Best Methods to Add Calcium to Soil To Grow Healthy Cannabis Plants

Created by
Added 10 September 2023

Calcium gives you strong bones, and similarly, calcium gives your cannabis plant strong branches and stems (among various other things). No wonder, calcium is a crucial nutrient for the cannabis plant that can either nourish it or turn it into a weakling that is quick to suffer from diseases, stress, and other problems.

However, the calcium factor of the cannabis plant is subtle. Most growers will never have to think about calcium because even tap water contains enough calcium to sustain cannabis plants. Some growers may have to supplement their plants with calcium for various reasons.

If you think you are one of the latter, read this guide on how to add calcium to your soil for the best growth. 

Why Does Cannabis Need Calcium?

Why Does Cannabis Need Calcium?

Cannabis plants require three macronutrients but they also require many secondary nutrients, and one of the most important secondary nutrients is calcium. Calcium is classified as a secondary nutrient because cannabis requires it in lesser quantity than nitrogen, phosphorus, and potassium, but that doesn’t take away the vitality of this mineral. 

Your cannabis plant requires calcium in all its growth stages in varying amounts, but this nutrient is primarily found in the cells of the plant, where it serves various functions. Calcium is used by cannabis in many ways, but here are some of the most important processes where calcium comes in handy:

  • One of calcium’s primary functions is to strengthen plant tissue. This nutrient is used in all cell structures of the plant, and a calcium-rich diet will lead to a sturdy, strong cannabis plant.
  • Calcium improves the absorption and translocation of nutrients; it is critical for nutrient uptake, so if your plant does not get enough calcium, it can suffer from other forms of nutrient deficiencies too.
  • This nutrient regulates the activity of hormones and enzymes within the cells, which makes a big impact on the plant’s energy generation and expression of the phenotypes.
  • Calcium also acts as a gatekeeper for plant cells by aiding membrane permeability; in other words, this nutrient is responsible for letting some nutrients move out of the cell and others stay in, as required. 
  • Since calcium makes your cannabis plants stronger, it also improves your plant’s resistance to pests and pathogens. 
  • Calcium also encourages the respiration of the tissue, and at the same time, it stimulates enzymes that protect the plant from environmental fluctuations, especially heat stress

How to Add Calcium to Soil for Cannabis Plants?

How to Add Calcium to Soil for Cannabis Plants?

You don’t need to apply a lot of calcium to make cannabis plants happy. Moreover, the application process is easy and manageable for even new growers. And thanks to modern gardening and farming techniques, there are many ways to add calcium to soil. 

The maximum amount or upper limit of calcium that cannabis plants can take is still unknown. However, most manufacturers recommend a concentration ranging from 150 to 300 ppm that you can apply either through a growing media or as a solution. Beware that calcium can react with any phosphate present in the solution, so you should not mix it with solutions containing phosphate. This is also why manufacturers send calcium as a separate fertilizer where you have to mix it and then apply it to the plant. 

Here are a few ways to add calcium to cannabis plants:

1. Add Lime to the Soil 

If you’re looking for an easy application of calcium to your cannabis plant, your best bet is to use lime — not the citrus fruit, the lime mineral — in the soil. Lime, also known as calcium carbonate, can be purchased from your local gardening store for cheap, usually in a powder form, which you can sprinkle on the soil during fall. 

Do note that adding lime powder to the soil will affect the pH of the medium, raising it slightly, if you don’t administer the supplement properly. So, it’s recommended that you test your soil’s pH using a pH meter before using lime. If your soil is close to the ideal pH range, it’s best to avoid using lime as it will increase the pH. 

You can even use dolomite lime if you want to supplement your soil with calcium and magnesium; it is rich in both nutrients. Even dolomite lime will increase the soil’s pH, so it is recommended to be used only on slightly acidic soil.

2. Get Your Plant Some Bone Meal

Bone meal is one of the best organic fertilizers for cannabis plants. This product is made from ground-up animal bones, and since bones are rich in calcium, bone meal is incredibly rich in calcium along with nitrogen and phosphorus. It’s a more balanced calcium source compared to many others on this list. 

Again, bone meal does raise the pH of the soil, so only use it if your soil is slightly acidic or you don’t mind rebalancing the pH every now and then. On the bright side, bone meal can be a rich source of calcium for up to four months in most cases! So, if you add bone meal to the soil at the beginning of the season, it can very well last the entire growing season. 

You can purchase bone meal from your local gardening store and follow the manufacturer’s recommendations on using it. Bone meal is a great product for cannabis gardens, even if your plant is not suffering from calcium deficiency, so it’s highly recommended that you use bone meal in your garden. 

3. Use a Calcium Fertilizer

One of the easiest approaches to adding calcium to soil is using a calcium fertilizer or a fertilizer that is rich in calcium, especially if your soil is naturally low in calcium. These fertilizers come in both organic and synthetic varieties, offering fast-acting nutrients that your cannabis plant can absorb easily. 

These fertilizers are straightforward to use — follow the manufacturer’s recommendations — but before using them, it is recommended that you test the nutrient levels of your soil to ensure you don’t end up applying a nutrient that your soil is already rich in. 

4. Use Dried Eggshells

Eggshells have many uses in a cannabis garden; many growers use eggshells to deter pests or add them to the mulch, but you can also use eggshells to give a calcium boost to your cannabis plant. This is because eggshells are incredibly rich in calcium, so even a few eggshells can be enough for your cannabis plant. 

Using eggshells is straightforward, too. Start by storing a few eggshells in a sealed container until they dry, then grind them in a food processor. Try to grind them as fine as you can — the finer the better. 

You can apply eggshells to your cannabis plants in two ways. The first is to sprinkle and mix the crushed eggshells into the soil, but if you’ve already planted your cannabis, you can mix two tablespoons of crushed eggshell powder with one gallon of water and use it as a soil drench. 

Another way to use eggshells is to crush them finely and pour them into a container of vinegar. If you remember your high school Chemistry, the calcium carbonate in the egg shells dissolve, resulting in calcium acetate. You can then store the solution, and mix an ounce of it into a gallon of water and water the plants. Many gardeners use this technique to combat bottom-end rot that’s prevalent in tomato plants. Also, this is a quick way to provide some calcium to your plants rather than just sprinkling eggshells on the soil and waiting for them to do their thing. 

5. Use Gypsum 

If you’re worried that some calcium sources will affect the soil’s pH and want an alternative, you can use gypsum, too. This excellent naturally occurring substance releases calcium at a relatively faster rate compared to other calcium sources like eggshells. This is also quite easy to find in most gardening and horticultural stores across the globe. 

Gypsum also offers various other benefits to your cannabis garden as it can prevent crusting or erosion of the soil, allows the roots to expand easily, and removes salt from the rhizosphere that may otherwise create problems for the roots. The latter is the reason growers living near the coast tend to use gypsum in their gardens even if their plant is not suffering from calcium deficiency.

6. Grind Down Some Oyster Shells

Oyster shells are a rich source of calcium carbonate, and you may already have it in your garden if you shelter chickens. If you want to use oyster shells, you must know that this will also increase the soil’s pH like lime, but at a slower rate since it takes a long time to break down entirely. 

The slow breakdown time may make you think, “What’s the point of using oyster shells, anyway?” Your concern is real, but oyster shells have a place in some cannabis gardens where the grower is looking for a long-term calcium source, not a quick fix. 

If that’s the case, feel free to grind up some oyster shells and add them to the soil. Remember, it won’t release calcium quickly, so it’s only recommended when you want to increase the calcium content of your growing medium for the long run.

7. Wood Ash

Wood ash is another substance that is rich in calcium carbonate, and it’s often easy to get it if you whip out your BBQ every weekend. However, you need to be careful about the type of wood ash you use; only use hardwood ash, which contains 50% of the calcium that’s in lime, but avoid softwood ash as that can be harmful to cannabis plants. 

Wood ash is also rich in other nutrients like potassium, phosphorus, and boron, making it an ideal bloom-time fertilizer for the cannabis plant. But it also has a downside — it increases the soil’s pH, so avoid using it if your growing medium is alkaline or neutral. 

To use wood ash, make a circle around your cannabis plant with the ash, ensuring it doesn’t touch the plant directly (it can burn the plant). A good rule of thumb is to always use twice as much wood ash as you would lime. 

8. Use Soft Rock or Rock Phosphate

Soft rock, rock phosphate, or colloidal phosphate — call it what you want — is a decent source of calcium if you only need to give your plant a slight nudge of calcium. Unlike lime or bone meal, rock phosphate does not increase the pH of the soil, so it’s an excellent amendment for many growers. 

Another benefit of rock phosphate is that it is rich in phosphorus, too, which is an essential macronutrient for cannabis plants. On the flip side, you may want to avoid it if you want something that’s quick to act as it releases calcium slower than lime. 

You’ll find rock phosphate in two types: soft and hard. If you plan to use this as a fertilizer, you should use soft rock as it tends to release calcium faster than hard rock. You should also check the instructions as some products are only suitable for soil slightly acidic soil. 

9. Apply Calcium as a Foliar Spray 

If you want to provide calcium instantly to your plants, organic fertilizers like eggshells or lime are not the best option since they release calcium slowly into the soil. If your plant needs a quick boost, you can give it in the form of a foliar spray. 

You can give your plant a calcium fix via a foliar spray that contains calcium chloride, calcium nitrate, and calcium acetate (eggshells + vinegar will work as well) as a foliar spray for a quick fix. These compounds are excellent sources of calcium for cannabis plants and the foliar application puts the nutrient exactly where the plant needs it — on the leaves and branches. 

You may also prefer using foliar spray if you don’t want the mineral to affect the soil’s pH range. To use calcium as a foliar spray, take an ounce of any calcium source mentioned above and add it to a gallon of water, shake the solution, and spray it on your plant. If you’re feeling lazy, you can even purchase pre-made calcium foliar sprays, which come at a cost but last a long time and contain precise amounts of calcium. 

10. Switch to Tap Water 

If you’ve been using RO or purified water for your cannabis plant, the only calcium fix you may need is to switch to non-RO water or tap water, which is already rich in calcium. Check the EC of your tap water; if it’s between 0.3 to 0.4, i.e., 150 to 200 PPM, it has enough calcium to sustain your cannabis plant. Sometimes, the simplest solutions work the best!

When to Supplement Calcium To Cannabis Plants?

When to Supplement Calcium To Cannabis Plants?

Cannabis plants need three macronutrients, namely nitrogen, phosphorus, and potassium. However, one of the most important nutrients apart from NPK your plant needs is calcium, which is why many growers refer to this nutrient as the fourth macronutrient. 

Regardless of the labels, calcium is crucial for your cannabis plant. It helps the plant create strong cell walls, which promotes strength and better fruit production, among other things. Cannabis requires this nutrient throughout its growth cycle, so you can add it pretty frequently. 

However, don’t overdo it as your soil can get saturated with too much calcium over time, leading to calcium toxicity and nutrient lockout. Instead, it’s better to add calcium to the soil at the right time, when it will have the best impact. 

Depending on the type of calcium supplement you use, you can either add calcium during the start of the growth stage if you’re using slow-release products or when your plant needs calcium the most. 

The calcium requirements of cannabis plants can vary depending on factors such as growth stage, strain type, specific growing conditions, and pH of the soil or the growing media. Calcium is considered a macronutrient, so you must ensure that you provide enough for the plants to grow healthy. 

To make this easier, let’s break down the use of calcium for cannabis throughout its various growth stages:

  • Seedling Stage

When your cannabis plant is a young seedling or a fresh cutting, it doesn’t need much calcium or any other nutrients. The plants are just beginning their development. At this point, you don’t need to add any type of nutrient, let alone calcium. Seedlings mainly depend on the nutrients stored in the seed for their growth. They also have the ability to absorb calcium from the growing medium. While calcium is crucial for plants, at this stage it's important to avoid excessive amounts. Excess calcium can possibly interfere with the absorption of essential nutrients

In general, a balanced growing medium or soil mix that contains some calcium should be sufficient for promoting seedling growth. If you absolutely need to add nutrients at this delicate stage, it’s best to start with 25% of the recommended dosage of a grown plant to seedlings. However, this is not really required, so adding slow-release calcium products during this stage is ideal as they won’t release enough calcium to be a problem for the plant. Avoid using products that act fast. 

  • Vegetative Stage

After the fourth week, your cannabis plant will have entered the growth or vegetative stage, when it needs a lot of calcium. This is when cannabis plants prioritize developing stems and lush foliage. During this stage, they require calcium along with vital nutrients such as nitrogen and potassium. Here, you should feed the recommended dosage of nutrients, depending on your strain. Fortunately, most cannabis fertilizers are rich enough in calcium but you can also give your soil a calcium boost like CalMag or use one of the products listed above. 

  • Pre-Flowering Stage 

During this stage, your cannabis plant will be vigorous and stretch considerably — this is called the flowering stretch — and your plant will need a lot of calcium and magnesium at this time. At this phase, the plants direct most of their energy toward producing buds, so calcium is very important because it contributes to the strength of cell walls, which can help prevent issues, like bud rot. 

While the plants may need more calcium during this phase, it doesn’t mean that you dramatically increase the levels. Instead, continue what you’re doing and add balanced fertilizers according to the manufacturer’s instructions. No matter what fertilizer you use, make sure that it’s specifically designed for the flowering stage and contains adequate amounts of calcium and other macro and micronutrients to support bud development.

  • Flowering stage

During the flowering stage, your cannabis plant won’t require a lot of calcium. If you’ve already added slow-releasing calcium supplements to the soil, you don’t need to worry much during bloom. However, keep a lookout for any signs of calcium deficiency as it can negatively affect the yield; always keep a calcium supplement handy. 

Again, keep in mind that the exact amount of calcium needed may vary depending on factors such as the strain of cannabis, the growing medium used, and environmental conditions. Make sure to check the health of your plants and watch out for any signs of deficiencies or excesses. 

Adjust your routine accordingly based on what you observe. It's also crucial to keep the pH level of your growing medium in the range ( around 6.0 to 7, for soil and 5.5 to 6.5 for hydroponics). This is important because pH has an impact on availability, including calcium. To ensure that your cannabis plants can effectively absorb calcium and other nutrients, test the pH of your soil or nutrient solution and make any adjustments as required.

What is Calcium Deficiency? 

What is Calcium Deficiency? 

It’s always a good idea to add calcium to your soil even though tap water and most fertilizers already contain calcium. Despite your best efforts, some cannabis plants may have calcium deficiency once in a while especially if you’re growing hydroponically. 

Calcium deficiency can be caused due to various reasons, such as the following:

  • Incorrect pH in the soil, which causes calcium lockout for the roots 
  • Using coco coir as a growing medium without coco-specific nutrients 
  • Using RO water that lacks calcium 
  • Using excess potassium can prevent the roots from absorbing calcium properly 
  • Poor quality growing medium that lacks calcium 

Your cannabis plant uses calcium in the cells, and therefore, it cannot relocate to newer growth — it’s not a mobile nutrient. So, if your cannabis plant lacks calcium, the problems start occurring on new growth first. In such cases, look for the following symptoms in newer growth to identify calcium deficiency:

  • The new leaves may twist, curl, or wither 
  • The leaves may also turn yellow or purple, or they develop brown spots or necrosis 
  • The flowers may grow distorted and too small or airy 
  • The new branches may not grow as fast as they should and appear weak or sick 
  • The roots may experience root rot or bacteria/fungal infections 

If your cannabis plant shows any of these signs, it is likely suffering from calcium deficiency. So, the first thing you should do is give your plant a calcium boost via a foliar spray. Next, check the pH of the growing medium, check the nutrient solution’s ratios, and add some slow-release calcium supplements to the soil to prevent this problem from occurring again. 

Which is Better: Organic or Synthetic Calcium Fertilizers?

Which is Better: Organic or Synthetic Calcium Fertilizers?

The organic vs. synthetic fertilizer debate is an old one and there’s no right or wrong answer. It all depends on your goals, preferences, and situation. When it comes to adding calcium to your soil, you have plenty of options as listed above. 

If you want to choose the organic route, you can use many materials that are rich in calcium, from tap water to oyster shells and so many more. Add these to your soil and let them work their magic, but do note that organic calcium supplements tend to be slow-acting and will take a long time to break down. That’s not a bad thing if your timing is right; slow-release supplements reduce the chances of toxicity. 

On the other hand, if you don’t mind using synthetic fertilizers or want a quick boost, you can use synthetic fertilizers that contain calcium chloride or calcium nitrate. These products act fast and can come in handy when dealing with calcium deficiency, but they can cause calcium toxicity if you’re not careful. 

Figure out what your goals and preferences are and pick the one that suits you best. 

Summary: Best Methods to Add Calcium to Soil To Grow Cannabis 

Calcium is a critical nutrient for cannabis plants, and while it is a secondary nutrient, it plays the role of a macronutrient. It makes your cannabis plant strong, healthy, and yieldful. So, you need to supplement it with enough calcium at all times. 

Some of the best ways to add calcium to the soil include limestone, bone meals, calcium fertilizers, dried eggshells, gypsum, wood ash, soft rock, and tap water. Use this guide to narrow down which amendment suits your plant and growing conditions and stick to it. And whichever method you use, always be wary of calcium deficiency and toxicity — it’s all about striking the right balance. 


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Thanks for the informative read! I'm curious, though, about the potential impact of different soil types on calcium availability for cannabis plants. Would the methods mentioned in the blog still be effective across various soil compositions? I'm eager to learn more about this aspect of soil nutrition. Also, I stumbled upon another interesting blog on specialized crop kits by Budmax. Have you come across it, and if so, how does it compare to the methods discussed in your blog?