The water from the soil enters into root hairs on account of

The water from the soil enters into root hairs on account of Suction pressure.
Suction pressure is also called Diffusion Pressure Deficit. If some solute is dissolved in a solvent, its diffusion pressure decreases.
It is DPD that tends to equate and represent the water-absorbing ability of a cell, it is also called suction force (SF) or suction pressure (SP). The actual pressure with which a cell absorbs water is called "suction pressure".