The macronutrients and micronutrients covered earlier are the main twelve essential for plant survival and growth, and the ones routinely included in complete fertilisers. A few other nutrients not often mentioned also have beneficial roles, and these are chlorine (Cl), silicon (Si), cobalt (Co), nickel (Ni) and vanadium (V). (Chlorine (Cl) is actually regarded as an essential micronutrient these days.)

Chlorine (Cl)

Elemental chlorine (Cl), like nitrogen (N), exists as a diatomic gas, Cl₂, but it’s the chloride ion Cl^- that plants uptake from soil. (Note that chloride refers to the ion while chlorine refers to the atom.) The chloride ion is widely distributed and rapidly recycled in nature, but does not adsorb to soil minerals and is readily leached.

As chloride is a component of regular salt (sodium chloride, NaCl), it can be found in high concentrations in soils exposed to salty water, whether from oceans, rivers or bore water.

Chlorine is important in photosynthesis as it regulates the opening and closing of the leaf pores (stomata) that allow gaseous exchange of carbon dioxide, oxygen and water vapour. Chlorine also aids in keeping leaves stiff, maximising surface area available for collection of light.

A deficiency can develop in inland, heavy rainfall regions or soils that are overly-irrigated. A deficiency shows as blotchy chlorosis (breakdown of chlorophyll leading to pale leaf colour) and necrosis (death) of leaf tissue. A chloride-induced chlorosis differs from others like nitrogen or iron in that there is a very clear boundary between healthy and affected tissue.

A chlorine toxicity is more likely in soils exposed to salt, and shows as stunted growth, smaller leaves than usual, and leaves with necrotic margins (edges). These symptoms usually appear on older leaves first.

Silicon (Si)

Silicon (Si) is the second-most abundant element in the Earth’s crust after oxygen (O). Pure silicon as an element is rare on Earth, but makes up over 90% of the crust in the form of silicate minerals, or rock minerals made of silicon and oxygen compounds. Silicon dioxide (SiO₂), or silica, is the major component of sand.

Despite its abundance, silicon isn’t always available in soil solution. Quartz minerals, which are of a hard crystalline SiO₂ structure, are extremely resistant to weathering for example. Soils with more weatherable silicate minerals will have more available silicon than those that don’t.

While not an essential element, in that most plants can grow well and complete a life cycle without silicon, some plants (especially rice) do respond to increased availability.

Plants uptake silicon in the form of monosilicic acid [Si(OH)₄]. Silicon deposits in stems’ epidermal layers, which strengthens them and improves drought tolerance and resistance to wilting. Such ‘reinforced’ stems are also more resistant to fungal and insect attacks.  Silicon can also benefit a plant by increasing its tolerance to high levels of manganese (Mn) — similarly a silicon deficiency can increase the likelihood of a manganese, copper (Cu), or iron (Fe) toxicity occurring.

Applications of silica-containing fertilisers can increase the availability of phosphate.

Silicon toxicity is not common, but too much available silicon can interfere with the uptake of essential nutrients.

Cobalt (Co)

Cobalt in soil lies within a range of 1 – 40 ppm (parts per million) but typically makes up just 0.02 – 0.5 ppm of dry plant tissue. This trace element is not an essential nutrient for plants so much as it is for other organisms, thus a plant deficiency in cobalt can create deficiencies in those that eat the plants. Cobalt is needed to make vitamin B12, for example. (We cannot make this vitamin but do obtain it via other organisms that can. Plant-eating animals’ meat, milk and eggs are good sources of vitamin B12.)

Cobalt is also needed by the nitrogen-fixing bacteria that associate with legumes, thus a deficiency may lead to a nitrogen (N) deficiency and reduced growth in legumous plants.

High levels of cobalt can produce an iron deficiency, while a cobalt toxicity in its own right is marked by leaf loss and pale-coloured leaves with discoloured veins.

Nickel (Ni)

Nickel (Ni), like chlorine, has been recently regarded as a micronutrient, but some older books will classify it as a toxin. It is an element that walks a fine line, so to speak, between being of a beneficial soil concentration and a toxic one.

A nickel deficiency can create non-viable seeds and a reduced crop yield, while too much nickel can impede iron uptake. Too much nickel can also inhibit seed germination and root and shoot growth. Decreased photosynthesis and deformed flowers can also arise from high nickel levels.

Vanadium (V)

Vanadium occurs in soil at around 10 – 250 ppm depending on soil type, with an average of 110 ppm in the Earth’s crust overall. Vanadium is not needed by plants per se, and does not have much of a role in biology generally. But it is included here for one very important reason: it is a vital component of vanadium nitrogenase, an important enzyme used by nitrogen-fixing bacteria to fix nitrogen. This enzyme converts nitrogen gas from the atmosphere into ammonia which is then further converted into plant-available ammonium by other microbes. (Having said this, nitrogen-fixers do preferentially use molybdenum nitrogenase when molybdenum (Mo) is available, but switch to vanadium nitrogenase when this is not.)