Mineral Nutrition

Biological nitrogen fixation, the reduction of atmospheric $N_2$ to ammonia, is catalysed by the enzyme nitrogenase:
$N_2 + 8e^- + 8H^+ + 16ATP \xrightarrow{nitrogenase} 2NH_3 + H_2 + 16ADP + 16P_i$
It is a Mo–Fe protein found in nitrogen-fixing bacteria such as Rhizobium, Azotobacter and cyanobacteria.
Nitrate reductase reduces nitrate, and transaminases transfer amino groups.
So the first enzyme of nitrogen fixation is nitrogenase.

The middle lamella, which cements adjacent plant cells together, is made mainly of calcium pectate (and magnesium pectate).
So calcium is essential for the formation of the middle lamella. It is also needed for spindle formation and membrane function.
So the element necessary for the middle lamella is Ca.

Most crop plants grow best in soil that is nearly neutral, with pH 6.5 to 7.5.
In this range most mineral nutrients are readily available, and useful soil microbes (e.g. nitrifying bacteria) are most active.
In strongly acidic soils, Al and Mn become toxic and P, Ca and Mg are less available; in strongly alkaline soils, Fe, Mn, Zn and P become unavailable.
So the best pH of the soil for cultivation is 6.5 – 7.5.

Root nodules of leguminous plants such as soybean contain a pink-red pigment called leghaemoglobin.
It has a high affinity for oxygen and binds free oxygen, protecting the oxygen-sensitive enzyme nitrogenase, while still supplying oxygen for respiration of the bacteroids.
So the roots of soybean contain this red pigment.