To lower soil pH in potted plants, you must first understand the dynamics of soil pH. Soil pH is a measure of how acidic or basic your soil solution is.
It is measured on a scale from 0 – 14 where 7 is considered neutral.
Most plants prefer slightly acid soils, so many soils are adjusted by using ground dolomitic limestone which contains magnesium as well as calcium and will therefore raise both the pH and the Mg levels in the soil.
If the desired level is 5.5 then half-strength dolomite lime would be used because at that strength it will only raise the pH without raising Mg levels too high, which can have negative side effects on some plants.
What you will need:
- A new plastic container (with clear sides) at least 12 inches in width and length.
- Potting soil that is correct for the type of plant you will be growing (many common potting soils are high in peat moss which increases acidity, i recommend Miracle-Gro Organic Choice Potting Mix).
- Water (use dechlorinated water if your municipal water has high chlorine levels).
- Time, patience, and love for your plants.
- I have only performed this on my succulents, so I cannot guarantee it will work with other types of potted plants or flowering/fruit-bearing plants. If you are willing to try this method please let me know how it works out.
- A sunny windowsill or some grow lights.
Step 1: Water your plant with dechlorinated water, wait 30 min to allow the soil to drink in the water.
Step 2: Dump out any excess water that is sitting at the bottom of your pot by tipping your potted plants upside down and dumping the water into a bowl (you can also put this dechlorinated water on your plants if they are looking thirsty).
After you dump out all excess water you should notice that there is about an inch of standing water at the bottom of your pot, this is normal. If your potted plant has more than an inch of standing water then go to step 3a (otherwise go to step 3b).
Step 3a: If you have already done the above and your potted plant has more than an inch of standing water at the bottom, take a coffee filter or paper towel and place it in your container (if your container is clear like mine, this is how much water should be left in your pot). Using a spoon, gently push down on the soil until all excess standing water goes into the coffee filter.
Step 3b: If you do not have more than an inch of standing water simply continue onto step 4.
Step 4: Fill your container with organic potting soil leaving about 1/4-1/2 inch empty space from the top (this will depend on how deep your plastic container is).
Step 5: Place your potted plant in its new container (be careful not to break the leaves!) and fill it with soil around your plant, try not to touch any of the roots.
Step 6: Water using only dechlorinated water, wait 30 min before you water again (if you are having trouble with watering then check out my article on why succulents need dechlorinated water or this one on how much water does a cactus need).
On average it takes me about 4 weeks for my soil pH to drop after I perform this method. If it has not dropped in 4 weeks then do not worry, keep doing the steps above and eventually, it will.
It is okay if there is still 1/4 inch of standing water at the bottom of your pot, this is normal. I have waited as long as 1 month to see a change. You should notice that your plant will be looking healthier and happier after you perform this step.
Step 7: Water with regular water every week or two depending on how hot it is in your house. If it gets really cold, then you might need to increase the watering frequency. Make sure to use dechlorinated water.
Dolomitic limestone should be added to the soil to lower the ph
If there are deficiencies in both magnesium and calcium, dolomitic limestone can be used to correct them. Dolomitic limestone contains magnesium as well as calcium so it will raise both levels thus correcting for a deficiency.
Soil with a higher pH level than desired can be treated by applying elemental sulfur, which lowers the pH but does not affect calcium or magnesium levels.
The application rate should be equivalent to one pound of actual (elemental) sulfur per 100 square feet of garden area or one-half pound per 25 square feet of greenhouse area.
If the soil is already deficient in either Mg or Ca then use horticultural gypsum at the rate of one tablespoon per gallon of soil and water it in well.
Potting soils are often acidic due to the high organic content so should be checked to see what their pH is before any amendments are added.
Ph levels for garden soils will depend on the needs of the plants being grown, if you need to lower your ph then amendments can be made but bear in mind that adding anything to your soil will change its nature slightly so be prepared for other factors such as drainage etc.
To measure your pH simply fill a container with some soil and add distilled water until the mixture becomes saturated (no dry bits remain) then stir thoroughly to get an even consistency throughout. Insert a ph testing strip into this solution stirring gently until all of it has been immersed.
Leave it for 4-5 minutes then carefully remove the strip and match it to the scale supplied on the container.
If you were unable to get a reading then wait an hour or two before checking again, how accurate you need to depend on what your soil is used for (e.g food growing) but generally, 10-15% of either side of the expected reading isn’t too bad if you are planting ornamentals rather than vegetables.
For potted plants use a good quality potting mix that will provide all the nutrients required by the plant in its lifetime.
If there is still doubt about whether there are deficiencies in magnesium or calcium then apply dolomitic lime at approximately 2 lbs per cubic yard (1kg per 4Lt) of soil and water it in well.
If a deficiency exists then after a couple of weeks check the levels again as dolomitic lime will slowly raise magnesium and calcium levels at first but only do this once, using it more than once can lead to other deficiencies.
If you need to lower your ph then use elemental sulfur or horticultural gypsum as stated above. The application rate will depend on the plant being grown, for most ornamental plants 1 tablespoon per gallon (60mL per 4Lt) is sufficient unless otherwise stated by the supplier/manufacturer.
After adding any amendments it is advisable to wait a few days before planting so that there can be no burning of roots from a high salt content etc., if you are adding dolomitic lime then it should be watered in well to avoid an alkaline burn.
There are many different factors that can affect the levels of calcium and magnesium within your soil but hopefully, this article will have given you a basic understanding of how ph, Ca, and Mg interact with each other as well as some simple methods for accurately measuring these vital nutrients.
There is an excellent book called ‘All you need to know about growing crops’ by George Rousseau which has extensive information on all aspects of gardening.
If you would like to read more on this subject then it is definitely worth looking out for.
Just bear in mind that there are many factors that can cause problems with plants even when everything appears to be done correctly so make sure you read up on all of them and take extra care not to make mistakes that others have already discovered.
The good news is that there are some simple steps that can help correct ph issues when growing potted plants:
1). Use container soil – Potting soil has additives such as composted bark which lower its overall pH level. This means potting soils will usually hold moisture better than regular garden soil and will be less affected by drought in a container.
2). Add compost – Adding organic materials, such as aged manure or compost, to the soil can help lower its pH level. Most gardeners use mushroom compost which contains a large amount of peat moss and can lower pH levels drastically.
In general, you should add two pounds of organic material for every square foot of potting soil in your container(s). You can also add more when repotting plants for added benefits.
3). Use fertilizers with acidic ingredients – Commercial fertilizers, such as Miracle Grow brand products, usually have an acidifying effect on the soil because they contain high amounts of nitrogen.
These products are designed to grow lush green lawns and not potted plants, but they can be used for house plants as well. Be careful not to over-fertilize though because too much nitrogen will burn the roots of your plants and kill them.
4). Use calcified water – Calcified water is a relatively new product that many gardeners use to help lower soil pH levels quickly.
Basically, calcified water is regular tap water combined with minerals from ancient seabeds which have been exposed to heat over time.
The minerals in calcified water have a neutralizing effect on soil and can lower pH levels by as much as 2 points within one hour of adding it to containers.
5). Plant alkaline-loving plants – Lastly, you can always just plant alkaline-loving plants such as palms, hostas, conifers, and cacti in your containers. These kinds of plants don’t mind alkaline soil nearly as much as their counterparts do.
You can use these methods without question on houseplants that you plan to keep inside; however, it’s important not to apply them to any outdoor container plants because of potential damage to nearby acid-loving plants.
The best method for outdoor container plantings is to amend the soil properly when planting or re-potting, allowing it time to stabilize after you’ve added the amendments.
Another word of warning; avoid using high amounts of peat moss (over 40%) which can actually make ph levels too acidic for most plants.
All things considered, there are certain types of plants that will always grow better in soil with a lower pH level.
Fortunately, there are several ways to achieve the right ph levels for most plants when growing potted plants.