Page:Tree Crops; A Permanent Agriculture (1929).pdf/356

 gen from the legumes. Thus nut trees, including oaks, might be permanently interplanted with the leguminous honey locust.

Tree crops have unusual merits for agriculture in some lands too dry for plow farming. If a competition were opened for the driest farmers in the world, I should enter as promising contestants the Berbers who live in the Matmata section of central Tunis. Their average rainfall is about eight inches a year. It is often less than this, yet they are the owners of the finest olive trees I have seen in my journeys through Spain, Portugal, Algeria, Tunis, Italy, Palestine, and Syria. These trees are of record-breaking excellence though growing in a climate of record-breaking aridity. Why? The Berbers build dams of dry stone wall across gullies in a limestone plateau (Fig. 117). At every sudden shower, water rushes down the gullies sweeping a certain amount of loose soil. This catches behind the dams. Olive trees are planted in this soft earth. Every shower that produces a run-off in the gullies soaks this evergrowing mass of collected top soil so that one-half inch of rain may give these trees in the rich gully pockets the equivalent of six, eight, or ten inches of rainfall because of the thorough soaking of the collected soil mass.

This practice of gully-shower irrigation could be used in the arid parts of America and every other continent. In a certain modified sense, it has already been copied in America. A Montana bulletin describes the building of barrages and the