Many agricultural soils contain abundant levels of manganese locked up in soil reserves. Yet, for the reasons of selective suppression of biology, oxidation and immobilization discussed in these blog posts most crops are constantly manganese deficient, which limits water hydrolysis for photosynthesis.
The reasons above are why field research seldom shows a positive crop response from manganese. In the short term, until microbial populations are established which can release the unavailable reserves of soil manganese, the only effective solution is to supply manganese with foliar feeding so it does not limit photosynthesis.
The symptoms of subtle manganese deficiency have become so commonplace today we don’t recognize them as anything outside of the ordinary.
When plants have adequate levels of manganese, the leaf veins will be the same shade of green as the area between the veins.
These cotton leaves are showing light colored veins, symptoms of inadequate manganese. Once you begin looking, you can see this almost everywhere, in undomesticated plants as well as crops. There are a very few plants which have light colored midribs genetically, even those will darken when supplied with manganese.
How many plants have you observed that did not have light colored veins?