Growing garlic for the first time

Sorry I can't give you a definitive answer Can, garlic needs a cold spell of under 10c  for 30 nights which helps the cloves develop properly that's why your friend is a bit critical about you growing in a tunnel as for the leaves yellowing it could be the watering, which is another reason for growing outside their growing requirements of pH 4.5 to 8.5 which gives quit a wide range, but if you want to drop the pH a bit add a small amount of vinigar to a full watering can  of water and soak the soil

Thanks Spec, I missed G&T do a ph test on the soil, BTW how long does it take for garlic to grow, and should I expect to see the bulbs above ground or below ?