India, June 17 -- In Luweero, 70 kilometres north of Kampala, Uganda, coffee farmer Gaddafi Nassour has doubled his income in a few years. The change was simple: A diesel pump gave way to a solar one, and water became available throughout the dry season. Over its working life, the solar pump will cost him half as much as the diesel one, and ask for far less maintenance. Stories like Nassour's are still an exception. Sub-Saharan Africa irrigates only 4% of its cropland; India irrigates 55%. The cost shows in failed harvests, food imports that strain household budgets and foreign exchange alike, and the paradox of a continent buying what its own soil and sunshine could grow.

The puzzle is not technology. Solar pumps work, and now, their ec...