Calcium and manganese are two soil abundant elements that are often not as plant available and may be deficient in plant cells. Calcium is used in cell wall membranes and often becomes limiting during critical pollination periods when cells are rapidly dividing. Manganese is used in photosynthesis to split the water molecule (H20) into H+ and OH.

Grain and fruit size are determined by calcium after pollination. The cell division process proceeds rapidly, lasting 5-40 days but most grain crops have a 10-14 day cell division window. Cell division occurs exponentially (2-4-8-16-32-64 etc.) as cells divide so calcium may become deficient quickly after pollination. After cell division, grain or fruit fill occurs as the cell is filled with proteins, sugars, and water.  A lack of calcium can limit cell division, grain or fruit size, and reduce yields.

 

https://www.ocj.com/2020/06/calcium-and-manganese-deficiency/