The statement, little of the nitrogen produced by the crop is available to subsequent crops does not result in the conclusion that the beans only produce nitrogen for themselves. The correct conclusion is that the legumes not only produce sufficient nitrogen for pod production but also produce excess, leaving some for subsequent crops, although that is little compared the nitrogen they use. They make a net contribution of nitrogen to the soil beyond their own needs. Other non-legume crops around them deplete nitrogen from the soil, so the relative benefit is therefore much greater.