Read the passage and answer the question that follows it.
Erosion of the vital top-soil is a very great problem confronting many nations. It was brought on by the carelessness and negligence of earlier generation. Their greed caused them to overcrop their lands, resulting in the gradual destruction of vast areas, which became deserts through continual exposure to wind, rain and frost. Once this erosion starts, it is very difficult and costly to stop.
It is foolish to think that now there is nothing to fear because we have more advanced implements and methods. Today, millions of people are in the East eking out a miserable existence with very little food. To feed all of them adequately, it will be necessary to utilize the land; tend it carefully for many years in order to return it to its former condition. More grass and trees must be planted to hold the soil together and preserve the moisture; humus must be introduced by keeping stock. Where violet storms prevail, terraces and dams must be built.
As used in the passage, the word ‘vital’ means___________.