Years ago I went to Kenya and Tanzania to asses some fields for trials of new cultivars my group was developing. There were a lot of issues with people seeing a yearly decrease in crop yield. But the major issue was actually the lack of crop rotation causing a buildup of disease in the soil which was weakening the plants each year.
I don't know this guy or his field. And, not carefully fertilizing fields can cause root burn for sure. But poor agricultural yield in Africa is definitely impacted by poor crop rotation.
Hmm, it seemed to be a problem with subsistence crops too. We were working on black rot resistance in sukuma wiki which isn't a cash crop. We were working on resistance specifically because it reduced the need to rotate crops. People could grow it more continuously without risking yield loss.