Arab or Middle Eastern countries are abundant oil producers and their people are known to be prosperous and rich, but that does not mean that all their people are prosperous and rich because there are some people who live well. although the country is known as an agricultural country because abundant harvests are not a guarantee that all its people will not starve. This may happen because most of the agriculture in the country is only controlled by a few people.
You are right, but in the case of Nigeria, poor people are known for farming, but they lack proper mechanization to grow their foods well that will be enough for consumption and exportation; if rich men or the politicians decided to farm, they will use their power and money to farm, instead of supporting the poor with food when there's is finished, they sell it to them very costly and they grow them in the natural land which is a free gift.
What i am saying in essence is that, the green land in Nigeria is fertile and can grow several varieties of food crops and cash crops, if government decide to support the farmers with mechanization and fertilizer, the country will never cry of hunger, but the government are selfish, they prefer to import their food than sponsor their local farmers.