The Basics and Acids of Composting

Quite commonly, pH is not a primary concern within a compost system; however, pH can impact microbial efficiency and compost quality. pH is a logarithmic scale from 0 to 14 that measures a substance’s acidity or alkalinity. A pH of 7 is considered neutral, while a pH below 7 is acidic, and above 7 is alkaline (also known as basic). The ideal pH for composting ranges from 6.0 to 8.0, with the optimal pH being closer to 7.0. A pH near neutral permits a greater diversity of microorganisms to thrive and break down organic matter. When a variety of organic material types (including a good balance of greens and browns) are added to an aerobic compost with ample oxygen, usually pH stays within its ideal range. However, there are instances where the pH of composts can be too acidic or too alkaline. This article discusses factors influencing compost pH and management options.

Factors influencing compost pH

 In general, the pH value changes throughout the composting process due to changes in chemical compositions as materials decompose. Initially, pH will fall below neutral (sometimes near a pH of 5) due to the formation of organic acids (e.g., carbonic, acetic, lactic acids and others). Later on, the pH will increase as microbes use these acids, and while proteins are being decomposed.  

What is added to the compost can also cause the pH to be out of the ideal range – as the saying goes, “you are what you eat.” For instance, abundant citrus peels, pineapple waste, pine needles, and/or oak leaves can lower compost pH. Conversely, incorporating a generous amount of wood ash or lime can increase the pH above 8.

A greater contributor to compost becoming too acidic is excess moisture. Anaerobic (oxygen-limited) conditions promote acetic and lactic acid production that lowers compost pH. When a compost has too much water and is very smelly, this indicates an acidic pH influences by the high concentration of organic acids produced by anaerobic decomposition.  

If an in-progress compost pH is too low (below 6.0), the breakdown of the organic material will be slower. Furthermore, composting in an acidic environment can promote the growth of harmful microorganisms that encourage plant diseases. On the other hand, if the pH is too high (above 8.0), the compost will lose nitrogen as it is released in the form of ammonia gas. A composted finished product that is too alkaline can also inhibit nutrient availability for plant growth. 

Determining compost pH

Whether there is an interest in monitoring compost pH for management purposes or science projects, there are tools available to measure pH. These tools include compatible soil test kits, electronic soil pH meters, pH strips, and/or sending compost samples to accredited laboratories for pH analysis. In addition, it is suggested that temperature and moisture be measured regularly along with pH. These three data types used together, provide a greater understanding of microbial activity and the composting environment.

Compost pH Management

Most often, compost pH can be corrected by promoting aerobic microbial activity. For instance, lowering moisture content (i.e., adding dry browns, such as shredded cardboard), adding oxygen through mixing, and using a variety of organic inputs. Another management option when correcting an anaerobic compost environment is to add some soil and/or mature compost to help regain beneficial aerobic microbes to the compost batch. 

Lowering compost pH (more acidic)

In some cases, having acidic compost is preferred. Purposefully acidic compost (pH 4 to 5) is called ericaceous compost. Flowering plants in the Ericaceae family (e.g., azaleas and rhododendrons) and other acid-loving plants (e.g., blueberries, highbush cranberry, and citrus) prefer ericaceous compost. Moreover, regularly adding neutral to slightly acidic compost to alkaline soils (soil pH greater than 7.5) will help lower the soil pH over time. In turn, lowering the pH of alkaline soils benefits crop growth by improving nutrient availability (specifically, phosphorus, boron, iron, copper, and zinc). Some management options to lower 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. Note that an in-progress compost should not exceed a pH below 6.0 as this can hinder decomposition. For situations where an acidic compost (pH 4 -5) is desired, it is better to add a sulphur amendment during the curing phase of the compost.

Increasing compost pH (more alkaline)

When a compost has a pH below 6 and is not preferred, management options exist to increase the pH. Seaweed, egg shells, bones, and shellfish remnants are organic wastes with alkaline pH. Wood ash and carbonates (dolomite and limestone) are also amendments used to increase compost pH. They also buffer substances; they resist pH changes when subsequent acidic substances are added and prevent the pH from becoming too low. Both limestone and wood ash are common pH buffers, though they are rarely necessary for aerobic decomposition and should be used with caution. Excessive use of wood ash and limestone can lead to an overly alkaline compost – impacting nitrogen content and nutrient availability. Compost with a neutral to slightly alkaline pH is beneficial when incorporated in acidic soils (e.g. soil pH below 6) – as the compost will help raise the soil pH over time. In general, soils with a pH between 6.5 – 7.5 are ideal for growing most plants, as this pH range supports plant nutrient availability, root growth, and microbial activity.

In summary

Under aerobic conditions, composting will likely be within a pH range of 6-8 without management intervention. However, management options exist to achieve a desired compost pH – whether for repairing composting conditions, creating ericaceous compost, or remediating soil. Monitoring pH, moisture content, and temperature will provide insight into the composting environment and determine the best uses for the finished compost product.