The principles involved in irrigation are more complex than one might imagine, and familiarizing yourself with them is beneficial to achieving the best crop yield. It may be helpful to begin with some basics of how water works in the soil. As water moves down through the soil, it fills up open spaces or pores between the "dirt" particles. When they are full, water descends past the root zone, or "reach" of the roots. This is referred to as gravitational water and is essentially useless to the plant, also leaching the soil of nutrients and minerals. Each type of soil has an intake rate, the rate at which it can accept water. Volumes exceeding this will result in ponding or runoff. The point to which soil is as moist as it can be without draining water past the root zone is referred to as the field capacity.
Plants absorb 70% of their water in the upper half of the root zone. Overwatering can be just as damaging as having too little water. At the point of saturation, the pores of the soil are full of water and plants have difficulty absorbing it due to the lack of oxygen. It can be a delicate balance between these two extremes. Soil moisture sensors can measure the available water in soil at different levels and can be an essential tool in determining when to water. Several farmers and some residential utilize such tools to signal the beginning and ending of irrigation water use.