In chemistry, pH is a scale used to specify how acidic or alkaline a substance is. Acidic solutions have a lower pH, while alkaline – or basic – solutions have a higher pH. If the pH level is exactly 7, such as that of pure water, it’s neutral.
Once you’ve decided what to plant, or decided which plant may benefit from lower pH it’s best to verify the soil’s current pH level before making any adjustments. This will give you the base line to start making amendments from.
How to lower pH in soil
In order to correct alkaline soil, you will typically need to introduce a source of acid. You can add compost, manure or organic soil amendments like alfalfa meal to increase the nitrogen level of the soil which will also gradually decrease the pH.
Organic gardeners commonly use elemental sulfur to decrease the pH level of their soil; however, sulfur requires some time (6 months+) for the soil bacteria to convert it to sulfuric acid.
The speed of the conversion is dependent on the particle size of the sulfur, the temperature and the degree of moisture of the soil and the amount of bacteria present.
Sulfur will only work during the warmest months of summer when bacterial activity peaks. It can therefore take up to several months for this method to decrease the soil pH value.
As it takes so long to act, element sulfur is best introduced at the end of the last planting season.
How to lower pH in soil fast
What is the fastest way to lower pH in soil? – While there are methods that will make the soil more acidic very quickly, their results may vary, and in some cases, they may over-correct the soil pH and do more harm than good. The following methods will lower the soil pH quicker than elemental sulphur but as they are fast acting, you should add them to the soil in measured doses.
Using Coffee to lower soil pH
While it’s a well-known myth that coffee grounds are a quick fix for lowering soil pH, in fact most of the organic acids in coffee are water-soluble, and flush out into the brew. Used coffee grounds have a pH around 6.8, which is so close to neutral that they won’t lower pH much; however, they do add a little nitrogen, so they can help reduce pH over time, just like manure or compost.
How to lower pH in soil with vinegar
Vinegar is a kitchen staple, because it has a wide variety of uses; it can be used as a condiment, to add flavor to cooked dishes, and even to clean sinks and counters when the cooking is done. This potent liquid is also useful to gardeners, and it can be used to naturally adjust the pH level of soil without the need for harsh, commercially manufactured products.
Vinegar is a diluted, liquid form of acetic acid, and depending on what the vinegar is made from and how it’s processed, it may also contain other things, like traces of vitamins and minerals. The average pH of commercially manufactured white vinegar, like that sold in supermarkets, is 2.4, making it highly acidic. Organic gardeners can find an organically-made vinegar.
Vinegar can be sprayed onto the soil or introduced through an irrigation system. A cup of vinegar mixed into a gallon of water is ideal for plants like azaleas and rhododendrons.
Aluminum sulfate to lower soil pH
One of the quicker-acting acidic soil additives is aluminum sulfate; it produces acidity in the soil as soon as it dissolves, which is basically instantly as long as moisture is present. If you need to urgently lower the pH level of your soil, aluminum sulfate is a great choice.
Keep in mind that using too much additive can be harmful for your plants, so it’s best to verify the usage details based on the starting pH of your soil. Aluminum sulfate shouldn’t be used for large applications because it can lead to aluminum accumulation or even aluminum toxicity in the soil.
Mix around 5lbs of aluminium sulphate around the base of the plant you want the soil ph lowered to reduce the pH by around 1 unit. Always check the dosage on your label before application. I have provided a link to the Aluminium Sulphate on Amazon here. [Full disclosure: If you buy a product on Amazon through one of my links i may receive a small commission at no extra cost to you. This helps keep the lights on. Thanks!]
Sulfur-coated urea to lower soil pH
A common ingredient in many slow-release commercial fertilizers, sulfur-coated urea is a fairly quick-acting soil additive. It can lower the pH level of the soil considerably over time, yet will produce some effect within a week or two of being introduced.
If you were already planning to fertilize the soil as well as decreasing its pH, simply choose a fertilizer that contains urea; the sulfur-coated urea content does vary from one brand of fertilizer to another, so remember to consult the mixing instructions to determine the proper amount to use.
Iron sulfate to lower soil pH
A good choice for heavily compacted soil with a high clay content, iron sulfate and aluminium sulphate rely on a chemical reaction to create acidity in the planting beds, making it less dependent on temperature conditions than elemental sulfur which relies on a slower biological reaction to begin any changes in soil ph.
Both Iron sulfate and Aluminium sulphate act faster than elemental sulfur and can significantly reduce pH in as little as three or four weeks; therefore, it can be used during the same season you decide to plant acid-loving plants.
It may take more than 10 pounds of iron sulfate per 100 square feet of soil to reduce the pH level by one; if you do need to add more than that, it’s best to split the quantity into two applications that are spaced a month or two apart. This will give the soil enough time to absorb the iron sulfate between applications.
Iron sulfate can leave rusty stains on clothes, so it’s best to wash any clothes that have come into contact with them separately to avoid damaging other items; they can also stain cement surfaces such as patios or sidewalks. Have a look at what other users had to say about Iron Sulphate on Amazon.com [Full disclosure: If you buy a product on Amazon through one of my links i may receive a small commission at no extra cost to you. This helps keep the lights on. Thanks!]
In summary i would say that the most important thing to do first is to measure the soil pH. (Many home user soil test kits are not very accurate – this soil analysis kit is tested in a proper lab – you just fill the bag with your soil and post it off). After you get your results back you can begin to amend the pH by whichever way you choose. The coffee and vinegar methods would be ok in small potted areas but not really realistic for large areas.
To lower the pH in larger areas i would use elemental sulphur if i wasn’t in a rush- but usually you do want the benefit within that growing season. In which case I would choose Iron Sulphate as my first choice (this will help to reduce aluminium toxicity in the soil) and if i couldn’t get that then my second choice would be Aluminium Sulphate. Thanks for reading and good luck- Richard.