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A firm favorite of many gardeners, lime is often used in gardens and to great effect. However, there are many plants that will not react well at all to lime use in the garden, so before considering the use of lime, it’s important to know which plants don’t like lime at all.
Many plants do not like lime, such as rhododendrons, azaleas, camellias, and heathers. Lime can significantly raise the pH level of the soil, so plants already in appropriately acidic soil will also be negatively impacted by lime.
It can be hard to know exactly how to judge whether your plants will benefit from lime or not. Keep reading to understand more about when — and when not — to use lime in your garden.
Why Do Some Plants Not Like Lime?
While lime actually refers to a number of different substances, the common property they share is that they are all very alkaline — meaning that on the pH scale, measuring the acidity or basicity of something — they score very highly.
Using lime for the garden is a very old process, and the impact that it has on the soil can be substantial. When added to the soil, the lime will react with compounds in the soil, which contribute to any acidity. This is called having a neutralizing effect.
As a result of these reactions, the pH level of the soil is increased. While it isn’t really possible to assess the pH level of solid limestone, it has been shown that when applied, it measures at a pH of around 8.5.
Most plants in home gardens have an acceptable range for soil pH in the following range:
- Lowest desirable pH – 6
- Highest desirable pH – 7
This shows that most plants prefer a pH range indicating slightly acidic to neutral soil. With this in mind, it’s easy to see that many plants will not be happy if lime is applied, as it will move their soil’s pH out of this range.
The difficulty with answering the question specifically is that it always depends on the current pH of the soil in which plants are situated. There are many plants that want acidic soils, and so — in theory — they will not react well to lime. If the soil is too acidic, though, then they might benefit. In contrast, even plants that prefer alkaline soil with a pH of 7.5 will not want lime applied if they are already at that level.
Why Do Plants Need a Specific Soil pH Level?
The pH value is a measure of the concentration of hydrogen ions. This can vary over a wide range, which is why the pH scale is a logarithmic measure.
The role of soil pH in the growth of plants is varied. The direct effect that the soil pH is on the solubility of chemicals and compounds in the water in the soil. This is referred to as ‘availability.’ Different nutrients are more available under acidic ranges of pH level, while others prefer a more basic soil pH.
For example, elements that are more readily available in acidic soils include:
In contrast, some compounds are more available in basic soils, such as magnesium and ammonium. We can see how the natural environment that a plant can be adapted to over thousands of years will determine its preferred level of acidity.
In addition, it’s also worth remembering that in both extremes, there are also elements that have much lower availability. For example, there are some negatives associated with strongly acidic soils, where conditions are such that the growth of the plant can be greatly inhibited by issues such as:
- Aluminum toxicity
- Manganese toxicity
- Calcium deficiency
- Magnesium deficiency
In alkaline soils, the deficiencies are more likely to be:
So, we can see that plants have quite specific pH ranges in which they operate, and adding lime can drive significant changes in nutrient availability. This is the reason why many plants are said to ‘not like’ the process of liming.
What Are the Different Types of Lime?
While lime is usually used in a generic context that implies it’s a single substance, it’s not. Lime is the name given to a calcium-containing substance that is inorganic. It is composed primarily of oxides and hydroxides, and so its calcium component is usually calcium oxide.
This means that when people refer to lime, there is some variation that is exactly what they are referring to. In most cases, they actually more likely are talking about the process of liming, which is the addition of a lime substance to the target area — in our case, the soil.
Some common types of liming material are:
- Ground limestone (garden lime)
- Calcified seaweed
- Ground chalk
- Ground magnesian limestone (dolomite lime)
- Calcium hydroxide (hydrated lime)
These different types of liming material are often interchangeably called ‘lime,’ but they have slightly different properties, so it’s important to understand the differences.
All forms of lime have a property that is known as its ‘Neutralizing Value’ or NV. This is a measure of the degree to which it is effective at neutralizing the acidity of the soil. This is a key consideration when selecting a type of lime that is right for your needs.
This video has more information on garden calcium.
Ground limestone is often called ‘garden lime’ because it is the most frequently found type of lime used in the garden. It does not require any additional processing beyond being ground from the source rock.
It is ground as very small particles in order to affect the soil, which generally holds only weak acidity and therefore is unable to react effectively with larger ground limestone pieces.
The NV of ground limestone is approximately 50.
Using calcified seaweed in place of garden lime is an increasingly popular practice, mostly as a result of the way in which the calcified seaweed takes effect. It is not actually seaweed but is instead a form of calcified cold water coral.
Calcified seaweed is approximately 50% calcium, with up to 10% magnesium content. It also contains trace elements that can benefit plant growth, such as:
This range of benefits to the nutritional requirements of the plant is the main reason for its popularity. Soils which are treated with chemical fertilizers are also more likely to see deficiencies of trace elements, making calcified seaweed a very useful replacement for garden lime.
Given the broader content of calcified seaweed, the NV is a little lower than that of ground limestone, coming in at 44.
Chalk is similar to garden lime, although it is geologically younger than limestone and partially, as a result, much less tightly compacted. This makes it especially effective at being ground into a dust-like level of particle size, which in turn increases its effectiveness of soil neutralization.
Ground chalk has an NV of approximately 50, putting it on par with garden lime.
Ground Magnesian Limestone
Magnesian limestone is formed through a process extremely similar to that of normal limestone used for garden lime. However, it has a much higher magnesium content as a result of the environment in which it is formed.
This higher level of magnesium content arrives through a geological process called dolomitization, where magnesium can take the place of some of the calcium in the soil. However, because magnesium is available in the soil as a particle known as a cation — that is, positively charged, just like calcium — it does not impede the ability of the substance to neutralize the soil’s acidity.
This quality of magnesian limestone means that it actually has a higher NV than the sources we’ve reviewed so far, coming in at around 56.
Calcium Hydroxide, also known as builder’s lime, is most frequently sold for use in construction. It is derived from slaking burnt lime or magnesian burnt lime and should not be used for gardening purposes.
Extreme care should be taken if using calcium hydroxide because unprotected exposure to the substance can cause many health issues, including:
- Severe skin irritation
- Chemical burns
- Lung damage
How Should Lime Be Applied?
The most important thing to remember when assessing the right timing to add lime to a soil bed is that changing the pH level of any area of soil takes time. The lime needs to work its way into the soil and be exposed to those elements providing the acidity. With this in mind, the best time of year to apply lime is generally considered to be the fall.
Applying lime in the fall gives plenty of time ahead of the following spring for the substances to disseminate throughout the soil and take effect. Experiencing a winter is also good for the spread of the lime in many areas, as freeze/thaw cycles will also help in the movement and efficacy of the lime.
The approach taken to apply the lime will generally depend upon the two states of the soil bed:
- A new bed
- An established bed
When applying lime to a new bed, it is best dispersed through tilling. This provides a direct distribution through the top six inches or so of the soil in order to maximize the distribution.
If it is being applied to an established bed, it is likely not worth the impact on the soil structure to mix the lime in too aggressively. In this case, it should be applied liberally to the surface of the bed, taking care to distribute it as evenly as possible. Using a fertilizer spreader is often a good idea to assist with this process.
Whichever approach is taken to apply the lime, moderate to heavy watering — provided the plants are happy with this — will help the lime work travel through the soil and be absorbed faster. This is a particularly useful post-spreading activity in windy areas, as the water will weigh down the lime and reduce the amount carried off by the wind.
How Do I Know If My Soil Will Tolerate Lime?
The best way to identify whether your soil will benefit or tolerate an application of lime is by using a test kit to understand the pH levels of your soil. There are also DIY alternatives to a commercial test kit that can help with this process.
In practice, though, for established beds, it’s often quite straightforward to assess the rough pH level of the soil by looking at which plants are growing successfully. If the plants you have in the bed are known to be most effective in quite acidic soil, such as:
Then the soil is likely acidic. Similarly, if there are plants thriving that are known to prefer more basic soils, then this is likely a strong identifier of a higher pH level of the soil.
Following on from the application, you should also be aware of the impact of applying too much lime. Most frequently, if the soil becomes too alkaline, plants will begin to struggle to take on the right levels of different nutrients, and chlorosis can occur. Chlorosis is identifiable by a yellowing of the plant leaves, often from the ground up, as the plant nutrients travel up the stem.
Should you believe that too much lime has been added, then the best course of action is to look at how best to alter the pH to become more acidic. With any application of fertilizer impacting the soil pH, however, the application should be conservative in order to slowly assess the impact so as to avoid substantial swings in pH levels, which will make the soil difficult to grow in.
Distribution is also key to avoid ending up with a bed that is, in part, very acidic while being very alkaline in other areas. This is an issue as it can make diagnosing problems extremely difficult.
Lime can be a very useful addition to the garden, providing the ability to significantly alter the pH of the soil and, in turn, change the environment in which your plants grow. This isn’t always what your plants need, though, so understanding the requirements of your plants alongside the type of lime you have available can give you the best chance of success.