I have several citrus trees from Kaffir lime to Meyer lemon, and while the majority are doing well, many of them are in soil that’s high in clay. Since clay is typically more alkaline, I was wondering if it was bad for citrus trees and if they preferred acidic soil instead. So, I did some research and testing to find out. Here’s what I found.
Citrus trees prefer slightly acidic soil with a pH of 6.0-7.0. While they can tolerate soil pH as acidic as 5.0, soil that’s either too acidic or too alkaline will bind nutrients in the soil and make them unavailable to the tree’s roots. You can amend alkaline soil with coffee grounds, sand, or peat moss.
So, while citrus trees prefer to have a slightly acidic soil pH, what exactly does this do for citrus trees, what’s the best way to measure soil pH, and how can it be adjusted? Let’s take a closer look.
Why Acidic Soil Is Good for Citrus Trees
A soil pH of 6.0-7.0 is good for citrus trees because it’s the right amount of acidity that dissolves the majority of minerals in the soil as nutrients for the tree’s roots. It also supports the beneficial soil bacteria which break down organic matter in the soil—providing even more nutrients for the tree.
Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.Donald Bickelhaupt, SUNY College of Environmental Science and Forestry
To recap, a slightly acidic soil pH of 6.0-7.0 is good for citrus (and most plants) because it:
- Makes minerals and nutrients available for the plant
- Supports healthy beneficial soil bacteria
While these two factors may seem small, they’re vital in breaking down larger pieces of organic matter and providing sufficient nutrients for the plants. Without them, the plants will quickly run out of usable nutrients in the soil and develop issues such as yellow leaves (more on this later).
Reasons Why Soil Becomes Acidic or Alkaline
Alkaline soil is commonly caused by calcite from the erosion of rocks and from water that contains calcium carbonate (the white residue after rinsing glassware). Acidic soil is typically caused by rainwater, carbon dioxide, and acids from organic matter and chemical fertilizers.
The reason why your backyard might have soil that’s alkaline or high in clay is due to the accumulation of calcite and sediment from billions of years of erosion (source). Let’s unpack this a bit.
Alkaline soil commonly comes from a few different sources:
- Calcite (calcium carbonate) from eroded rocks
- Water containing calcite
- Dust accumulation
Since calcite is water-soluble, it can easily add up as it’s filtered through the soil.
On the other hand, acidic soil is commonly caused by:
- Rainwater leaching minerals
- Too much carbon dioxide in the soil
- Strong acids such as nitric and sulfuric acid
Rainwater leaches away basic minerals such as calcium, magnesium, sodium, and potassium which stabilize the pH of the soil. Without these minerals, the soil becomes imbalanced and leans more acidic.
Carbon dioxide occurs from decomposing organic matter in the soil. The accumulation of too much matter too quickly can be trapped in the soil and raise its acidity.
Lastly, strong acids also occur naturally from decaying organic matter, but also unnaturally from fertilizers high in ammonium and sulfur. This is a big reason to use organic fertilizers such as manure and compost.
But what happens to the citrus tree when the soil is imbalanced? What are some symptoms and signs that you should keep a lookout for?
Issues Caused By an Imbalanced Soil pH
Soil that is either too acidic or alkaline will restrict the nutrients available to citrus trees and potentially burn their roots. This commonly leads to issues such as yellow, brown, and dropping leaves and fruit. For best results, check the soil’s pH after amending or every 1-2 months with a pH meter or strip.
When a soil has an imbalanced soil pH, it affects the plant by:
- Binding nutrients in the soil
- Killing beneficial soil bacteria
- Potentially burning the roots
As nutrients are necessary for just about every function of the tree including growth, flowering, fruiting, immune system, and water retention, having a proper pH is incredibly important.
A common example of a growing issue from imbalanced soil pH is when alkaline soil binds the iron in the ground, causing chlorosis, or yellowing of the leaves. If you’d like more information about yellow leaves on citrus trees, feel free to check out my recent post: How to Fix Yellow Leaves on Your Citrus Tree.
Fortunately, measuring and amending soil pH isn’t too tricky.
How To Measure Soil pH
The best and easiest ways to measure soil pH are either with a pH meter or pH strips. pH meters can be inserted into the ground, and pH strips require stirring the soil sample with distilled water, filtering it, and testing the liquid with the strip. While pH meters are easier to use, pH strips can be more accurate.
As you may have guessed, pH isn’t something that is just for plants. Animals also require a balanced pH to stay healthy. Like plants, most of our internal systems thrive off of a balanced, or neutral pH (around 7.0).
For context, here are some common materials and their pH values:
|Extremely Acidic (Less Than 4.5)||Soda (2-4), Stomach Acid (2.0), Lemon (2.5), Vinegar (3.0)|
|Very Strong Acid (4.5-5.0)||Tomatoes (4.5), Beer (4.5-5.0)|
|Strongly acid (5.1–5.5)||Coffee (5.0), Cabbage (5.3), Boric Acid (5.2)|
|Moderately acid (5.6–6.0)||Potatoes (5.6)|
|Slightly acid (6.1–6.5)||Salmon (6.2), Milk (6.5)|
|Neutral (6.6–7.3)||Saliva (6.3-7.3), Shrimp (7.0), Blood (7.3)|
|Slightly alkaline (7.4–7.8)||Eggs (7.7)|
|Moderately alkaline (7.9–8.4)||Sea Water (8.2), Sodium Bicarbonate/Baking Soda (8.4)|
|Strongly alkaline (8.5–9.0)||Borax (9.0)|
|Very strongly alkaline (Greater Than 9.1)||Wood Ash (9.0-11), Ammonia (11.1), Lime Powder (12)|
You can measure the pH of your citrus tree’s soil with two primary methods.
1. pH Meter
pH meters are fairly easy to use as they have a probe that can be simply inserted into the ground. For best results, test different areas of your garden.
To see which pH meter I use and recommend, you can visit my recommended tools page.
2. pH Strips
While pH strips require more steps, they can be slightly more accurate than pH meters.
Here are some steps to test your soil with pH strips:
- Dig for a soil sample
- Add 2 teaspoons of the soil in a clean glass
- Pour in distilled water
- Stir or swirl the sample
- Pour the sample through a coffee filter and into another clean glass
- Dip the pH strip into the liquid
As above, it’s a best practice to test multiple samples of your garden’s soil to get a better idea of its average pH. Alternatively, before testing, you can mix equal amounts of soil from different areas of your garden and test it once to obtain the average.
How To Amend Acidic or Alkaline Soil for Citrus Trees
|Acidic Amendments||Alkaline Amendments|
|Peat Moss||Wood Ash|
If you test your soil’s pH and find that it’s either too acidic or alkaline, it can be helpful to amend it. This is easier to do with smaller amounts of soil, such as potted citrus trees. For larger amounts of soil, it can be better to grow the trees on mounds of soil and mulch (more on this below).
For acidic soils (below 5.0-6.0), use alkaline amendments such as charcoal, wood ash, and lime. For soils that are alkaline (above 7.0), use acidic amendments such as coffee grounds, peat moss, or sand.
For example, many gardeners use lime to increase the alkalinity of their soils as it has an extremely high pH (12) and contains nutrients such as calcium and magnesium. You can use lime supplements including Calcic limestone (ground limestone) and Dolomitic limestone (ground limestone high in magnesium).
Knowing how much amendment to use is tricky as many variables contribute to it. Generally, start small and test the pH as you go.
If you are working with a large amount of soil, and you’d like more information about growing citrus trees on mounds, check out my recent post: Can Fruit Trees Grow in Clay Soil (If So, Which Ones)?.
For a more accurate soil analysis, including how much soil amendment you should use, consult your local nursery, professional orchard, or cooperative extension service.
Other Fruit Trees That Like Acidic Soil
If you have other fruit trees in addition to your citrus trees, and you’re wondering if they prefer a similar soil pH, you’re in luck.
Here are other fruit trees that like slightly acidic soil:
|Fruit Tree||Preferred pH|
Generally, most plants do best with a soil pH of 6.0-7.0. Again, this is because a slightly acidic soil is great for dissolving the minerals and nutrients needed to be absorbed by the plant’s finer roots.
If your soil is in this range, you’re most likely safe to grow just about any plant.
However, some other plants like even more acidity, such as azaleas, rhododendrons, blueberries, white potato, and conifer trees.