There are large amounts of ice underground on mars. Some are closer to the surface then others. Assuming this ice has been mined and is not safe for human consumption without purification, is it safe for irrigation?
Do the intermixed dust particles provide nutritional value for the plants? Or is the risk of toxins (to both the plant, and the human assuming the plants produces food) too great?
Mars Ice Deposit Holds as Much Water as Lake Superior
with a composition that's 50 to 85 percent water ice, mixed with dust or larger rocky particles.