By nature no agricultural land is unproductive even before the advent chemical fertilizers. It degradation especially in recent past has been due to man's consistency in poor farming practices such as shifting cultivation, slash and burn, monoculture, none respect of crop ecological zones, etc and this is mainly in developing countries.
Uniquely, all crops fall under ecological zones and does well when cultivated under this zone, for example the yield of millet cultivated in the northern lowland of Cameroon can not be the same as that from the western highlands in the same country. Also, if farmers can be able to stop the slash and burn system which greatly kills soil microorganism that facilitate the decomposition of organic matter thus rendering the soil ineffective and unfertile in the long run.
Furthermore, shifting cultivation which constitutes working and abandoning farmlands especially when yields drops in favor of new areas can be alternated by crop rotation specifically with legume crops. These crops do replenishes the soil with the major nutrients in the soil such as Nitrogen.
Sometimes, the seed material used are ware seed as such improved seed should be used as well planting distances respected. More often, intercropping other crops with legumes is a good practice and does increases yields especially in areas of limited land.
Bush fallow which farming on a piece of land and allowing it in the following year . This practice is good in areas with plenty of farmlands and during this process, it should be accompanied by the planting of Tephrosia. But this practice is too uncommon especially in rural Cameroon today.