In many areas of the world, food shortages are common. to improve crop growth, what should be done to acidic soil

In acid soils you can select the crops that can growth in this conditions (pH 6.0 - 6.5 is slightly acidic, and you can find cultures that can be cropped under this conditions). If soil is acid to highly acidic , in some areas of Northern Spain the use to increase the pH by liming the soil in a gradual / gentle way. You can find different product for liming fron rocks, wastes (from marble industries; animal bones; etc