Using Soil Water Measurements to Schedule Irrigation

Article reproduced with permission
Citrus Industry Magazine, February 1999

By Larry R. Parsons, K. T. Morgan and T. Adair Wheaton

Note: Figures mentioned in this article are not available at this time.

Irrigation is one of the most important practices for a grower to focus on in the spring. During the dry months of March, April and May, irrigation is essential for improving fruit set and yield. Early research by Robert Koo showed that irrigation could increase yields of Marsh grapefruit, Hamlin, Valencia and Pineapple oranges by 52, 38, 35 and 14 percent, respectively, when compared to non-irrigated controls. Time spent in understanding the concepts behind irrigation is worth money to the grower. The purpose of this article is to discuss aspects of soil water holding properties and soil moisture measuring devices. With this knowledge, growers can improve irrigation management.

Florida soils and their water holding properties The main characteristics describing soil water holding capacity are field capacity, permanent wilting point, and available water. Field capacity is defined as the amount of water held in the soil after drainage has essentially ceased and water content has become relatively stable. This is also described as the amount of water held against gravity. In most Florida sands, drainage of the top foot of soil can be completed in less than 24 hours, and thus field capacity can be reached in that time. It may take three or four days for drainage at a five-foot depth in ridge soils to be completed as water from upper layers percolates down. Florida citrus soils usually have less than three percent clay and less than one percent organic matter. Average field capacity values in the top foot can range from six to eight percent in ridge sands and eight to 12 percent in flatwoods soils. At deeper depths in ridge soils where there is less organic matter, field capacity values can be as low as four to six percent. Permanent wilting point is the soil water content at which plants remain wilted overnight or in a humid atmosphere. It is considered to be the point where plants cannot extract more water from the soil. Hence, plants remain wilted and will not recover from the wilt unless water is added. There is no sharply defined lower limit for water availability, but -15 bars (approximately -15 atm) is commonly used as an estimate of permanent wilting point. Average permanent wilting point values in the rooting zone for ridge sands range from 0.4 to 2.5 percent and 1.5 to five percent for flatwoods soils. Available water is the amount of water between field capacity and the permanent wilting point. The relationships among field capacity, permanent wilting point and available water for sand, loam and clay soils are shown in Figure 1. Notice that the darkened area, which represents available water, is very small in sands and much larger in loams and clays. Figure 1 illustrates how notoriously low the amount of available water is in Florida sands. Part of the reason for the low available water in Florida sands is the very low amount of organic matter and clay that sands contain. Citrus is grown in Texas and California on loam and clay soils that have two to three times the water holding capacity of Florida sands. There is water in the soil below the permanent wilting point value, but it is unavailable to the plant. Notice that both field capacity and permanent wilting point values increase as one moves to the right in Figure 1 from sand to loam to clay. Clay soils hold much more water than sands, and permanent wilting point values of clays can even be higher than the field capacity values of some sands.

Soil Water Measurement
There are a number of soil water measuring devices on the market today. These devices usually measure soil water content or water tension. Water content on a volumetric basis is the foundation for irrigation scheduling using a budget system. A second way to express soil water is based on the tension required to remove water from the soil. This is directly related to the availability of water to the tree. Soil water content and tension are related by the soil moisture release curve, which is shown in Figure 2. Water is lost from the soil and leaves by evapotranspiration (ET) during the day when the stomata are open. As water moves out of the leaves, it creates a tension which is transmitted to the roots. This in turn pulls water from the soil into the roots. As the soil dries and the water content decreases, soil water tension increases; and it becomes more difficult for the roots to pull water from the soil. The relationship between water content and water potential (or tension) is important and is shown in ridge and flatwoods soils in Figure 2. This shows that after the soil reaches field capacity, a small decrease in water content can cause a large increase in tension. Flatwoods soils typically hold more water than ridge soils, but even a moderate decrease in water content in flatwoods soils can cause a noticeable increase in tension. It is important to understand this relationship in order to know why certain instruments respond the way they do as the soil water content changes. Some of the devices on the market that measure water content are capacitance devices. Sensors are attached to probes which are inserted into access tubes in the soil. Daily changes in soil water content can be shown by plotting water content at different soil depths over time. These instruments give a near-continuous measurement of soil water content and are useful for scheduling irrigation.

Tensiometers
A tensiometer is a closed tube filled with water that has a hollow ceramic tip at one end and a stopper at the other end. A vacuum gauge attached near the stopper end measures soil water tension. As the soil dries, water moves out through the ceramic tip, creating a partial vacuum inside the tube which is registered on the vacuum gauge. When the soil is irrigated, water is drawn back into the tensiometer by the vacuum and the gauge reading is lowered. Most tensiometer gauges read from 0 to 100 centibars (cb) or kilopascals (kPa). One hundred centibars equals one bar which is nearly one atmosphere. One bar equals 14.5 psi and one atmosphere equals 14.7 psi.

Where should tensiometers be placed and how many should be used?
Tensiometers should be placed in the active root zone and in the spray pattern of the irrigation system. The majority of the roots are found in the top two to three feet on the ridge and top one foot on the flatwoods. Even on the ridge, roots involved in much of the tree's water uptake are located relatively close to the soil surface. Soil dries most rapidly at the six-inch depth because of evaporation and high root density there. For these reasons, we recommend a minimum of four tensiometers Ð two groups with one tensiometer at a six-inch depth and another at a 12-inch depth. An extra tensiometer can be placed at the 18- or 24-inch depth to check the deeper roots on the ridge, but it will respond more slowly than those at shallower depths. If the soil dries to tensions greater than about 85 cb, the water column will cavitate, air bubbles will form, and the tensiometer will give invalid readings. This may occur if irrigation is not frequent enough, particularly at the six-inch depth. When these things happen, water may need to be added to the tensiometer and a vacuum applied to remove air bubbles from the water column. In Florida sandy soils, recent research suggests that tensiometer readings higher (drier) than 30 cb may be somewhat inaccurate. Hence, to get good readings with tensiometers, it is best not to let the soil dry out frequently or excessively. With proper servicing, tensiometers should give good readings for a number of years. For more information on tensiometers, look at the University of Florida IFAS Bulletin 319 and Circular 487. These can be obtained from the Extension Digital Information Source (EDIS) at http://edis.ifas.ufl.edu/. To view a copy of these publications, click on 'Search Publications Database' and type 'tensiometer' in the Publication Title box. Ideally, a pair of tensiometers should be placed in a representative location in each different soil type. Since this is not always possible, at a minimum, two sets of at least one six-inch and one 12-inch tensiometer should be placed in each irrigation block. Two pairs of tensiometers provide backup in case one or more units cavitate or lose vacuum. Obviously, more tensiometers will allow for more precise irrigation management.

Recommendations
Since many Florida citrus groves are now irrigated with microirrigation systems which wet only part of the grove floor, irrigations should be initiated at 33 percent depletion from February to June (during the critical flowering and fruit set stage) and at 50 percent depletion from July to January. Depending on soil type, this corresponds to a trigger point for starting irrigation at 10 cb on the ridge and 15 cb on the flatwoods during the spring. In the less critical fall and winter when a little more water stress is acceptable, trigger points can be around 15 cb on the ridge and 30 cb on the flatwoods. Adjustments in trigger points can be made for specific soil types. It is important to note that tensiometers require periodic maintenance; if they are not maintained properly, misleading readings can be made. Tensiometers should be serviced every few weeks, particularly those at the six-inch depth that may have cavitated water columns from too much drying. Common servicing does not take long and usually involves adding water to the unit and applying a vacuum to eliminate air bubbles from the column. A knowledge of soil water holding capacity is critical for good irrigation management. By using tensiometers or other soil water measuring devices, better irrigation control can be developed and some irrigation cycles may be eliminated. This can save the grower money and potentially make more effective use of rainfall.

Parsons and Wheaton are professors and Morgan is a project manager at Lake Alfred's Citrus Research and Education Center.

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