Some trees can survive a long time without water. Think of trees that grow in the desert. But other trees may need more moisture  than they can get from rainfall or from the air . Trees and other plants can look thirsty. Leaves can become weak and hang downward. They can also turn yellow. Yellowing can be a sign of too much water. But it can also be a warning sign of too little water. With a newly planted tree, the roots have not yet spread out from the root ball. The root ball can become dry faster than the dirt around it. So put water on the area of the root ball and the surrounding soil until the roots become established. Once a tree is well established, water deeply instead of watering often. The amount of water needed depends on the tree and the soil. Clay  soils hold water for longer periods while sandy soil holds less water. During the hottest season, a deep watering may satisfy a tree for anywhere from ten days to four weeks. Ted Swiecki is a plant scientist . He says people should not water established trees at the base of the trunk . This can harm the tree. Too much water in the soil at the base of a tree can lead to the growth of fungi . If the area is too wet, harmful organisms have a better chance to invade the tree and cause disease. Mr. Swiecki says this is true especially in Mediterranean and semi-dry climates. Many trees in these climates have adapted to having a dry area near their base during the hottest season. He said, "Water displaces (,)air in the soil. And roots are aerobic ; they require oxygen for the soil to function properly. So if you keep the soil completely wet and there's no air there, then the roots are starving for oxygen." What is the passage mainly about?
It introduces to us different kinds of trees It instructs us how to water trees correctly It teaches us how to tell whether a tree needs water It analyses why different trees need different amounts of water