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 (CO32- 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 (PO43-) 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.
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Comment from: airlie Visitor
You say - "As the organic matter decomposes, CO2 is released into the soil."
It’s been my understanding that decomposing organic matter releases its CO2 into the atmosphere - not the soil. Isn’t that what the current climate change/global warming issue is all about?
The climate change thing is more about burning fossil fuels, which does release CO2 into the atmosphere as this is where they’re burnt (ie, whether in a plane, car, factory, whatever, they are still ultimately being burnt in the greater environment that is the atmosphere).
But soil can be regarded as a unique environment too (’environmental medium’ is a better phrase). (The sea is another medium again.)
Once you realise these regions are their own environments, things make a lot more sense! It also helps to think small - really small! I’m talking microscopic and atomic levels here. (That is so second-nature for me that I really do have to stop and realise that it is not an obvious concept for most people.)
So think of soil as not some solid non-divisible lump, but as something made up of many small particles of sand, silt, clay, and other particles including very small pieces of organic matter that have been broken up by (eg) earthworms and arthropods. Don’t forget too that these pieces don’t fit together tightly like bricks, and have air pockets between them.
Now imagine tiny bacteria and fungi chomping away on those small pieces of organic matter. Just as we expire CO2 from metabolism, so do they.
CO2 is heavier than air, and unless soil is physically disturbed, that CO2 is not likely to enter the atmosphere. Those molecules will stay where they are, or perhaps diffuse into those air pockets mentioned above, ready to react with other molecules encountered.
You might be thinking of peatlands releasing vast amounts of CO2 into the air? This is only because they are being disturbed, eg by draining or burning the peat. If left alone, these are actually very good carbon sinks.
Thanks for that - this snippet, taken from Wikipedia - confirms (in part only as I note the word ’some’ is used) - your above comment:
"The main natural carbon sinks are plants, the ocean and soil. Plants grab carbon dioxide from the atmosphere to use in photosynthesis; some of this carbon is transferred to soil as plants die and decompose. The oceans are a major carbon storage system for carbon dioxide."
It would be interesting if it could be determined as to exactly how much of the CO2 remains in the soil and how much is released into the atmosphere. Any views?
BTW, a most impressive academic history…
Here’s a succinct pdf showing concentrations of CO2 at different depths:
0.03% in air, and up to as much as 5% about a metre down in warm, wet soils.
Interesting article, but I don’t think it answers the question as to how much of the (100%) CO2 contained in decomposing organic matter remains in the soil and how much is released into the atmosphere.
CO2 isn’t contained in organic matter, it is a byproduct of decomposition of the carbohydrates, fats,and proteins in that organic matter. Like CO2 isn’t in the food we eat, but we still exhale it as a byproduct.
Another complication is what constitutes ‘organic matter’ is going to differ from environment to environment. Central Australian desert vs lush Amazonian rainforests for example. One will have not much organic matter at all, if any, while the other abounds in it.
Another complication is the type it is made of. Very woody material will take longer to decompose than the same mass of leaf litter.
Still another complication is the composition of the soil - what materials are in it that can or can’t react with CO2 will determine how much CO2 is left.
And still another complication is the temperature, amount of moisture, and amount of oxygen in the soil, and how these influence the microbial, arthropod and earthworm populations that break organic matter down.
So unfortunately there’s no one answer as to how much CO2 stays in soil and how much enters the atmosphere except, it depends!
When I first raised the question, I knew really that there is no specifically correct answer, but, it keeps you on your toes!
Gee, thanks, I think!!