It is easier to keep moist soil moist, rather than trying to get dry soil moist. When you do irrigate give enough water to do a good job of wetting the soil to a good depth. The heavier the soil type the better the water will spread sideways. Many books say apply an inch (2.5 cm) each watering. Crops vary in water needs. Some crops – Potatoes, need irrigation to increase of crop, Carrots need irrigation to increase root size Leafy salad crops need to grow fast to retain fresh texture and need regular watering. Maize is deep rooted and can survive some drought conditions.