Emphasize the basic concept: One cannot harvest a crop without, as a result, also removing those mineral nutrients (chemical elements) that the plants withdrew from the soil. Therefore, adding these elements back to the soil (fertilizer in one form or another) is essential. For most of commercial agriculture, mineral nutrients are mined from resources and blended into the fertilizer. Nitrogen is a peculiar case: Plants other than legumes cannot utilize the nitrogen gas present in the air. It must be made into a “mineral” compound as explained in the following video.