Calcium in Soil: Uses and Signs of Deficiency
Most gardens have calcium in soil blends or native soils, but what does it do? Soil expert Ben Anderson dives deep into what calcium provides for plant growth and how to identify deficiencies.
Contents
In humans, calcium is essential for building strong bones and teeth. Similarly, in soil, calcium plays a crucial role in maintaining structure and supporting healthy plant growth throughout the season. As an important macronutrient, it is a key component of plant nutrition.
This article will explore calcium’s role as a plant nutrient, how to identify a calcium deficiency, and its impact on soil structure. As with most soil-related topics, factors such as climate, soil texture, and organic matter will all come into play.
Let’s take a deep dive into this essential nutrient and discuss the best ways to add calcium to the soil.
Calcium as a Plant Nutrient
Calcium serves several essential functions in plants. It helps build strong cell walls, providing the rigid structure and protection that plants need. It also influences the permeability of cell membranes, helping to prevent toxic compounds from penetrating plant cells. Additionally, calcium activates certain enzymes involved in biological reactions and plays a crucial role in cell division and elongation.
To understand how plants absorb calcium, it’s important to first look at the vascular system, which consists of the xylem and phloem. The xylem transports water and dissolved minerals from the roots to the stems and leaves, while the phloem distributes sugars and nutrients produced through photosynthesis to other parts of the plant.
Although plant vascular systems are highly complex, calcium uptake follows a specific process. Calcium cations (positively charged ions) are absorbed by young root tissues, which have more permeable membranes than older roots. From there, the xylem carries calcium upward to support plant growth and development.
Which Plants Need Calcium?
All plants use this nutrient, but the amount of calcium needed 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 diagnosing nutrient deficiencies, it’s important to understand how calcium interacts with each plant. Some deficiencies present similar symptoms but appear on different parts of the plant. For example, one deficiency may affect only older leaves, while another affects new growth. These small details are crucial for accurate diagnosis.
A classic example of calcium deficiency is blossom end rot in tomatoes. This condition is easily recognizable, as the bottom of the fruit develops a rotten appearance due to poor calcium distribution. Since calcium is not efficiently transferred to the fruit, deficiencies become visible in that area.
Calcium deficiencies can also affect growing points. New leaves may appear stuck together, and root growth can be stunted due to inadequate calcium levels. Since examining roots is more difficult, checking the growing tips and fruit is often the easiest way to identify a calcium deficiency.
Calcium’s Role in the Soil
Calcium is essential for maintaining healthy soil structure, particularly through a process called flocculation. Flocculation occurs when calcium pulls clay particles together, helping them separate from other soil particles. This can be observed in a basic home soil test, where soil is shaken in water. Well-flocculated clay aggregates settle, leaving the water clearer compared to soil with fewer aggregates.
Flocculation plays a crucial role in reducing runoff and erosion, as water leaving the soil is clearer and carries fewer sediments. Among soil cations, calcium is the most effective at binding clay particles together. While magnesium also has a +2 charge and shares some similarities with calcium, the two differ in their effects on plant function and soil structure, with calcium being the superior flocculant.
How Calcium Cycles
Calcium enters the soil in several ways. One is through the breakdown of minerals like calcite, limestone, and dolomite. These and other soil parent materials release calcium into the soil. Calcium is a base cation essential for cation exchange, which refers to the soil’s ability to hold and exchange positively charged ions. Cation exchange capacity (CEC) measures the number of cations the soil can retain.
The exchange process occurs as calcium ions (Ca²⁺) trade places between soil particles and the soil solution. Many other cations are also involved in this process, each with varying exchange abilities. Cation exchange capacity is one way calcium becomes available to plants.
Another way calcium naturally enters the soil is through atmospheric deposition, where soot and ash settle into the soil, contributing calcium in the process.
Calcium can also leave the soil in two ways. Leaching occurs when water washes away calcium that isn’t bound to clay particles. Additionally, plants absorb calcium from the soil as they grow, removing it from the system.
Why Your Soil is Not Calcium Deficient
Seeing blossom end rot may lead you to believe your soil lacks calcium, but in most cases, this isn’t true. Most soils contain enough calcium to support plant growth. The primary cause of blossom end rot is typically insufficient or inconsistent watering in the root zone.
As mentioned earlier, calcium is absorbed by young roots through water. Once root cells mature, they can no longer take in calcium. Consistent watering ensures calcium uptake and proper distribution to the fruit. Additionally, most garden soils, substrate mixes, and generic fertilizers already contain calcium, making an actual deficiency even less likely.
One scenario where calcium might be unavailable is in highly acidic soils. When soil pH drops below 4.5, aluminum, iron, and manganese toxicity can occur. In these conditions, calcium binds tightly to clay particles, making it inaccessible to plant roots.
This issue is usually easy to identify. In highly acidic soil, several other nutrients are also deficient, and plant growth is noticeably poor. A soil test is the only way to confirm whether your soil lacks calcium.
Extremely sandy soils can also contribute to calcium deficiency, as they contain fewer clay particles to bind to calcium. However, it is unlikely that garden crops would be grown in soil this sandy. In both acidic and sandy soils, overall nutrient availability is low, making successful plant growth difficult.
Calcareous Soil
While rare, excessive 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 but generally do not significantly hinder healthy plant growth.
It’s important to note that calcium itself is not the cause of increased pH. Instead, the carbonate in lime is responsible for raising soil pH, though calcium is often mistakenly blamed.
Treating a Calcium Deficiency
Appropriate irrigation is the key to addressing most calcium deficiencies in the garden. However, if a true calcium deficiency does exist, there are several options available to correct it.
Foliar sprays, however, will not fix blossom end rot in tomatoes. This is because the deficiency is not in the leaves but in the fruit, and calcium is not effectively transported to fruit tissues through foliar application.
Soil pH
Calcitic lime and dolomitic lime are the two main amendments used to raise soil pH. When calcium is truly deficient, the soil is already extremely acidic. Of all the calcium fertilizers, calcitic lime is usually the best choice, depending on the situation. Agricultural lime is commonly used to increase soil pH.
Lime is calcium carbonate, and it is the carbonate component that raises soil pH. Dolomitic lime is similar but contains magnesium in addition to calcium. Since calcium plays a key role in soil structure and flocculation, adding only dolomitic lime could lead to an excess of magnesium.
Applying calcium-based lime periodically to acidic soil helps maintain a balanced calcium ratio. Garden lime is available in several forms, including hydrated lime and burnt lime, all of which contribute calcium to the soil.
Soil Amendments That Are Not Agricultural Lime
Gypsum is a commonly used soil amendment that contains calcium sulfate. It is often marketed as a way to break up clay, though calcium sulfate itself does not have this ability. However, gypsum is an excellent choice for adding both sulfur and calcium to the soil without significantly altering pH.
Bone meal, made from ground-up bones, is another widely available calcium amendment. As an organic fertilizer, it provides calcium along with other nutrients, particularly phosphorus (3-15-0 NPK). It contains some nitrogen but is primarily valued for promoting fruiting and flowering in crops.
Wood ashes have long been used as a natural fertilizer. Hardwood ashes contain a significant amount of calcium carbonate, making them a useful soil amendment. However, if soil pH is already balanced, excessive application of wood ash can raise pH beyond optimal levels.
Calcium chloride, a highly soluble calcium source, is sometimes promoted as a calcium fertilizer. However, due to its high chlorine content, it is not recommended for plant health. High concentrations of chlorine can be harmful, making calcium chloride a poor choice as a soil amendment.
Similarly, calcium nitrate is an inorganic fertilizer that combines limestone and nitric acid, mixed with ammonia. While it does provide calcium, it is best to choose organic sources instead, as synthetic fertilizers can contribute to runoff and more readily affect soil pH.
Other amendments, such as calcium acetate, soft rock phosphate, and colloidal phosphate, also provide calcium. While these are sometimes included in general fertilizer blends, they are not primarily formulated to supply calcium but offer it as an added benefit.
Eggshells and Oyster Shells
Eggshells and oyster shells are often promoted as fertilizers that can be easily added to the garden, much like bone meal or wood ash. While they do contain calcium, plants can only absorb it once the shells have broken down. Plants take up calcium in its cation form, not as part of a compound, meaning the calcium must be released through natural decomposition before it becomes available to plant roots.
This process takes time, so the calcium from eggshells or oyster shells is not readily available within a single growing season. Ground eggshells will degrade faster than whole eggshells but still take a significant amount of time to break down. Other organic amendments provide calcium to the soil more quickly and efficiently. However, adding eggshells or oyster shells can still benefit the garden by supporting next season’s growth.
Water, Water, Water
Calcium is a crucial component of plant nutrition, supporting strong growth, cell division, cell elongation, and enzymatic activation. However, like many soil-related topics, calcium has been widely misrepresented online.
Eggshells are frequently promoted as a solution to blossom end rot, but while they won’t harm the garden, they also won’t provide an immediate benefit. Many gardeners apply lime to the soil to add calcium, but the key to preventing blossom end rot is consistent and sufficient watering.
Proper watering not only improves nutrient uptake during the early growth stages of tomato seedlings but is also often the most effective solution for preventing blossom end rot.
Frequently Asked Questions
How can I get calcium in my soil?
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.
What does high calcium in soil mean?
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.
Is high calcium in soil good?
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.
Does calcium change soil pH?
Calcium is not the reason pH increases with lime applications. The carbonate in calcium carbonate causes reactions with the soil chemistry that raises pH.
Is clay soil high in calcium?
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.
