lime raises the ph of the soil. when ph is low, nutrients bind to the soil and plants have trouble taking them up. most soil ph is naturally low.

the lime you see at the lumber yard may be calcium carbonate or calcium hydroxide. either way, it's probably a couple dollars for a 50 lb sack.

at quarries, at least around here, calcium carbonate or ag lime is about $6 per ton. it typically takes 1~2 tons per acre of lime.