In a subsurface drainage system, often called a tile system, all parts except the outlet are located below the surface of the ground. It provides better drainage than a surface system because it removes water from the soil to the depth of the drain, providing plants a greater mass of soil for root development, permitting the soil to warm up faster in the spring, and maintaining a better balance of bacterial action, the air in the soil, and other factors needed for maximum crop growth. The smallest component of the subsurface system, the lateral, primarily removes water from the soil. The laterals may be arranged in either a uniform or a random pattern. The choice is governed by the crop grown and its value, the characteristics of the soil, and the precipitation pattern. The primary decision required for a system with uniform laterals is their depth and spacing. In general, the deeper the laterals can be emplaced, the farther apart they can be spaced for an equivalent degree of drainage. Laterals usually are spaced from 24 to 91 metres (80 to 300 feet) apart and 0.9 to 1.5 metres (3 to 5 feet) deep. Subsurface drainage systems are as important in many irrigated areas as they are in humid areas. A drainage system is needed on irrigated lands to control the water table and ensure that water will be able to move through a soil, thus keeping salts from accumulating in the root zone and making the soil unproductive.