[TheMagician0]

"I really don’t understand the physics that makes that system work, but it has a lot to do with water pressure..."

For those who are curious: In simple terms, pressure is proportional to depth. Assuming the air pump is outputting constant pressure, the pressure exerting on the top surface of the water would be constant. On the other hand, the amount of pressure experienced at a point near the bottom of the container depends on the its depth- the deeper the more pressure.

When there is more fluid, there is more pressure near the bottom due to the weight of the fluid itself, which allows greater output (water going into the soil). As the fluid's depth decreases, so will the pressure experienced by a point near the bottom.