WHAT IS THE SOIL IN YOUR PLANTS MADE OF? LOOKS LIKE SEED HUSKS.
That is a hard question to answer simply because there are so many possibilities! And, it comes down to the preference of each grower. Without seeing exactly what you are looking at, my guess is it may be rice hulls, which are commonly used in growing media. Rice hulls hold moisture in soil very well as they break down, and increase the porosity and infiltration of the media.
There are many other components as well based on the area of the country and grower choice.