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Fertilizer, Soil Improvement

Calcium in Soil: How It Helps Plants


14 min read

In humans, calcium serves as a critical part of building healthy bones and teeth. Calcium in soil plays a critical role in developing garden soil structure. For plants, calcium promotes healthy plant growth throughout the growing season. It is, after all, an important macronutrient in the plant nutrition profile.

This article will outline calcium as a plant nutrient and help readers determine how to spot a calcium deficiency. We’ll also discuss its role in soil structure. As with most soil-related topics, variables like climate, texture, and organic matter, in addition to other properties, will come into play. 

Understanding your garden soil structure is important when considering soil fertility. Calcium is a nutrient that often gets a lot of traction on the internet. Gardeners are prompted to add egg shells and oyster shells to prevent blossom end rot. In most cases of adding it to the soil, amendments alone will not fix the deficiency. 

Let’s take a deep dive into this critical part of plant nutrition and discuss the proper way to add calcium to the soil.

Calcium as a Plant Nutrient

Xylem and phloem
The xylem and phloem carry nutrients and plant food to where they’re needed. Source: sgharvey

Calcium serves several functions in a plant. It helps plants build strong cell walls. Plant cell walls provide a rigid structure and protection a plant needs. Calcium plays a role in the permeability of the cell membrane. Fortified membranes help a plant keep toxic compounds from penetrating cell walls. Calcium also activates certain enzymes involved with biological reactions within the plant. Finally, it plays a role in cell division and cell elongation. 

A critical part of understanding calcium is how plants uptake this nutrient. First, we need to understand the function of the xylem and phloem, the vascular system of the plant. The xylem is responsible for distributing water and dissolved minerals from the roots through the stems to the leaves. The phloem carries the sugars and food from photosynthetic regions to other parts of the plant. 

So much more than this brief explanation is involved in the complex vascular system of plants. Calcium cations (positively charged ions) are taken up from young tissue in the roots. The xylem then carries the calcium to the plant from the roots. Here is where young roots are key, as young roots have more permeable membranes than older root tissues. 

Calcium Requirements

The amount of calcium needed for plants varies by species and family. Monocot plants (grasses) like corn, sorghum, and alliums require low amounts of calcium. Garden crops in the brassica family (cabbage, broccoli, collard greens, kale, etc.) require more calcium. Trees need the most calcium. This is because calcium is important in the formation of woody trunks. 

Spotting a Calcium Deficiency

When it comes to nutrient deficiencies, it is important to know how calcium interacts with each plant. Some nutrients present similar deficiency characteristics, but on different parts of the plant. For example, a deficiency may show up only on old leaves and not new ones. These small details are important when diagnosing a deficiency. 

The prototypical example of calcium deficiency is blossom end rot in tomatoes. The deficiency is obvious in this case, as the bottom of the tomato has a rotten appearance. Calcium is poorly transferred to the fruit, so deficiencies are visible at that location.

Growing points also can display deficiency characteristics. New leaves may appear stuck together. Under the soil, roots can experience stunted growth from a lack of calcium. Needless to say, looking at the growing points and fruit is an easier way to diagnose the deficiency than examining the roots. 

Calcium’s Role in the Soil

Calcium is critical for healthy soil structure. It is specifically important for a process called flocculation. In the flocculation process, clay particles are pulled together, separating from other soil particles. This can be seen in the most basic home soil test where the soil is shaken up in water. The clays are aggregated and leave the water less murky compared to soil with fewer aggregates.  

Flocculation helps with runoff and erosion as the water that comes off the farm is clearer with fewer sediments in it. Calcium does the best job of holding these clay particles together compared to other cations like magnesium which also has a +2 charge. Magnesium and calcium often shadow each other in many ways but differ in plant function and flocculation ability. 

How Calcium Cycles

Calcium enters the soil in several ways. One way is by degrading minerals like calcite, limestone, and dolomite. These and other soil parent materials release calcium into the soil. Calcium is a base cation critical in cation exchange. Cation exchange capacity is the number of cations the soil could hold. 

The exchange process occurs as cations (or positively-charged calcium ions) trade places between the soil particle and the solution. There are many cations at play besides calcium, all of which vary in their ability to be exchanged. Cation exchange capacity is another way calcium is made available to plants in the soil. 

The third main way calcium naturally enters the soil is through atmospheric deposition. Soot and ash from the atmosphere are deposited into the soil, and add calcium to soil in the process. Calcium can leave the soil in two ways. The first is by leaching, as water washes away the calcium not attached to clay particles. The second is through plants, which take up the calcium absorbed from the soil. 

Why Your Soil is Not Calcium Deficient

Blossom end rot in tomato, which isn't a symptom of low soil calcium
While blossom end rot is a calcium deficiency symptom, it isn’t due to low calcium in soil. Source: cromely

Seeing blossom end rot may make you think your soil is devoid of calcium. In most scenarios, this is not the case. Most soils have enough calcium to support the garden. The reason you have blossom end rot is likely due to insufficient and/or inconsistent watering in the root zone. 

As I mentioned before, calcium is taken up by the young roots of the plant via water. Once the plant cells in the roots mature, calcium is no longer absorbed. Consistent watering allows calcium to enter the plant easily and make its way to the fruit. Most garden soil, substrate mixes, and generic fertilizers contain calcium anyway, making it even less likely that a lack of calcium is the issue. 

One of the only scenarios where calcium may not be a part of the soil structure is in highly acid soils. Once the pH of your soil is low enough (around 4.5 and below), aluminum (iron and manganese) toxicity occurs. In this case, the calcium in the soil is being held so tightly onto clay particles that roots cannot access them. 

This scenario is usually easy to pinpoint. When the pH of your soil is acidic, several other minerals and nutrients are lacking, and plants grow poorly. A soil test is the only way to know for sure if your soil structure is lacking calcium. 

Extremely sandy soils may also cause a lack of calcium because of a lack of clay particles available to bond to calcium. It’s unlikely anyone would grow garden crops in soil this sandy. In both acidic and sandy soils, nutrient availability is low in general. The likelihood of attempts to grow in either condition is low.

Calcareous Soil

While a rare occurrence, too much calcium can be present in some soils. Soils high in calcium carbonate are known as calcareous soils. These soils have a pH above 8.0 and will not significantly hinder the healthy growth of plants. It’s worth noting that calcium is not the cause of increased pH. Carbonate in lime is why soil pH is increased, rather than calcium which is often blamed. 

Treating a Calcium Deficiency

Dolomitic lime is used to add calcium in soil
Dolomitic lime is a viable way of increasing the calcium in soil. Source: dreamexplorer

Appropriate irrigation is the key to fixing most of the calcium deficiencies present in the garden. If for some reason, there is a calcium deficiency, there are many options out there to help. Foliar sprays, however, will not fix blossom end rot in tomatoes. That’s because the deficiency isn’t present in the leaves but is present in the fruit.

Soil pH

Calcitic lime and dolomite lime are the two main amendments used to raise soil pH. In the situation where calcium is actually deficient, the soil is already extremely acidic. Of all the calcium fertilizers, calcitic lime is usually the best, depending on the situation. Agricultural lime will raise the soil’s pH. 

Lime is calcium carbonate, and the carbonate salt in this molecule is responsible for increasing the soil pH. Dolomitic lime is similar but contains magnesium instead of calcium. As mentioned before, calcium is responsible for structure and flocculation in the soil. Adding only dolomitic lime could increase magnesium levels. 

Add calcium-based lime every so often to acidic soil to improve the calcium ratio. Garden lime comes in a variety of options, including hydrated lime or burnt lime – all of which add calcium to the soil. 

Soil Amendments That Are Not Agricultural Lime 

Gypsum is a common soil amendment. This calcium sulfate-containing amendment is commonly advertised as a way to break up clay. Calcium sulfate doesn’t have this ability, but it is a great amendment for adding sulfur and calcium to the soil without significantly changing the pH. 

Bone meal, made of ground-up bones, is another common calcium fertilizer amendment available to gardeners. It is an organic fertilizer that contains calcium along with other nutrients. It is comprised of a little nitrogen and more phosphorus (3-15-0 NPK) than other fertilizers, which have much more nitrogen. Bone meal is often touted as a way to promote fruiting and flowering in crops.

Wood ashes have long been a method utilized for fertilizing crops. Wood ash fertilizer comes from the remains of burning wood products. Hardwood ashes contain a good amount of calcium carbonate. If the soil is already at an acceptable pH then applying too many wood ashes can cause the development of a higher than optimal pH. 

Calcium chloride, or chlorine, is an easily soluble option often used as calcium fertilizer. I would not recommend this as a fertilizer, however, as a high concentration of chlorine is bad for a plant. Using calcium chloride as a source of calcium is a bad idea, but it is often touted as a viable option when talking about calcium amendments. Given all the information and amendment options, opt for something else. 

Similarly, calcium nitrate is an inorganic fertilizer used to add calcium to soil. Calcium nitrate is made up of a combination of limestone and nitric acid, which is mixed with ammonia. While this fertilizer does add calcium to soil, it is best to opt for a fertilizer that is made of organic matter rather than chemicals that can run off and more readily alter the pH of your soil.

Other amendments like calcium acetate, soft rock phosphate, and colloidal phosphate are available calcium amendments. Some of these are included in normal fertilizer mixes. Most are not developed with an aim to mainly supply calcium, but they have that added benefit.

If you are looking for ways to add bioavailable nutrients to your soil, we recommend the Espoma fertilizers in the Soil and Amendments section of our store.

Eggshells and Oyster Shells

An eggshell on the soil's surface
Eggshells do not immediately improve calcium in soil as they have to break down. Source: cobaltfish

These two have often been touted as fertilizers that can simply be thrown into the garden, much like bone meal or wood ash. While eggshells and oyster shells do contain calcium, they would only be available to a plant once broken down. Plants take up calcium in its cation form, not by way of a compound. This means the calcium needs to be isolated via reactions over time to be absorbed into roots and plant tissues. 

This process takes time, so the calcium would not be available for an entire growing season. Ground eggshells would degrade quicker than whole eggshells but still take a while to break down. Other organic amendments add calcium to the soil more quickly and efficiently. However, you can add these to your garden to support next season’s growth. 

Water, Water, Water

Calcium is a critical part of plant nutrition, promoting strong growth, cell division, cell elongation, and enzymatic activation. Overall, like many soil matters, calcium has been misrepresented by many on the internet. 

Eggshells are constantly advertised as the solution to blossom-end rot. While eggshells ultimately won’t harm the garden, they will not immediately help either. Many of us apply garden lime to the soil which will add calcium, but the key is watering consistently and sufficiently.

Not only does proper watering help with nutrient uptake in the early season growth of your tomato seedlings, but it’s also often the best solution for blossom end rot. 

Frequently Asked Questions

Q: How can I get calcium in my soil?

A: If you lime your soil, you are already adding calcium in the form of calcium carbonate. As I have explained, for most people reading this, the calcium levels in their garden are most likely perfectly fine. The problem lies in inconsistent watering, which prevents the uptake of calcium. 

Q: What does high calcium in soil mean?

A: High calcium means the soil is most likely calcitic. Have your soil tested and look for a pH that hovers around 8.0. While this pH should not impact the growth of the plant significantly, a pH soil test may give you some idea of how much of this nutrient is present. 

Q: Is high calcium in soil good?

A: Calcium, along with other nutrients, is an important part of soils. Too much of anything has the potential to be bad for the soil solution as a whole. Calcium at high levels can indicate other problems like increased pH. 

Q: What is the best source of calcium for plants?

A: Calcium is taken up as the Ca2+ cation. Organic sources of this nutrient are best for most gardens – especially organic gardens. See the treatment options listed above for info about organic amendments. 

Q: What is a good calcium level in soil?

A: There is not a right answer to this question. Looking up soil studies about the kind of soil you grow in should tell you what the average amount of calcium is in that soil type. Your climate and ecosystem are usually general indicators of calcium levels. 

Q: Does calcium change soil pH?

A: Calcium is not the reason pH increases with lime applications. The carbonate in calcium carbonate causes reactions with the soil chemistry that raises pH. 

Q: Is clay soil high in calcium?

A:  Clay soils will have more clay particles that conduct cation exchange. Calcium deficient soils often occur in areas with very low pH or sandy soils. Low pH makes the Ca less available, much like other cations in the soil solution. Sandy soils have less clay to conduct cation exchange, making it difficult to hold calcium in the soil. This is why only certain plants do well in xeriscaped gardens, for instance.

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