Hydrangeas with blue flowers are gorgeous, and they also happen to have a soft spot for acidic soils. Many soils are naturally acidic, a condition that comes down to several factors. However, if your soil is too alkaline for hydrangeas, you’ll need to take steps to reduce the overall pH.
To make soil more acidic for hydrangeas, use soil amendments like coffee grounds, vinegar, sulfur, and peat moss. Aluminum sulfate and nitrogen-rich fertilizers can also be effective.
In this article, I’ll be going over all the ways you can make your soil more acidic for hydrangeas. I’ll also discuss the science behind soil pH, testing your soil, and some other ways you can ensure your hydrangeas are happy and healthy.
1. Add Coffee Grounds To Your Soil
This may surprise you, but coffee grounds can be an excellent soil amendment. They naturally affect acidity by exposing aluminum substrates in the soil to your hydrangeas’ roots – making them bloom big, blue, and beautiful.
Coffee grounds can also strengthen microbial communities within the soil, so they become more active and break down organic matter quicker and more efficiently.
The extra organic matter will help heavy soils (especially clay-based soils) become lighter, improving aeration in the soil and moisture retention.
When adding coffee grounds to your soil, follow the steps below:
- Make sure the coffee grounds are in a fine powder. You can also try used coffee grounds, but you’ll need to ensure that any debris has been removed.
- Sprinkle the grounds with a few teaspoons of water to moisten them up. This will help them mix better with the soil and promote moisture retention in the topsoil.
- Mix them into the first few inches of the soil. You can do this by hand or use a small hand tiller to make sure they’re evenly distributed. You need to spread them throughout the topsoil since leaving them on the top layer won’t do much for your hydrangeas.
2. Use Vinegar To Alter the pH
Vinegar is a naturally acidic substance, so it makes sense that it will alter the acidity of your hydrangeas’ soil. If you use too much of it, you could kill the roots — so only use a little bit at a time if you choose to try this method. A few spritzes should do the trick.
Vinegar has properties that naturally lower the pH of the soil, but it doesn’t last as long as other methods. It’s a good method to try if you don’t have any other option.
Additionally, you’ll likely find that vinegar has the magical ability to destroy weeds and other pathogens in the soil (in small doses), which is an excellent way to ensure your soil stays free and clear of alien plants.
Some types of vinegar contain ingredients other than acetic acid, such as essential minerals, so this will also be helpful for your hydrangeas in small doses.
3. Add Sulfur to the Soil
Using sulfur to alter soil pH is one of the most popular ways to maintain soil acidity. Elemental sulfur is cost-effective and environmentally friendly. It works by using microorganisms in the soil to initiate chemical changes that lead to the creation of sulfuric acid – which acidifies the soil.
If you’re not sure where to buy sulfur to acidify your hydrangeas’ soil, you can purchase it at any local nursery or online.
I’ve written an extensive guide about using soil acidifiers such as sulfur and how long they take to work. Don’t miss it: How Long Does Soil Acidifier Take to Work?
4. Acidify Your Soil With Peat Moss
Peat moss is popular among many gardeners, and there’s a good reason for it. Peat moss comprises decayed organic matter such as animal and plant material. It is naturally very light, so it makes for an excellent addition to heavy clay-based soils.
It also acts as a soil conditioner and will lower the pH naturally. It lasts for a very long time and creates an environment with better aeration and drainage channels.
The idea behind adding peat moss to your soil is simple: the more you put in the soil, the more acidic it will become.
Before adding peat moss, however, ensure it has absorbed enough water. You’ll need to drench it slowly in a few cups of water and squeeze out the excess before distributing it among the top few layers of soil.
Downsides of Pea Moss
However, if you’re planning on using peat moss for your hydrangeas’ soil, you must be mindful of the downsides. Peat moss doesn’t have any nutrients by itself, so it can’t be used as a natural fertilizer. You’ll need to add some organic matter, like compost or well-rotted manure.
Additionally, the process of harvesting peat moss is not considered environmentally friendly. Peat bogs worldwide have undergone extensive cultivation to the point that they’re no longer viable habitats for the wildlife who call them their home.
As a result of these processes, peat bogs are now releasing carbon dioxide into the atmosphere, irreversibly damaging the earth.
Therefore, if you’re planning to use peat moss in your garden, it’s best to be mindful of how much you use and how it is sourced.
5. Aluminum Sulfate Will Change the Soil’s pH
Aluminum sulfate in the soil acts quickly to neutralize very alkaline soils and, over time, will make them more acidic. Your hydrangeas will benefit from this, and the natural acidity, as a result, will make the flowers more blue and beautiful than ever.
The best way to use aluminum sulfate is to dilute it with water. This creates a chemical reaction resulting in sulfuric acid forming, which will naturally acidify your soil. Additionally, since hydrangeas love aluminum-rich soil, the solution will act as the perfect catalyst for growth.
However, it would help if you were mindful of using aluminum sulfate without the proper protective equipment. Always use gloves when handling the mixture; if it touches your skin, wash it off immediately. Be mindful not to use too much, and always follow the instructions on the packaging.
6. Apply Nitrogen-Rich Fertilizer to the Soil
Fertilizers rich in nitrogen and ammonium are perfect for affecting the pH of soils. Ammonium in the fertilizer goes through a process called nitrification, which unleashes hydrogen molecules into the soil. This process is what changes the acid levels in the soil.
This is all because nitrogen is one of the primary foundations of pH changes in the soil. If you can find a good fertilizer with these properties, your hydrangeas will thank you.
Understanding pH and Testing Your Soil
One of the most fundamental aspects of soil health and chemistry is the pH level. The pH determines how efficiently your plant roots can access essential nutrients through the soil, so measuring the acidity of your soil is one of the best ways to ensure your plants can thrive.
Testing your soil acidity every few months is an excellent way to maintain the correct levels for the plants you’re growing. If your plants, like hydrangeas, love acidic soil, the pH level needs to be below 5.5. If your plants prefer more alkaline soil, then your soil should be 6.5 and above, depending on the plant.
Checking your soil’s pH levels is vital because it’s a great way to predict how well your crops will grow, how long they will last, and how nutrient-rich your soil is. If you don’t have a healthy soil balance with an ideal pH, your plants won’t do well at all.
A good time to test your soil for its pH level is around September or October. This is because it gives you enough time before winter frosts to alter the pH levels if needed, bringing you around to a productive spring.
A good acid soil for your hydrangeas lies around 5.0 to 6.0. This is relatively acidic but not so much so that the nutrients are too soluble and can be easily eroded by heavy rainfall.
To test your soil, I recommend using a 2-in-1 soil pH tester tool that tests soil moisture levels and acidity. This also makes your life easier by letting you know how often you need to water your plants. It’s straightforward to use and doesn’t need a battery.
How To Test Soil pH Without a Soil pH Meter
It may seem surprising, but you can actually test your soil’s pH levels without a meter. It’s much easier to do if you have a tester, but if you’d rather spend the time to do it yourself without spending the money, then follow the steps below:
- Grab a few handfuls of soil and place them on a flat surface. Spread out the soil to form a very thin layer and leave it for about 24 hours to dry.
- After it has dried out, take some of the soil and place it in a small bowl of water. Mix it up so that the soil turns into a thick sludge.
- Add a few tablespoons of vinegar to the sludge. If it begins to froth, you may need to add an amendment to the soil to make it more acidic for your hydrangeas.
This is a relatively easy way to check for soil pH, but it is rather time-consuming. This is why we prefer the Fosmon Soil Tester – but if you have the time, this method is an excellent alternative.
What Affects Soil pH?
The primary characteristics of soil come from its parent material which is the rock formation it used to be. Over time, erosion, weathering, temperature, and other factors can create a specific soil structure that is either alkaline, acidic, or neutral.
In very hot and humid climates, soil pH tends to be more acidic since the mixture of hot weather, and heavy rains can affect how nutrients are released from the soil. On the other hand, in cooler climates with low levels of humidity, soil pH tends to be more alkaline than acidic.
If you have very heavy clay-based soil in your garden, you may find it slightly more impervious to soil amendments like peat moss. Peat moss will do its best to break the alkaline bonds within a clay-based soil, but it may take more time to create the proper levels of acidity that hydrangeas thrive in.
One thing to note is that you need to be careful when altering soil pH. While acidic soil can be healthy for your hydrangeas and give you that beautiful blue color, dropping the pH too low can cause problems for your soil and your plants.
As a result, it’s important to always be aware of your soil pH before adding anything to it. This is why knowing how to measure your pH levels is vital.
What if My Soil Is Too Alkaline for My Hydrangeas?
Believe it or not, if your soil is too alkaline for your hydrangeas, they won’t die – instead, the flowers will be pink rather than blue. The aluminum in very acidic soils creates good conditions for the soil compounds that make blue flowers.
On the other hand, if your soil is very alkaline, this compound won’t be present, and the flowers will instead be pink.
Therefore, it may not be the end of the world if your soil is too alkaline for blue hydrangeas – unless that is your preference. A very neutral soil pH will allow the flowers to bloom in shades of blue, pink, and white.
What Do Hydrangeas Need To Thrive?
Your hydrangeas need more than just a good soil structure to thrive. They also need magnesium sulfate, a compound that allows your plant to become bigger and more green through photosynthesis. It also gives your hydrangeas a better survival rate and higher resilience to insects.
One of the best ways to ensure your hydrangeas are getting enough magnesium is to provide them with Epsom salts. It is a natural and cost-effective way to help your hydrangeas grow big and strong and create a mineral-dense soil texture that helps spur growth.
Hydrangeas are generally not fans of direct sunlight for extended periods, so placing them in a shaded spot that gets a little bit of sunlight is your best bet. Since hydrangeas love a moisture-rich environment, too much direct sunlight will dry out the soil, and the roots will have more difficulty accessing those all-important nutrients.
If your hydrangeas are indoors, it’s a good idea to place them in a window spot that gets direct sunlight for a couple of hours a day. This way, you’re not leaching them of the moisture they need, and they’ll still get adequate sunlight for photosynthesis to take place.
Additionally, you’ll need to prune your hydrangeas once a year to ensure they don’t grow too big. If you leave your hydrangeas without pruning them, you’ll find they’ll have all their beautiful flowers right at the top of the shrub and hardly any flowers below. This can also make your shrub fall to the side in extreme cases due to top-heavy flowering buds.
The best time to prune the hydrangeas is between late winter and early spring. You just need a good pair of small garden scissors to trim off any dead buds, leaving the healthy ones intact and ready to bloom in the summer months. Remove any stems that are growing too wild outside of the shrub, and you’re good to go.
To make soil more acidic for hydrangeas, you can use organic soil amendments such as coffee grounds and peat moss. You can also use ammonium sulfate, sulfur, and vinegar.
Looking after your hydrangeas requires more than a regular watering schedule – you also need to maintain a good pH level in the soil and ensure you prune your shrub every year.
Keep your hydrangeas away from too much direct sunlight, and use Epsom salts for an extra helping of magnesium sulfate. Hydrangeas are beautiful when looked after properly, so test your soil often.