Liebig’s Law of the Minimum: plants need many nutrients, but their growth rate is limited by the one least present in the soil. In most cases, that nutrient is nitrogen. At first blush, the notion of nitrogen being a limit seems odd; there is more nitrogen in the world than carbon, oxygen, phosphorus, and sulfur combined. Unfortunately, more than 99 percent of that nitrogen is nitrogen gas. Nitrogen gas—N2 in chemical notation—consists of two nitrogen atoms bound together so tightly that plants cannot split them apart for use. Instead, plants are able to absorb nitrogen only when it is in
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