Growing the same crop repeatedly in the same place eventually depletes the soil of various nutrients. One way that farmers can avoid a decrease in soil fertility is to practice crop rotation, by which different crops are planted in a regular sequence so that a crop that leaches the soil of one kind of nutrient is followed during the next growing season by a crop that returns that nutrient to the soil. If crop rotation is done properly, farmers can keep their fields under continuous production, without a need to let them lie fallow or to apply artificial fertilizers, both of which can be expensive.
Plants of the family Fabaceae, for instance, have nodules on their roots which contain nitrogen-fixing bacteria. It therefore makes good sense agriculturally to alternate them with Poaceae and other plants that need nitrogen. It also makes good nutritional sense to grow beans and grain at the same time in different fields.