Like roothair length roothair density is a trait that can increase the root surface area at very little energy investment. Increased roothair density is positive for the uptake of immobile nutrients like phosphorus. Roothair density and roothair length are synergistic traits. The root surface area and thereby nutrient uptake increases with roothairlength*roothairdensity not roothairlength+roothairdensity.

Roothair Density