The relation of soil moisture to pear tree wilting in a heavy clay soil

One group of workers believes that trees may suffer for lack of water while the moisture content of all material portions of the roots zone is still well above the permanent wilting percentage. A second group believes that soil moisture is as readily available near the permanent wilting percentage as it is near the field capacity. A pear tree on Meyer clay adobe soil near Medford, Ore., was observed to wilt and partially defoliate while the average moisture content of each foot depth of soil and of almost all individual soil samples was well above the permanent wilting percentage. When this tree showed serious suffering from water shortage, intensive soil moisture sampling showed that no material portion of the root zone was within 3.2% of the permanent wilting percentage. Some days later the rate of extraction of soil moisture became markedly slower while the average moisture content of each foot depth of soil ranged from 1.7 to 4.2% above the permanent wilting percentage. Still later, withdrawal of soil moisture at all depths, except for the first foot, ceased before the average content of any foot was depleted to the permanent wilting percentage. The movement of water through the soil by capillary action was too slow to maintain a uniform condition in large masses of soil. The roots do not seem to occupy the entire soil mass. The following hypothesis appears to be valid: The soil moisture content of the soil in contact with the feeding roots may be at or near the permanent wilting percentage, while at the same time the moisture content at some distance, perhaps only a few millimeters away, may be much higher thus allowing the average content for an ordinary soil sample to be well above the wilting percentage at the time a tree shows serious distress for need of water.