Last week’s discussion on iron became very ‘chemical’ very quickly with quite a crash-course on oxidation, reduction and oxidation numbers thrown in out of nowhere! I do appreciate that it would have been heavy going for people without a chemistry background beyond yr 8 – 10 at school.

But at the same time I feel it’s important to not gloss over that kind of detail if you really want to understand the interactions in soil that enable nutrients to enter (or not enter) plants.

Let’s quickly revisit the Periodic Table. The micronutrient iron is element number 26, and immediately to its left is element 25, manganese (Mn), the topic of this post. These two elements differ by just one proton in size and have several properties in common. Iron and manganese tend to pop up a lot together in soil interactions as you’ll see.

Manganese, like iron, is a metal. In fact, all the trace elements, with the exception of boron (B), are. (Boron is a metalloid, with the properties of both metals and non-metals). (Just half of the macronutrients are metals: potassium (K), magnesium (Mg) and calcium (Ca). Nitrogen (N), phosphorus (P) and sulfur are non-metals.)

Manganese, again as with iron, has more than one oxidation state (though this is by no means unique to these two and is very common for many elements). Mn²⁺, Mn³⁺ and Mn⁴⁺ ions are the common ones in soil, and the levels of each are very dependent on which oxidation-reduction reactions occur. And just as we can write Fe(II) and Fe(III) for iron, so too can we write Mn(II), Mn(III) and Mn(IV) for manganese. (And it’s easier to type too!)

Availability to Plants

Manganese in soil derives from weathered rocks, and is typically in the form of Mn(II) ions, manganese(IV) oxide (MnO₂), manganese(III) oxide (Mn₂O₃) and manganese(III) oxide-hydroxide [MnO(OH)]. There is a manganese cycle in soil involving these three oxidation states due to reduction and oxidation processes:

Mn²⁺ may oxidise to Mn⁴⁺ (by losing two electrons and forming MnO₂) or it may oxidise to Mn³⁺ (by losing one electron and forming a Mn(III) oxide)

Mn⁴⁺ in turn may reduce to Mn³⁺ (by gaining an electron and forming a Mn(III) oxide) or it may reduce to Mn²⁺ by gaining two electrons

Mn³⁺ in turn may reduce to Mn²⁺ (by gaining an electron) or it may oxidise to Mn⁴⁺ (by losing an electron)

These oxidation-reduction reactions are heavily determined by soil pH, the amount of organic matter, the amount of soil water, and microbial activity.

Mn²⁺ is the most important form for plants. It adsorbs onto clay minerals and organic matter, and enters an equilibrium with the Mn²⁺ dissolved in soil water. In waterlogged soils, the absence of oxygen drives reduction, and causes Mn²⁺ levels to rise. In waterlogged acid soils (pH less than 7), Mn²⁺ can rise to toxic levels, as low pH increases the solubility of manganese compounds. The solubility of Mn²⁺ decreases 100-fold for each unit increase in pH, to the extent that in high pH soils (alkaline, or calcareous soils with pH greater than 7), Mn²⁺ availability may not be high enough for a plant’s needs.

Increased pH also favours Mn²⁺-organic matter complexes which further locks manganese away.

Some soil microbes oxidise manganese to obtain energy (known as chemotrophy, or ‘nourishment from chemicals’). These have optimum activity around pH 7, which would reduce manganese availability in soils at this range.

Mn²⁺ in acid soils is readily leached.

Manganese in Biochemistry

Manganese is needed to activate many enzyme reactions. It is involved in the splitting of water molecules into hydrogen and oxygen for photosynthesis, and it also has roles in respiration and nitrogen assimilation. Pollen germination and pollen tube growth both require manganese, as do root cell elongation and lignin synthesis. Lignin in root cells acts as a barrier against pathogenic attack by soil fungi.

Manganese Deficiency Symptoms

A manganese deficiency shows as interveinal chlorosis (insufficient production of chlorophyll between the veins), with yellow leaves with green veins. This chlorosis resembles iron and magnesium deficiencies.

Magnesium is mobile in a plant, whereas manganese is not. Thus a magnesium deficiency shows up first in older leaves but a manganese deficiency shows up first in newer leaves.

Iron however, and like manganese, is not very mobile in a plant. It is harder to diagnose an iron deficiency from a manganese one, as deficiencies in both appear in newer leaves first. Often though, the entire leaf turns yellow in an iron deficiency, while a manganese deficiency is marked by more spotted leaves except in severe cases. Another difference is that light brown regions may appear between the veins in a manganese deficiency, while in an iron deficiency this region may turn a more uniform pale-yellow/almost-white.

Manganese Toxicity Symptoms

Manganese toxicity symptoms can resemble iron toxicity ones. Tips and edges of leaves have a burnt look, and dark-brown to black or purple spots appear on older leaves, surrounded by chlorotic areas.

Mn2+ ions compete with iron (Fe²⁺), magnesium (Mg²⁺) and calcium (Ca²⁺) ions for uptake, and an excess of manganese may create deficiencies of the others.

And just as excess iron can lead to a manganese deficiency, but be misdiagnosed as an iron deficiency, so too can excess manganese lead to an iron deficiency and be misdiagnosed as a manganese one.