There are a wide range of garden and landscaping plants which require soil with a higher acid content, or a lower pH, just as there is some soil that has high alkaline and needs to have the pH balanced so plants can thrive. Luckily, there are a number of commercially-available remediation methods that can lower the pH of soil. The problem is that not all of these commercially available methods are appropriate for organic gardening.
Options for Organically Lowering Soil pH
There are two commonly used methods for decreasing the pH of soil, each with a range of different amendment options. One of these methods is to utilize a compound that encourages soil bacteria to produce acids that, in turn, reduce the pH of the soil where the amendment is applied. The other common method is to directly apply organically-produced acids that would naturally occur as a result of bacterial metabolizing of the soil amendments.
Amendments that Encourage Acid Production in Soil Bacteria
- Elemental Sulfur, or "Soil Sulfur" - Sulfur is a common and very effective way to lower soil pH organically. When the sulfur is applied, naturally-occurring bacteria in the soil metabolize it and produce a byproduct called sulfuric acid. Using sulfur is considered to be a measure that should be taken when results don't need to be faster than with other common organic methods of lowering soil pH. To apply "soil sulfur" to your plants, you should:
- Measure the area you want to acidify.
- Measure the current pH of your soil using pH test strips.
- Match your soil acidity to the soil acidity you are trying to achieve-- this tells you how much sulfur to use per 100 square feet. The Perdue University Cooperative Extension offers a table to help with this process.
- Make sure you till the top six inches of top-soil before applying the sulfur.
- Sphagnum Peat - Peat is a naturally occurring moss that can be found in bogs and swamps, though it is also farmed commercially for use as a soil amendment. To use sphagnum peat to lower the pH of your soil:
- Measure the area to be acidified.
- Get 2.5 lbs of peat moss for each square yard you want to lower the pH in.
- Till the top six inches to one foot of topsoil.
- Mix in the peat.
- Organic Coffee Grounds - Coffee grounds are moderately to very high in acid, meaning they can have a very low pH. According to Purdue University, coffee grounds have roughly the same pH as sphagnum peat. This means that these grounds should be applied to your garden in roughly the same density and using the same steps as with sphagnum peat.
Directly-Applied Organic Acid: Humic Acid
Humic acid is a natural byproduct of the decomposition of organic materials. Any compost left to decompose for long enough will produce humic acid. Instead of waiting for organic material to break down and produce humic acid, gardeners can simply buy the humic acid on its own and use it to lower the pH of their soil very quickly.
The process for using humic acid will vary considerably depending on the types of plants you grow and the existing pH of your soil. It will also depend on the strength of humic acid you purchase. Whenever you buy humic acid from a reputable dealer, it will come with a Material Safety Data Sheet (MSDS) by law. This will tell you everything you need to know about specific application rates.
The overall trend toward more organic gardening practices has made interest in these pH amendment options skyrocket. As interest grows, so too will the amount of options available to organic gardeners everywhere. Utilizing the methods outlined here will get you on your way to having a banner year for your azaleas, blueberries and all of your other acid-loving plants.