Why does Watney want to double the floor area and height of the tent?

From the text:

Once I get some Martian soil in here, I can mix in the shit and spread it out. Then I can sprinkle the Earth soil on top. You might not think that would be an important step, but it is. There are dozens of species of bacteria living in Earth soil, and they're critical to plant growth. They'll spread out and breed like... well, like a bacterial infection..

Within a week, the Martian soil will be ready for plants to germinate in. But I won’t plant yet. I’ll spread it out over a doubled area. It’ll
“infect” the new Martian soil. After another week, I’ll double it again. And so on. Of course, all the while, I’ll be adding all new manure to the effort.