Composting is one of the best things you can do for your garden. By recycling yard and food waste into nutrient-rich humus, you can boost your soil health, retain moisture, and reduce your reliance on chemical fertilizers. But not all compost is created equal. The pH level of your compost pile – whether it turns out acidic or alkaline – will determine how useful it is in your garden. In this article, we’ll take an in-depth look at compost pH to help you understand how it develops and how to control it.
What is Compost pH?
pH is a measure of how acidic or alkaline a substance is. It ranges from 0 (very acidic) to 14 (very alkaline) with 7 being neutral. Most plants prefer a slightly acidic soil in the 6.0-7.0 range. Compost pH works the same way. The microbes that break down organic matter in your compost pile produce acids as they decompose the materials. This generally results in a pH around 5.5-6.5 in finished compost – mildly acidic and perfect for most gardens. However depending on what goes into your compost and how it is managed, the pH can veer out of this ideal range.
Why Compost pH Matters
If your compost ends up too acidic or alkaline, it can negatively affect your plants when you go to use it Very acidic compost can actually burn plants when applied directly in large amounts And extremely alkaline compost can keep beneficial micronutrients bound up in insoluble forms, making them unavailable to plant roots.
Getting your compost pH within the optimal range means it will safely break down in the soil and provide nutrition to your plants. So testing and adjusting the pH of your compost is an important step before using it in your garden.
How to Test Your Compost pH
Testing compost pH is quick and easy with a soil pH tester. Here are some tips for getting an accurate reading:
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Use a quality electronic pH meter or test strips designed for soil testing. Home pH tests for liquids won’t give accurate results.
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Take samples from a few different spots in the compost pile or bin since pH can vary.
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Use finished, cured compost. Don’t test unfinished materials.
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Follow the product testing instructions carefully for best results.
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Compare the results to the ideal compost pH range (5.5-6.5).
How Compost pH Changes During Decomposition
Compost starts off slightly alkaline as microbes break down simple sugars and proteins. As decomposition continues, it becomes more acidic. The final, cured compost rests in the ideal slightly acidic pH range. However, depending on the balance of materials in your pile, the pH can swing too far in either direction at different stages. Knowing how pH fluctuates can help you manage your compost more effectively.
What Makes Compost Too Acidic?
Certain materials and composting methods can lead to very acidic conditions. Here are some of the top causes of overly acidic compost:
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Excessive nitrogen-rich greens (grass clippings, food waste, manure) without enough carbon-rich browns.
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Too much coffee grounds or pine needles. These are very acidic.
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Anaerobic decomposition from compacted piles or closed containers. Lack of oxygen causes acid-producing bacteria to thrive.
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Adding acidic fertilizers like ammonium sulfate or aluminum sulfate. Don’t put chemical additives in active compost piles.
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Overwatering the compost pile leads to anaerobic conditions.
What Makes Compost Too Alkaline?
On the other end of the spectrum, some circumstances can make compost too alkaline. Some potential causes include:
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Not enough nitrogen-rich greens to balance out carbon-rich browns.
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Wood ash raises pH. Limit ash to small amounts only.
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Lack of water prevents full decomposition and pH drop.
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Lime or crushed eggshells will raise pH over time. Don’t add until compost is finished.
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Urine and herbivore manure (like cow) tend to be alkaline.
Adjusting Your Compost pH
If your compost pH is outside the ideal 5.5-6.5 range, you can tweak it by balancing out the “brown” and “green” materials. Here are some simple adjustments to make:
If your compost is too acidic:
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Mix in browns like dry leaves, straw, or wood chips.
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Let the pile dry out a bit between waterings.
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Turn the pile to introduce more oxygen.
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Avoid adding more acidic greens or fertilizers.
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Add a small amount of wood ash or lime after compost is cured.
If your compost is too alkaline:
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Add more nitrogen-rich greens like grass clippings or food scraps.
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Limit ash and bonemeal additions.
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Check moisture and water pile if needed.
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Add acidic materials like pine needles or coffee grounds in moderation.
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Use acidic fertilizers only after compost is finished.
With a little testing and tweaking during the composting process, you can bring your pH into the optimal range. Patience and proper curing also helps finished compost mellow towards a neutral, beneficial pH.
Using Compost with Extreme pH
If your compost just doesn’t seem to reach an ideal pH level, don’t despair. You can still use it to improve your soil over time. Here are some options:
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Mix acidic compost into alkaline soils to help neutralize pH.
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Likewise, blend alkaline compost into acidic soils. Opposites attract!
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Screen compost to remove large particles, then work it into soil instead of top-dressing.
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Limit very acidic or alkaline compost to just 10-20% of total soil volume.
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Allow compost to further break down and mellow after application.
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Test soil pH frequently and amend as needed. The compost will gradually adjust the pH.
With proper testing, troubleshooting, and application methods, even compost outside the ideal range can safely improve the fertility and structure of your garden soil. Getting the basics of compost pH under your belt is an important step on the road to gardening success!
pH measurement of compost
“Compost just happens” and it is not necessary for the gardener or recycler to know how or why. Action on the composter’s part to control the pH during composting is not normally necessary provided the process remains aerobic; you can just add the Greens and Browns (See Compost C: N Ratio) to the bin and leave them until they are turned into compost. However a greater understanding of the process can add to the enjoyment of composting and regularly and checking the pH during can enable the composter to monitor the process of decomposition. The pH levels in a compost bin will vary throughout the aerobic composition process as the microorganisms act on the material, it will also vary in different parts of the bin unless the material is regularly mixed. If anaerobic decomposition occurs and the contents start to turn black, wet and smelly, it will become more acidic due to the production of organic acids.
pH is the measure of the acidity or alkalinity or hydrogen ion activity of a soil or compost (on a logarithmic scale). from 0 to 14 with a pH or 7 being considered “neutral”. A pH change of one unit on the scale means a 10-fold increase or decrease of acidity. A pH of less than seven means that the substance is acidic while a value of above seven that it is alkaline. More information on this subject is a published by Washington State University on the “Acidic and Alkaline pH Levels in a Compost Bin” and while I have drawn on its contents in this page I would recommend that those interested in the pH changes during the composting process follow the link for more information. http://whatcom.wsu.edu/ag/compost/fundamentals/biology_acid_alkalinity.htm
pH changes in compost
Most finished composts are about neutral pH within a range from 6 – 8. However, the initial pH of home compost, being made with the usual mix of vegetable trimmings, food waste and other household compostable material, is likely to be more acidic perhaps between 5.0 and 7.0. If a significant proportion of the material being composted is wood shavings or peat moss, perhaps from chicken bedding, the pH may be as low as pH 4.5.
If the has been collected and stored to be added with other waste in a hot composting system and has started decomposing anaerobically e.g. in a heap of grass clippings, the anaerobic organisms will have produced acids lowering in the pH.
The initial pH of composts will be more alkaline, with a pH as high as 8.5, where the material contains a significant proportion of manure or wood ash.
Compost microorganisms grow and multiply within the pH range of 5.5 to 8. Organic acids are formed in the initial stages of the aerobic decomposition process and these acidic conditions favour the growth of fungi and the breakdown of lignin and cellulose. This means that if the initial pH of the material was six or seven it may fall due to the formation of these acids during the initial stage of the decomposition.
Where the initial pH is 5.0 or 5.5 the pH remains relatively constant during this period.
It is during these initial slightly acid conditions that the activity of composting microorganism causes the compost temperature to increase rapidly during the first few days and with it, the pH begins to rise, as the organic acids become neutralized giving mature compost generally a pH of between 6 and 8.
In addition to the initial microbial activity, compost can also become acidic when there is insufficient oxygen and the microbe activity becomes anaerobic. This may be due to having too many nitrogen rich greens in the bin e.g. grass cuttings, or lack of aeration.
Traditionally lime was incorporated into the compost heap and while lime will act as a buffer, keeping the pH from becoming too acidic or too alkaline. Adding lime is not necessary provided aerobic decomposition is maintained and even then, it is no longer favoured by many composters. Indeed, there is a view that the addition of lime may have an adverse effect because it causes nitrogen to be lost to the atmosphere as ammonia gas reducing the nitrogen content of the compost. Aerating or remixing the anaerobic material while including more “Browns” should return an anaerobic the mixture to aerobic decomposition
The compost pH will become neutral or alkaline as it matures pH values of less than six and above eight may indicate a problem with the composting process or the source materials. If you use maturation bin and have been monitoring the pH during the composting process continue to take readings until completion of the maturation process.
Garden Hack! Find out if your soil is Acidic or Alkaline.
FAQ
Does compost make soil more acidic?
What compost is alkaline?
Traditionally made with well-rotted stable manure, mushroom compost is now generally made using composted straw. Mushroom compost often contains chalk and is alkaline in nature.
What is the pH of compost in the kitchen?
Most finished composts are about neutral pH within a range from 6 – 8. However, the initial pH of home compost, being made with the usual mix of vegetable trimmings, food waste and other household compostable material, is likely to be more acidic perhaps between 5.0 and 7.0.
What to add to compost to reduce acidity?
… a compost’s pH are adding acidic organic inputs (e.g., green pine needles, beech and oak leaves, citrus peels, and peat moss) or adding a sulphur amendment