Hello Helen. Sorry for the delay of this reply, caused by a busy schedule.  A number of crops (beans, peas, etc.) belong to the leguminous plants, known for their ability to stock nitrogen from the air and to transform it into proteins, which are then broken down to nitrates, fertilizing the soil.  Such nitrogen fixating crops are very useful indeed.  They should be grown at a larger scale to fertilize the soil, particularly in the drylands.  So, it is not your imagination !

Prof. Dr. Willem Van Cotthem Beeweg 36 - B 9080 Zaffelare (Belgium)