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Will Compost Lower pH? How Compost Can Change Soil pH

A soil pH between 6 and 7 is most optimal for general plant growth. Due to this, many gardeners and farmers in dryer climates struggle to find ways to lower their soil pH to fall within these levels. Often, they are left wondering if compost will help lower their pH levels? 

As a general rule, compost does not lower soil pH. However, it does act as a buffer which causes the soil to function as if it has a lower pH. How you create the compost greatly affects its ability to buffer your soil levels though. 

Compost can change your soil’s pH to function at a lower level to promote optimal plant health. The climate in which you are growing and what goes into your pile will play a key role in how well your compost reaches those optimal soil levels. Read on to find out how to effectively use compost to help your plants thrive. 

Does Adding Compost Change Soil pH?

Remember in science class when you learned about buffers? They are essentially solutions that can neutralize small additions of acids or bases to maintain a specific pH level. Well, your compost acts as one to change soil pH. 

There are positively charged hydrogen ions present in compost, that when released into alkaline soil, lower the pH. Compost also provides many negatively charged attachment sites to bind hydrogen ions and raise the soil pH if the soil is more acidic.  

Since compost’s chemical makeup is so complex, it can act as a buffer against sudden pH changes by providing both positively charged ions and places for ions to attach. Due to this, it can very minimally change the pH but mostly acts as a buffer against drastic pH changes. 

What Is The pH of Compost?

Compost goes through a cycle of varying pH levels as it matures. In the early stages of compost, the pH is around 5.5 due to low-temperature bacteria breaking down organic matter and turning it into amino acids. As the compost matures, bacteria known as mesophiles take over and bring the pH up to a more neutral level, between 6 and 8. 

However, the mesophiles need oxygen to thrive. If you do not properly aerate the pile anaerobic bacteria take over and contribute to the buildup of more amino acids. As a result, the pH of the compost stays down instead of becoming more neutral. 

How To Control Your Compost’s pH Levels

You can control the pH of your compost by how much you aerate it, how long you let it mature, and what you put within it. 

As we just mentioned, mesophiles need well-aerated compost to exist and neutralize the pile. You can keep the pH of your compost down if you limit the number of times you aerate it.

Also, your compost will naturally have a lower pH if you decide to spread it onto the soil before it fully matures.

What you put into your compost plays a big role in the final pH of the mature compost as well. You can use the following materials to help lower your compost’s pH. 

  • Coffee grounds
  • Grass clippings
  • Pine needles 

These materials will not significantly lower the mature compost’s pH but can ensure it is more towards the acidic end of the mature pH range. 

Why You Don’t Want Your Compost to Be Too Acidic

Compost that is too acidic will develop an unpleasant odor and slow down the decomposition process. The pH can fall as low as 4.5 and cause the microbial activity to essentially stop. With no microbial activity leading to further decomposition and a pH below most plant’s optimal level of 6, the compost becomes worthless to you.  

To prevent this from happening, you should aerate your compost often and be sure to add carbon-rich ingredients such as:

  • Dry leaves
  • Woodchips
  • Hay

The ideal ratio of carbon-rich ingredients (those that make the compost more basic) to nitrogen-rich ingredients (those that make the compost more acidic) is about 30:1. So, even if you are looking to make your compost more acidic, you still need to add quite a bit more carbon-rich ingredients than nitrogen-rich ones. 

How To Know The pH Of Your Compost

Knowing the pH of your compost can help you to determine whether you need to add more carbon or nitrogen-rich materials to reach optimal levels for decomposition.

You can use a variety of tests to determine the pH but remember that your pile is not homogeneous. Therefore, you should test a variety of sites throughout the pile because they are likely to have varying pH levels, especially if you do not mix the pile often. 

A few pH tests you can use on your compost pile include:

If you plan to take pH readings, do so at regular intervals throughout the composting process.

That way you can keep track of the trend from acidic to more neutral and ensure it is happening at a steady pace. 

How To Know What Compost pH Level You Need

If you live in a dryer climate, your soil is likely on the basic side. If you live in a wetter climate, your soil is likely more acidic. Therefore, you may desire a more acidic compost in a dry climate, and a more basic one in a wet climate to achieve the optimal pH range for plant growth. 

You can also use the above testing methods to test the pH of the soil that you plan to add the compost to determine whether you need to buffer it higher or lower on the pH scale. 

Also, what plants you plan to grow can affect the pH levels (source) you need.

Plants that love more acidic soil include:

  • Strawberries
  • Azaleas
  • Blueberries

Whereas plants that love more basic soil include:

  • Beets
  • Hosta 
  • Lavender

Using Compost to Buffer Soil pH

Compost can be an excellent way to maintain a desirable pH level (source) in your field or garden. What you add to it, how well you aerate it, and when you decide to spread it onto your soil will greatly affect its pH levels. So, be sure to know what your desired pH levels are before making and spreading your compost. 

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