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.