I'm asking because I want to know how we can be sure the vitamin content present in foods today are the same as when the vitamin content was determined for RDA purposes. Many people believe that the chemicals used to increase yield and the lack of replenishing the soil with nutrients in commercial farming have caused a decrease in the vitamin content of our fruits and vegetables, whereby supporting the need for vitamin supplements. Interested in Vegetable Nutrients Dear Interested in Nutrients, You're absolutely correct in saying that the mineral content of vegetables can vary depending on the amount of minerals present in the soil in which they are grown. The vitamin content of vegetables is not directly determined by the soil in which they are grown. However, soil makeup does influence vitamin content indirectly by controlling the size and quality of the plant.
Most commercial farmers use fertilizers to increase crop production.
These fertilizers contain plant nutrients which replenish the soil with phosphorus, potassium, and nitrogen. All these elements are normally needed for plants to grow well, and are removed from the soil by the plant, and lost to the soil when you harvest the produce. These fertilizers do not affect the vitamin content of the food. However, as you suggested, the genetic makeup of the vegetables does determine the level of vitamins found in the vegetables. Also, the maturity and time of harvest can affect the vitamin content of a vegetable.