Overview
Maize is a valuable commodity that is geographically dispersed across Nigeria and suitable for most Nigerians. It is perhaps the most common staple food in developing countries, providing food for 900 million people earning less than US $2 per day.
Economics
The worldwide consumption of maize is more than 116 million tons, a testament to the fact that there is a high consumption of maize globally. In Sub-Saharan Africa, 50% of the population consumes maize while the entire African continent accounts for 30% of global Maize consumption.

One would imagine that a continent that has a heavy maize consumption rate will be a major maize producer but on the contrary, Africa accounts for only 6.5% of the Global Maize production with Nigeria being the largest African Maize producer, her production is slightly over 10 million tons of Maize, followed by South Africa. However, it is considerably small when compared to the USA’s corn production (384 million metric tons).

If Africa produces very little and consumes so much, then it confirms to us that there is a heavy reliance on importation to bridge the production deficit which is why Africa’s total consumption of maize accounts for 28% of the global maize imports.
Read more... https://www.commodity-port.com/blog/index.php/2018/11/13/harvest-spotlight-maize/

America's land mass is more than 10 times that of Nigeria and also one of the most naturally fertile soil on earth, are you suggesting that Nigeria will use all of her available land in cultivation of maize alone?

Every country do import food.