Manganese

The presence and concentration of manganese in plants is an important trace element, and is linked to processes such as electron transport, chlorophyll production and enzyme activation. It deactivates harmful free radicals and is highly dependent on soil pH, where a high pH causes a manganese deficiency and a low pH causes manganese toxicity. 

Manganese shows similar properties to other alkaline soil cations, for example calcium, magnesium, zinc and iron, and as such the concentrations of these ions are strongly correlated to the concentration of manganese uptake by plants. Manganese deficiencies usually occur in arid or semi-arid regions and cause the efficiency of photosynthesis to decrease, meaning crop yields and quality also suffer. The premature aging of old leaves (senescence) and dark spots on leaves are a sign of insufficient manganese. Manganese toxicity also reduces photosynthesis, causing leaf necrosis, plant roots turning brown and brown spots to appear on the leaves. The most common form of manganese to add to soil is manganese sulfate (MnSO4·3H2O) and manganese oxide (MnO). 

Hooda, P. (2010). Trace elements in soils. Wiley.