December 1, 1997
- soil water tension,
- field capacity,
- available water-holding capacity
The University of Florida-IFAS recommends using tensiometers to help schedule irrigation. Tensiometers indicate when plants should be irrigated based on a pre-determined allowable depletion of the available soil water, which is calculated as the difference between "field capacity" and "permanent wilting point" soil water contents. The soil-water characteristic curve must be used to determine the soil water tension level that will trigger the need for irrigation. We determined soil-water characteristic curves for six citrus grove soils. Most of the drainage of flatwoods soils occurred between about 2 and 8 cb of soil water tension, while most of the drainage of ridge soils occurred between 0 and 5 cb. Almost all plant-available water in both soil types was depleted at 15 to 20 cb of soil water tension. If 10 cb was used as field capacity when calculating available soil water, the tensiometer reading trigger points for 33% available soil-water depletion would be 24 and 27 cb for flatwoods and ridge soils, respectively. These are about 10 to 15 cb higher that what is currently used in the field. We feel that the traditionally-accepted 10 cb soil water content value as field capacity was too high for the soils considered, and propose field capacity soil water tension values of 5 and 8 cb for ridge and flatwoods soils, respectively. If the proposed field capacity values are used in the same calculation, the 33% depletion trigger points become 15 and 9 cb for flatwoods and ridge soils.