前面说过，岛上的粮食本来是自给有余的。

As indicated before, there is an oversupply of food on the island.

实现了粮食自给有余，农民卖粮难，仓库装不下，这是一个历史性的胜利。

In that year, it was difficult for the farmers to sell their grain and the barns could not store all the grain, which could be called a historical success.