Finishing off the secondary nutrients we will combine the post to cover both magnesium and sulfur since there are far less talking points on these compared to other nutrients. The levels of these nutrients are well below those of the macronutrients or calcium.
Sulfur is Found in Some Secondary Metabolites
Sulfur is generally associated with foul smells from rotten eggs to burned hair so it stands to reason that compounds which contain it also have a strong taste. The most noteworthy of these are thioketones in onions and garlic which give the characteristic flavour and glucosinolates which are responsible for the hot spicy taste of mustard and horseradish.
*Did you know?*
Chopping onions causes eye irritation resulting in tears. This comes from propanethial S-oxide which breaks down into a weak sulfur based acid which in turn irritates the eyes causing them to water.
The Primary Function of Sulfur is Very Limited
This is not to imply that it is not important however. Sulfur is a component of cystine, cysteine, and methionine which are essential amino acids which in turn form proteins. These form the basis of any number of metabolic reactions and structural components both in plants as well as other types of organisms.
Volcanoes are a Significant Source of Sulfur Fertilization in Nature
Most people realize that volcanic soil tends to be some of the most fertile in the world. In the short term a volcanic eruption is destructive, however in the longer term it is helpful to the environment since new nutrients are brought to the surface from deep in the earth. Unlike the other nutrients expelled in an eruption, sulfur gets dispersed over a very wide range as it's released as sulfur dioxide gas which then comes down in the rain.
Magnesium's role in Photosynthesis
Chlorophyll is a large photosensitive molecule very similar in structure to hemoglobin which is found in the blood of all vertebrates. The key difference however is that hemoglobin uses iron as its central part of the structure and chlorophyll uses magnesium instead. Chlorophyll is able to trap the energy in light and help to convert it into chemical energy in the form of sugars which the plant can then use as energy or as building blocks. Because chlorophyll is green a magnesium deficiency will show up as a yellowing of the leaves.
Magnesium is Calciums "little brother"
Chemically calcium and magnesium are quite similar. Calcium has some solubility issues and is incompatible with many nutrients especially phosphates and sulfates. Since they are related magnesium is able to stay in solution together with calcium. The popularity of "Cal Mag" products is partially due to the fact that there are so few elements that can be blended together with calcium.
Where they differ however is the fact that magnesium is able to stay in solution with most other nutrients. The most common source is magnesium sulfate (epsom salt) which is completely soluble whereas calcium sulfate is nearly insoluble. Phosphates on the other hand are a bit more complicated, magnesium won't react with monopotassium phosphate but will fall out when mixed with the ammonium phosphates.