Eli5 why plants which need nitrogen from the ground do not take it from the air which is 70% Nitrogen
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A useful analogy here is burning wood.
Why don’t dry forests burst into flame when there’s all those dry needles or leaves and wood around? The answer is because it takes a certain amount of energy to activate the process of chemically converting wood into carbon dioxide and water vapor gases, so you need to invest that energy to start the process.
Nitrogen has the same initial problem. In order to use it, plants need to bring it out of the air and convert it into a more useful form. But plants don’t have the high amounts of energy required to perform that process.
Some plants have developed a relationship with certain bacteria that can do this special conversion for them. Clover is a good example – it has nodules on its roots that these bacteria live in. They specialize in grabbing nitrogen and converting it into a more useful form that the plant can take advantage of. And that’s why farmers like to plant clover to let a field “rest” for a season – it’s effectively recharging the soil.
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