during composting, organic acids may accumulate rather than break down.