How To Lower pH In Soil Fast: what to add 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 level is exactly 7, such as that of pure water, it’s neutral. If your soil is too acidic you’ll need to learn how to lower pH in soil fast!

Different types of plants need different levels of soil pH to survive and thrive. And although soil pH varies from region to region and one garden to the next, 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, peppers – which need and thrive in slightly acidic conditions.

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.

Elemental sulfur is the best choice for lowering the pH of very dense soil, such as soils with a heavy clay component. Also by 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, 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.

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 houseplants or container vegetables.

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.

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.

Summary

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 bought online are not very accurate – I would recommend you to fill a bag with your soil and take it to your local agricultural shop for soil analysis.

When 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.

If you would like more information about soil, its profiles and horizons, take a look at my article “How Soil is Created” for more info. Thanks for reading and good luck- Richard.

Further reading:

https://hortnews.extension.iastate.edu/1994/4-6-1994/ph.html

https://www.extension.purdue.edu/extmedia/ho/ho-241-w.pdf

16 Comments
  1. This was helpful as my soil is alkaline and Ill try the 3 methods Here to try and get my ph to around 7 . Thanks for the info.

  2. I have garden . Check the PH in the soil and found that the PH level is around 8
    Should I lower it by adding vinegar water solution?

  3. I have 12 tomato plants already in the ground for 2 weeks. I’m a novice gardener and just learned they like acidic soils. I bought a tester and found my soil has a ph of 8 to 9. I will try the vinegar first and give it a week or so. I will let you know what happens. Many thanks.

  4. good article. will try vinegar. thanks

  5. Hi Richard,

    How soon after using vinegar or coffee, should soil pH be tested?

    Thanks,
    Maria

  6. How much vinegar/water would you apply to each azalea plant? how often should it be applied? if you use the vinegar how and when should you use iron sulphate to get the ph lowered permanetly? great article, thx

    • Hello Marcia, add 50ml of vinegar to 1 litre of water then test the soil the next day. Iron sulphate can be used if you have a larger area or if you are in less of a hurry to plant.

  7. Thank you Richard.

    Maria

  8. Hello Richard, I take care of a common area, an island in our cul-de-sac. For the winter I planted annual rye. In June I hope to seed it with Argentine Bahia. The 1700 Sq. ft. island has no watering system. So I chose the Bahia grass. The soil test done by UF in Gainesville showed a PH level of 7.7 down from a higher level a year ago. What is the best fertizer to use to lower the PH level before seeding in June? Ammonium? Iron Sulfate? How much would I need for the area?
    Harmen

    • Hello Harmen, look for a high ammonium based Nitrogen fertilizer to add to the lawn as this will lower the ph. The rates will be indicated on the bag.

      Richard.

  9. Can Iron Sulfate be safely used in an organic vegetable garden? Would it be better to use ammonium sulfate?

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