ph indicator with probe into soil

How To Lower pH In Soil Fast : what to add to lower pH and why

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 is exactly 7, such as that of pure water, it’s neutral. If your soil is too alkaline some plants won’t grow well.

Different types of plants need a certain pH to survive and thrive. Although soil pH varies from region to region, most plants need a pH between 6.5 and 7. But there are other plants, such as flowers like azalea, marigolds, or heather, and fruits and vegetables such as blueberries, potatoes, and peppers – which need and thrive in slightly acidic conditions..

Test your soil

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 baseline to start making amendments.

How to lower pH in soil

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 growers commonly use elemental suphur to decrease the pH of their soil; however, sulphur requires some time (6 months+) for the soil bacteria to convert it to sulphuric acid.

The speed of the conversion is dependent on the particle size of the sulphur, the temperature and the degree of moisture of the soil, and the amount of bacteria present.

Sulphur 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.

Sulphur is the best choice for lowering the pH of very dense soil, such as soils with a heavy clay component. Also adding organic materials such as manure to heavy clay soils will help it become more workable, and as a bonus, the manure will gradually lower the soil pH naturally. 

As it takes so long to act, sulphur is best introduced at the end of the last planting season.

How to lower pH in soil fast

What is the fastest way to reduce 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 sulphur but as they are fast-acting, you should add them to the soil in measured doses.

Coffee to lower soil pH

While it’s a well-known myth that coffee grounds are a quick fix for reducing pH, most of the organic acids in coffee are water-soluble and flush out into the brew. Used coffee grounds have a pH of around 6.8, which is so close to neutral that they won’t bring down pH much; however, they do add a little nitrogen, so they can help reduce pH over time, just like organic matter does with manure or compost.

Coffee grounds are almost neutral pH, the acidity is flushed out with the hot water.
Coffee grounds are almost neutral pH, the acidity is flushed out with the hot water.
Freshly ground or brewed coffee has an average pH of about 4.5, depending on the region in which it was grown. So if you need to drop soil pH more quickly, try watering your plants with leftover (cold) coffee that is diluted 50-50 with water. This method works especially well for smaller volumes of soil such as for garden soil, houseplants or container vegetables.

Vinegar to reduce soil pH

Vinegar is a kitchen staple because it has a wide variety of uses; it can be used as a condiment, to add flavour to cooked dishes, and even to clean sinks and counters when the cooking is done. This potent liquid is also useful to growers in small plots, and it can be used to naturally adjust the pH of the 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 growers can find organically-made vinegar.

Vinegar can be sprayed onto the soil or introduced through an irrigation system. A cup of vinegar mixed with a gallon of water is ideal for plants like azaleas and rhododendrons. Vinegar can be bought in bulk much cheaper than small bottles like this Heinz Multi-Purpose Vinegar (1 Gallon) from Amazon.

Aluminium sulphate

One of the quicker-acting acidic soil additives is aluminium sulphate; it produces acidity in the soil as soon as it dissolves, which is instantly as long as moisture is present. If you need to urgently lower the pH level of your soil, aluminium sulphate is a great choice.

Keep in mind that using too much additive can be harmful to your plants, so it’s best to verify the usage details based on the starting pH of your soil. Aluminium sulphate shouldn’t be used for large applications because it can lead to aluminium accumulation or even aluminium toxicity in the soil.

Have a look at this Aluminium Sulphate from Bonide Products to see if it works for you.

Sulphur-coated urea

A common ingredient in many slow-release commercial fertilisers, sulphur-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 effects within a week or two of being introduced.

If you were already planning to fertilise the soil as well as decrease its pH, simply choose a fertiliser that contains urea; the sulphur-coated urea content varies from one brand of fertiliser to another, so remember to consult the mixing instructions to determine the proper amount to use.

Iron sulphate to lower soil pH

A good choice for heavily compacted soil with high clay content, iron sulphate, and aluminium sulphate rely on a chemical reaction to acidify the soil, making it less dependent on temperature than elemental sulphur which relies on a slower biological reaction to begin any changes in soil ph.

Both Iron sulphate and Aluminium sulphate act faster than elemental sulphur and can significantly reduce pH in as little as three or four weeks; therefore, it can be used during the same season.

It may take more than 10 pounds of iron sulphate per 100 square feet of soil to reduce the pH level by one point; if you need to add more than that, it’s best to split the quantity into two applications, spaced a month or two apart. This will give the soil enough time to absorb the iron sulphate between applications.

Iron sulphate can leave rusty stains on clothes, so it’s best to wash any clothes that have come into contact with it separately to avoid damaging other items; they can also stain cement surfaces such as patios or footpaths.

If you have a smaller area of soil you would like to treat, take a look at this 4lb bag of Iron Sulphate from High Yield – It says it treats 1000 square feet which seems a lot for 4lbs – I would still apply one dose at a time and check your levels again afterwards.


The most important thing to do first is measure the soil pH. – I would recommend you buy your own soil pH meter or fill a bag with your soil and take it to your local agricultural shop for soil analysis.

If you’re not sure which pH meter to buy – I can only say avoid cheap meters – usually the more accurate meters are not the cheapest, the most accurate pH meter I have ever used is the Bluelab Soil pH Pen – which I wrote a short review about.

When you get your results back you can begin to amend the pH whichever way you choose. The coffee and vinegar methods would be okay 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 this 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.

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