If a fruit or vegetable isn’t grown in dirt, can it be organic? That is the question roiling the world of organic farming, and the answer could redefine what it means to farm organically. At issue is whether produce that relies solely on irrigation to deliver nutrients to plants — through what is known as hydroponic and aquaponic systems — can be certified organic. And the National Organic Standards Board, an advisory group that makes recommendations to the federal secretary of agriculture, will get an earful on the topic at its meeting in St. Louis this week. On one side are the growing number of big and small growers raising fruits and vegetables in these soil-free systems. They say their production methods are no different from those of farmers who grow plants in dirt — and, they add, they make organic farming more sustainable by, for instance, reducing water use. “Soil to me as a farmer means a nutrient-rich medium that contains biological processes, and that doesn’t have to be dirt,” said Marianne Cufone, an aquaponic farmer and the executive director of the Recirculating Farms Coalition, which lobbies for aquaculture. Not so, say the farmers who have spent years tending their soil so that it produces the nutrients plants need. They argue that organic production is first and foremost about caring for the soil, which produces environmental benefits that go beyond growing plants.