PAICHO, Uganda — Just after dawn, Patrick Ogik placed a wooden yoke over the bulging necks of his two oxen and attached a metal plow behind them, the ropes, fraying from wear, tied to the animals.

The 44-year-old farmer guided the oxen to a fallow field, where he was preparing to plant peanuts in a tiny patch of land he owns outside Paicho, a village in northern Uganda.

As the metal plow drove through the soil, it struck something hard. Mr. Ogik reached down and pulled the metal casing of a mortar shell from clumps of soil. He wiped the surface with his hand.

This one was spent, he explained, though he has sometimes come across live ammunition. He tossed it aside and continued his work.