Suitable soils for irrigation are deep, permeable, and hold lots of water. Factors like soil depth, texture, and plant type determine how much water plants can access. Techniques like observing plant wilting, feeling the soil, and using meters can help decide when to water plants. It's better to water plants before they wilt, as wilted plants grow slower. Soil moisture can be roughly estimated by how the soil feels, with categories ranging from dry to very wet.