Control the water table at the right level for each crop.

Water levels are: 40 cm. for radish, parsley, blueberry and cranberry;
60 cm. for celery, sod and cereals;
just over 60 cm. for potato, cauliflower and bean;
80-90 cm. for onion, beet, lettuce, corn and spinach;
and 90 cm. for carrot.