The yam is the main food crop of Southern Nigeria, and is an important food in the coastal belts of the other colonies. In Southern Nigeria one might almost say that all other crops are subsidiary to it. Deep cultivation, and the best land, are always reserved for this crop. The commencement of the yam harvest is an important event in the social life of the native, and the actual day is often formally fixed by the chief of the tribe. The varieties of yams are innumerable, most districts having a special variety which is locally much more popular than any other; but, in the main, the differences between the. varieties are not very great. Some are earlier than others, some have whiter flesh, some are more suitable for pounding than others, and so on. The most important of these characters is that of earliness or lateness. Where yams are interplanted with cotton, or where they are grown in a rotation which incorporates green manuring, there is obviously a great advantage in growing a variety which can be harvested as early as possible.

The yam crop often occupies the land for nearly a whole year. The best time for planting varies somewhat in different localities, but generally the crop can be planted either in November or early December, or in March or April. Yams are not often planted in the interval between these two periods, and experiments indicate that this is not merely a question of convenience, as, if they are planted then, they do not germinate well. Also, yams planted too early do not do so well as those planted in November. The choice between the two good periods mainly depends on the proportion of land which can be prepared at the end of the rains; for yams planted in the earlier period yield better than those planted in March.

The farmer starts working on his yam land just before the rains cease and continues until the hardness of the soil makes the work too laborious. Such areas are planted in November-December. If he then has insufficient land prepared, he waits until the rains break when the ground must be prepared and the yams planted as quickly as possible. Any further delay results in a very serious loss of yield.