Irrigation Reduces Soil Quality and Rains Usually Don't

Irrigation reduces soil quality and rains usually don't. Why?

A) Rains deliver more water to soil than irrigation does.
B) The fast water flow during rains is able to remove toxic materials from topsoil.
C) Rainwater contains less inorganic material.
D) Water used for irrigation contains pesticides that affect soil quality.
E) The fast water flow during irrigation removes organic material from soil.