A vegetable grower notices that his tomato plants are producing fruits with a hard, dark, sunken patch at the bottom (blossom) end, even though he has been regularly applying nitrogen and potassium fertiliser. An agriculture extension officer explains that this disorder is not caused by any fungus, bacterium or virus, but instead results from an irregular supply of a specific nutrient to the rapidly growing fruit tissue, often aggravated by inconsistent irrigation. Which nutrient deficiency is most likely responsible for this disorder?