Legumes

Often land which was once thought excellent is left to grow up in weeds. The owner says that the land is worn out, and that it will not pay to plant it. What does "worn out" mean? Simply that constant cropping has used up the plant food in the land. Therefore, plants on worn-out land are too nearly starved to yield bountifully. Such wearing out is so easily prevented that no owner ought ever to allow his land to become poverty-stricken. But in case this misfortune has happened, how can the land be again made fertile?

Source: Agriculture for Beginners