Aerial roots, from what I've seen microcarpa (along with a few other Ficus) don't produce much in the neighborhood of them. It also takes a mature tree to begin to produce them.
I would suggest crusing the web and typing that subject into Google.

Bob

Some folks use straws to grow/direct the roots down into the soil.

Josh

The term "air roots", is a common term for adventitious roots, roots which do not originate from existing roots. Air roots have their origins in a type of tissue other than root tissue, called parenchyma. The primary cultural stimulus for the formation of air roots in species known to produce these roots is high humidity. Plants growing with good vitality are also much more likely to produce air roots, but again, humidity is usually the limiting factor, no matter how robust the plant. Age of the plant, especially when Ficus is the subject at hand, is not a factor in the formation of these roots. If anything, age would be a limiting factor. All species of Ficus I know of, including carica (hardy/edible fig), readily form adventitious roots under conditions of high humidity.

You can 'get' air roots by providing an environment high in humidity and keeping your plants in good vitality. Some growers build a wall around their trees & fill it with very loosely packed chopped or milled sphagnum moss (not sphagnum peat) and keep it moist at all times. The moss and high humidity in combination are stellar at stimulating adventitious root formation. After roots have formed & are anchored to the soil, the moss is gradually removed so the roots can harden off.

Al