Elemental calcium (Ca) makes up 3.64% of the Earth’s crust, making it the fifth most common element in the crust after oxygen (O), silicon (Si), aluminium (Al) and iron (Fe). This makes it the most abundant of all plant nutrients after oxygen and iron. In dry plant tissue, calcium ranges in concentration from 5 – 30 mg per gram.

Availability to Plants

Calcium in soil comes from weathered rocks, and the amount of naturally-occurring calcium will depend on the parent rock, how much that rock has weathered, and how much leaching has occurred. For example, limestone-derived soils are usually very rich in calcium, present as calcium carbonate (CaCO₃). Yet their surfaces may be low in calcium where leaching has been high. Old, weathered soils in wet environments may be very low in calcium due to excessive leaching, while in arid regions calcium may accumulate in the upper soil in the form of gypsum (calcium sulfate dihydrate, CaSO₄.2H₂O).

Soils high in calcium carbonate are high in pH (alkaline), and calcium ions (Ca²⁺) when in the presence of high pH tend to form Ca-humus complexes. These give such soils a dark colour.

Low pH (acid) means the presence of H⁺ ions, and these increase the weathering of calcium-containing rock minerals, which releases calcium ions into the soil. H⁺ ions also exchange two-for-one with Ca²⁺ ions on soil colloids, which releases the latter into soil. These calcium ions will leach in wet climates, the rate of leaching increasing with increasing rainfall and increasing amounts of calcium-containing minerals. 200 to 300 kg of calcium per hectare typically leach from temperate soils!

Weathering of rock minerals which contain carbonates (CO₃^2- ions) is very dependent on the presence of carbon dioxide (CO₂) in the soil. Calcium carbonate (CaCO₃) is a rather insoluble compound, with just 10 – 15 mg Ca dissolving per litre of water. But in the presence of CO₂, the much more soluble calcium bicarbonate [Ca(HCO₃)₂]  is formed:

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

Ca(HCO₃)₂ dissociates into calcium (Ca²⁺) and bicarbonate (HCO₃^-) ions in the soil water and become prone to leaching.

Surprisingly, adding organic matter can also increase the leachability of calcium. As the organic matter decomposes, CO₂ is released into the soil. This is converted into carbonic acid (H₂CO₃) which dissociates into bicarbonate (HCO₃^-) and H⁺. H+ ions, as mentioned above, can exchange two-for-one with Ca²⁺ ions on soil colloids, which again releases Ca²⁺ ions into the soil where they are subject to leaching.

The formation of nitrate (NO₃^-) also affects the availability of calcium. Going back to the nitrogen cycle, nitrate forms from ammonium (NH₄⁺) via nitrification:

2NH₄⁺ + 4O₂ → 2NO₃^- + 4H⁺ + 2H₂O

This process also produces H⁺ ions. A H⁺ ion is a H⁺ ion, and whether from nitrification or carbonic acid dissociation, is equally capable of displacing calcium ions from colloids no matter what the source. H⁺ ions are produced at a greater rate from nitrification than from carbonic acid dissociation however, and this has implications wherever ammonium fertilisers are applied. Such fertilisers, through nitrification, will not only acidify the soil (acidity is a direct result of the concentration of H⁺ ions) but those same H⁺ ions will increase the removal of calcium from soil via leaching.

It follows that climatic factors that affect nitrifying microbes, such as moisture and temperature, will also affect nitrification and the rate of calcium leaching (and acidification). This is why soils in warm, humid environments (the tropics especially) naturally increase in acidity with year-long exposure of microbes to warm, wet conditions and plenty of lush green (nitrogen-rich) fallen vegetation. This abundance of organic matter also leads to the formation of carbonic acid and yet more H⁺ ions as described above.

Some soils high in organic sulfur may also become more acidic as this sulfur is converted into sulfuric acid.

Acidification may also increase in the presence of increased root production, as roots can excrete H⁺ ions in the presence of ammonium (NH₄⁺).

Fossil-fuel burning leads to the formation of acids such as sulfurous acid (H₂SO₃), sulfuric acid (H₂SO₄), nitrous acid (HNO₂) and nitric acid (HNO₃). All of these precipitate out of the atmosphere and into soil, where they rapidly break down soil minerals via chemical action and acidify the soil in the process.

Whether naturally-caused or man-made, it’s clear that soils will generally trend towards a slowly-increasing acidified state. Increasing acidification not only contributes significantly to soil degradation (for many reasons best covered in a future post), it also renders many nutrients either unavailable to plants or at toxic levels. And as soils accumulate more H⁺ ions, those ions will displace calcium, and those calcium ions will leach away in the presence of water.

A time-honoured way of ensuring calcium remains in soil, is accessible to plants, and slows the natural acidification of soil is to apply liming materials, which are both calcium-rich and very alkaline. Crushed limestone or chalk (both of mostly calcium carbonate) are the primary sources of ‘agricultural lime’. In fact, this artificial application of alkaline materials to soil by people is the only way a soil’s pH can be raised and calcium leaching reduced. As mentioned above, soils naturally become more acidic with time, and lose calcium when wetted.

The above may sound like soils are doomed to forevermore deplete in calcium without human interference, but in reality there is a calcium cycle in nature that returns calcium to soils — it is only our intensive agricultural practices that place a heavier load on the land. I’d like to cover this cycle in a future post.

Liming, and the importance of calcium to soil structure, also is very much the subject of a future post — I’m already getting carried away with the chemistry enough as it is! But it should be quite apparent that calcium is more than just a nutrient, and has a multifaceted role when it comes to soil.

Calcium in Biochemistry

Calcium is needed for cell elongation and cell division and has important roles in cell wall strength, and cell membrane stability and permeability. (Note: plant cells have an internal cell membrane and an outer cell wall. The cell wall provides structural strength. Animal cells only have a cell membrane.)

A strong cell wall is more resistant to attacks by insects and the enzymes secreted by many bacteria and fungi intended to pierce cell walls. Calcium helps maintain the structural integrity of cell membranes by regulating the ions that leave and enter cells and ensuring membranes don’t become too ‘leaky’.

Calcium has roles in enzyme and hormone processes, and helps in transpiration and heat regulation by regulating the pores in leaves called stomata (plural form of stoma).

Calcium Deficiency Symptoms

Reduced growth is one sign of a calcium deficiency as this mineral is vital in the elongation and division of cells. Calcium is not mobile in a plant, and thus a deficiency first appears in the growing tips and the youngest leaves. The leaves become deformed, chlorotic (become yellow from a lack of chlorophyll production), and at an advanced stage show necrosis along the leaf margins (edges). Affected tissues become soft as the cell walls weaken.

Growing root tips are especially affected and will die in severe cases. This has consequences for the entire plant as it is only the growing tips that are able to take up calcium ions. Anything preventing the tips from growing, such as temperature, moisture content, or soil quality, will also reduce calcium uptake.

An absolute calcium deficiency as described here is actually quite rare, but deficiencies will likely show in the fruit and storage tissues where calcium is under-supplied to a plant. Blossom-end rot of tomatoes and bitter pit in apples are two such examples. These arise as calcium is only moved through a plant via the xylem (the dedicated transport tissue in a plant, which moves water from the roots to the stems and leaves via capillary action). If the xylem sap is low in Ca²⁺, fruits being last in line to receive this sap will be the first to suffer.

High amounts of potassium (K⁺) and magnesium (Mg²⁺) will interfere with calcium uptake, even if Ca²⁺ is of sufficient amounts. Calcium should be applied at the same time if high amounts of K⁺ and Mg²⁺ are applied as fertilisers. High applications of ammonium fertilisers can acidify the soil and lead to leaching of calcium away from plant roots. Sodic soils, those containing high amounts of sodium ions (Na⁺) attached to clay particles (not to be confused with saline soils, those with salty soil water) also restrict Ca²⁺ uptake.

High applications of phosphorus will cause Ca²⁺ to react with excess phosphate (PO₄^3-) and form insoluble calcium phosphate [Ca₃(PO₄)₂], thus locking calcium away.

Calcium Toxicity Symptoms

Calcium toxicity does not exist, but soils with high levels of calcium carbonate will have a high pH, and it is this high pH that may cause problems if the supply of other nutrients is affected.