the fertile arable land available is running out.

Fertile arable land is made out of rock. The earth is made out of rock. If the earth - as you imply - is running out of itself, where do you suppose it - rock - might be going? Does rock get used up by farming in it, and thereby disappear?

Normally, when something is becoming scare ("running out"), people 1) make more of it, 2) use it more efficiently, and 3) substitute other things for it. In the case of arable land, we have observed people 1) making fresh arable land out of rock, 2) reclaiming arable land from wetlands/lakes/oceans/hillsides/mountainsides, and 3) substituting in the forms of vertical and soil-free agriculture and other forms.

As I noted previously, "A mine is not a permanent use of land." http://www.juliansimon.com/writings/Ultimate_Resource/TCHAR0...

Both ignorance and mysticism enter importantly into conventional thinking about farmland. For example, one hears that "once it's paved over, it's gone for good." Not so. Consider the situation in Germany, where entire towns are moved off the land for enormous stripmining operations. After the mining is done, farmland is replaced, and the topsoil that is put down is so well enriched and fertilized that "reconstituted farmland now sells for more than the original land." Furthermore, by all measures the area is more attractive and environmentally pure than before.

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that is a very shallow investigation. [...] Cities grow usually on fertile land

Did you read it? Here is the link again: http://www.juliansimon.com/writings/Ultimate_Resource/TCHAR0...

But what about the fertility of the land used for human habitation and transportation? Even if the total quantity of land used by additional urban people is small, perhaps the new urban land has special agricultural quality. One often hears this charge, as made in my then-home town in the 1977 City Council election. The mayor "is opposed to urban sprawl because 'it eats up prime agricultural land.'"

New cropland is created, and some old cropland goes out of use, as we have seen. The overall effect, in the judgment of the U.S. Department of Agriculture, is that between 1967 and 1975 "the quality of cropland has been improved by shifts in land use ... better land makes up a higher proportion of the remaining cropland."

The idea that cities devour "prime land" is a particularly clear example of the failure to grasp economic principles. Let's take the concrete (asphalt?) case of a new shopping mall on the outskirts of Champaign-Urbana, Illinois. The key economic idea is that the mall land has greater value to the economy as a shopping center than it does as a farm, wonderful though this Illinois land is for growing corn and soybeans. That's why the mall investors could pay the farmer enough to make it worthwhile for him or her to sell. [...]

The person who objects to the shopping mall says, "Why not put the mall on inferior wasteland that cannot be used for corn and soybeans?" The mall owners would love to find and buy such land - as long as it would be equally convenient for shoppers. But there is no such wasteland close to town. And "wasteland" far away from Champaign-Urbana is like land that will not raise whornseat - because of its remoteness it will not raise a good "crop" of shoppers (or whornseat or corn). The same reasoning explains why all of us put our lawns in front of our homes instead of raising corn out front and putting the lawn miles away on "inferior" land.