Potential fruit or grain size is determined during the cell division period immediately after pollination. The cell division process can continue for as little as 5 days, to as long as 40 days, but most crops have a 10-14 day cell division window. During this window, the cells in the embryo are rapidly dividing, 2-4-8-16-32-64 and so on. At the end of this 10-14 day window, cell division stops completely, and the remainder of the fruit fill or grain fill period is focused on cell expansion, filling each cell with proteins, sugars, and water.
Fruit that are tightly packed with more smaller size cells are firmer, store better, are crisp, and crunchy. Fruit with more cells can be much larger in size when all the cells are filled with water and nutrients. Fruit with more cells are resistant to cracking and splitting. In general, almost all the fruit quality characteristics we seek can be improved by increasing the number of cells formed during the cell division period, with the exception of some fruit where excessive size is a negative.
The nutritional factor which limits the number of cells formed during the cell division period is calcium, because calcium is needed to form the cell membranes for all the rapidly dividing cells in the fruit embryo.
An easy step to produce exceptional quality and yield is to ensure a peak of available calcium during the cell division period.
This means any soil applications of calcium need to be timed so the peak of the release curve coincides with the crops peak demand curve during cell division. Applying gypsum or limestone on tree fruit in the spring is much less effective than a fall application, because it doesn’t release quickly enough to be available during cell division right after pollination.
In almost all cases, when fruit express physiological symptoms of inadequate calcium, which we call blossom end rot, bitter pit, or cork, it is because there is inadequate calcium supply during the cell division period.
Quite often, this inadequate calcium level in the plant or in the embryo may not be the result of low calcium in the soil. Poor calcium absorption can be the result of excessive potassium, low boron, or low manganese availability in the soil. Any of these conditions will limit calcium absorption, and thus negatively impact fruit quality.
I have been framing the discussion around fruit, but these concepts hold equally true for grain crops.
These grapes are still 3-4 weeks from harvest, and each berry is about 60% of mature size. When was the last time you bought grapes like these? Would you like to grow crops at an equivalent level of health and quality? If so, managing calcium and the associated nutrient interactions during the cell division stage becomes a top priority.
The first thing you will cut is all potassium applications until after the cell division stage is completed. To achieve this, you likely needless fertilizer application, not more. And most likely also timed very differently.