toGrowing blueberries in a pot is not such a whacky idea. Along the coast, they never get as big as the Central Valley or other places where they are grown. That's because they are in almost continuous flower and fruit production. So when they are small, the pots can be put more closely together, reducing water use and weeds. As the plants grow, the spacing can be increased. Also, blueberries are very sensitive to high soil pH which is easier to correct with artificial substrates. They are also prone to Phytophthora root rot, the pesticide for which can't be used by organic growers, but can be controlled by careful irrigation of a pot. So the easier control of weeds and the easier control of root rot would be worth it to an organic grower, even though the initial expenses are higher. Better control typically lead to higher yields. Being able to control plant spacing might also make them less prone to frost damage because they could more easily be covered up when frost is forecast.
- Author: Jim Wolpert - UC Davis
Soil Moisture Sensors
Jim Wolpert, University of California, Davis
Soil Moisture Content
The quantity of water in soil is called the soil moisture content. After rainfall or irrigation, some water drains from the soil by the force of gravity. The remaining water is held in the soil by a complex force known as surface tension and varies depending on the amount of sand, silt, and clay. Sands, with larger particles and smaller total surface area, will hold less water than clays, which have much smaller particles and larger total surface area. The drier the soil, the greater the surface tension, and the more energy it will take for a plant to extract water.
Vineyard managers often measure soil water content as a guide to determine their irrigation timings and amounts. There are several methods for monitoring soil water content. Correlating these methods with actual inches of moisture per foot of soil is very complicated (see Recommended Links) but at the very least can help a grower to identify patterns of water use, depth of irrigation, and soil water content trends over time.
A tensiometer, as its name implies, is a device for measuring soil moisture tension. The design is a simple tube with a porous cup at the lower end and a vacuum gauge on top. The tube is filled with water, sealed airtight, and placed in soil. As soil dries, water is pulled from the porous cup into the soil, creating a vacuum and causing the gauge to move. As soil continues to dry, more water is pulled out and the suction increases. As soil re-wets after a rain or irrigation, water moves back into the cup and the suction decreases. Installing tensiometers in soil requires attention to detail to obtain accurate readings (see Recommended Links for installation downloads).
Tensiometers are usually placed as a pair with the shorter tube positioned in the middle of the rooting zone (e.g., 18 inches deep) and a longer tube positioned near the bottom of the rooting zone (3 to 4 feet deep). Growers can use the difference between the two tubes to monitor water usage and determine the effective depth of irrigation. At least two stations (two tubes per station) are recommended per field, or more depending on soil variability.
Tensiometers have the advantage of being inexpensive, and easy to install, maintain, and read. They are better in fine-textured soils where good contact can be made between the porous cup and the soil. They do not work well in coarse sands where good contact may not be possible. Because the gauges are aboveground, the units are prone to damage by vineyard equipment.
Electrical Resistance Blocks
Electrical resistance blocks are also known as gypsum blocks or soil moisture blocks. They are simple devices with two electrodes embedded in a block of gypsum or other similar material. When blocks are buried in soil, water moves into or out of the block, depending on the moisture of the soil, changing the resistance between the two electrodes. Like tensiometers, gypsum blocks are cheap and easy to install. They are usually installed in at least two stations per field, at two depths, and must be installed correctly to provide accurate readings. Some block designs perform better under wet soil conditions and some correct for soil temperature. The meter used to read the blocks can be moved from field to field, but is specific to the block design (i.e., it is not a simple ohm meter). The wires aboveground are much less prone to damage by equipment compared to tensiometers.
A neutron probe uses a radioactive source for measuring soil moisture. A tube, usually made of PVC or aluminum, is installed in soil to a depth of interest and the radioactive probe is lowered into soil to measure soil moisture at as many depths as desired. The probe emits fast neutrons that are slowed by water in the soil in a way that can be calibrated to the soil water content. The probe has a significant advantage, especially for perennial crops, because access tubes are easy to install and relatively permanent. Another advantage is the reading accounts for a spherical area about 10 inches in diameter, much greater than other methods. The major limitation to this method is the probe itself; it is expensive and the presence of a radioactive source triggers requirements for operators to be trained and licensed in handling, storage, and use. In some production regions, service providers are available, usually at a fixed cost per access tube for a growing season.
Di-electric sensors measure the di-electric constant of soil, a characteristic that changes with changing soil moisture. A common method is called time domain reflectometry, or TDR. The theory behind how this method works is too complicated to be discussed here. The advantage of these types of systems is that they are designed to be left in place and provide continuous readings of soil moisture. The disadvantages are that the units are expensive and read soil moisture only a very small distance from the unit.
All measures of soil moisture suffer from the same limitation — the value of the information is dependent on the extent to which the soil where the measurements are taken reflects the rest of the field. Where soil variability is high, growers must exercise caution in relying too heavily on relatively few measurements.
Irrigation of Winegrapes, University of California
Irrigation Basics for Eastern Washington Vineyards, Washington State University
Reviewed by Ed Hellman, Texas AgriLife Extension and Eric Stafne, Mississippi State University
California is suffering historic drought conditions. The information on this webpage offers farmers and ranchers links to valuable resources, carried out by researchers and specialists, on a vast array of issues they are facing during this extremely dry and difficult year. For general drought information please click on the links below.
For drought issues with a particular crop, please see the left navigation buttons where you will find resources for your specific agricultural needs.
UC ANR California Institute for Water Resources Drought Information
UC Davis Rangeland Watershed Laboratory Managing Drought http://rangelandwatersheds.ucdavis.edu/main/drought.html#RancherPerspectives
Coping with Declining Groundwater Levels
CIMIS Drought Tips
View film excerpts from University of California Researchers and Academics on a large variety of expert water and drought topics
Insights: Water and Drought Online Seminar Series
Irrigation Scheduling Tools
UC Drought Management-Evapotranspiration Scheduling
Soil Moisture Monitoring
UC Drought Management-Soil Moisture Monitoring
Irrigation Scheduling during a Drought
John Letey, Jr.
University of California–Riverside (UCR) Distinguished Professor of Soil Physics and Soil Physicist Emeritus John Letey, Jr. passed away on 14 September. He was 81 years old.
He received his B.S. degree at Colorado State University and Ph.D. degree at the University of Illinois. He joined the faculty in the Department of Irrigation and Soil Science at UCLA in 1959, but with the phasing out of agriculture at UCLA, he elected to join the Department of Soil Science at UCR in 1961 and enjoyed a distinguished career in research, teaching, administration, and service at the university. During his tenure, Dr. Letey served as chair of the Department of Soil and Environmental Science from 1975 to 1980, director of the Kearney Foundation of Soil Science from 1980 to 1985, and director of the University of California Center for Water Resources from 1999 to 2003. He was instrumental in the establishment of the Environmental Sciences undergraduate major at UCR, which was one of the first of its kind in the United States. He recognized and appreciated the critical link between science and policy and built teams and research to address it.
Letey's research focused on all aspects of water quantity and quality related to irrigated agriculture that provided both applied and basic information critical to establishing sound water resource management. Topics of research included irrigation, drainage, salinity, pesticide transport, plant–water relations, nitrogen, soil aeration, and polymers. He was also recognized as one of the world authorities on water-repellent soils and the utilization of surfactants. He wrote a biography of his education and professional career that can be accessed on the following website: http://envisci.ucr.edu/downloads/johnleteycareer.pdf.
Letey was a fellow of SSSA, ASA, and AAAS and was awarded the SSSA Soil Science Distinguished Service Award in 2005 and the SSSA Soil Science Research Award in 1970. He authored or co-authored more than 300 technical publications concerning chemical, water, and gas movement through soil before retiring from UCR in 2002. In 2007, he published a fictional book titled, The Folly of Fearing Death (PublishAmerica, Baltimore).
In 2003, Letey and Ardyth Stolzy, wife of the late Professor of Soil Physics Emeritus Lewis H. Stolzy, combined the Letey Soil Environmental Fund and the Lewis Stolzy Memorial Fund into the Stolzy–Letey Endowment in Soil and Environmental Science. The Stolzy–Letey Fund is now used for the benefit and support of the students in the Department of Environmental Sciences at UCR.
Letey was a friend and mentor to students, visiting scholars, and faculty across the world. He served his research community, church, and family with great love and personal integrity. He is survived by his wife, Sonia; three children, Laura Petersen, Don Letey, and Lisa Smith; 10 grandchildren; and 11 great-grandchildren./h3>
Impacts of the recent drought conditions on Central Coast avocado production, and potential impacts of continued drought conditions
Avocados are the most salt and drought sensitive of our fruit tree crops. They are shallow rooted and are not able to exploit large volumes of soil and therefore are not capable of fully using stored rainfall. On the other hand, the avocado is highly dependent on rainfall for leaching accumulated salts resulting from irrigation water. In years with low rainfall, even well irrigated orchards will show salt damage. During flowering there can be extensive leaf drop due to the competition between flowers and leaves when there is salt/drought stress. In order to reduce leaf damage and retain leaves, an excess amount of water is required to leach salts out of the roots zone. The more salts in the water and the less rainfall, the greater leaching fraction. Drought stress often leads to diseases, such as black streak, bacterial canker, and blight (stem, leaf, and fruit). Defoliation leads to sunburned trees and fruit which can be severe economic losses.
Strategies to address drought conditions
Ensure that the irrigation system is at its greatest potential and is maintained. Avocados are grown on hillsides and pressure regulation is extremely important and is frequently neglected.
Significantly prune trees to reduce leaf area. Avocado can be a very large tree, and if half the canopy is removed, there can be as much as 1/3 reduction in water use. When trees are about 15 feet tall, removing half the canopy can reduce water use by one half.
In extreme drought conditions, the canopy can be reduced to just the skeleton branches which are white washed to prevent sunburn. Water use drops to zero, and then gradually as the tree leafs out, water can be slowly reapplied, but at significantly less amounts than with the full canopy. Stumping typically results in three years' worth of crop.
In orchards that have low producing areas, because of recurrent frost, high winds, shallow soils, disease, etc. the grower could decide to completely remove those trees, thereby saving water.
White kaolin applied to leaves has been shown to reduce leaf temperatures and water loss. This can be used, but under the direction of the packing house, since if it is applied to fruit, it is very difficult to remove.
Impacts of the recent drought conditions on Central Coast citrus production, and potential impacts of continued drought conditions
Citrus is much less sensitive to salts and drought than avocado, partially because of its greater rooting depth. However, it is much more sensitive than deciduous fruit trees, resulting in smaller fruit and lower prices when drought cannot be addressed with adequate irrigation water. Drought also makes the trees more susceptible to leaf drop, and sunburned fruit.
Strategies to address drought conditions
The strategies for citrus are very similar to those for avocado. It is much more sensitive to pruning to reduce water use than avocado. Typically removing half the canopy results in half the water use. Because of thus greater control, citrus is rarely stumped.
By reducing canopy size, production can be maintained, often without loss of fruit size.
Kaolin clay can effectively reduced water use and can be applied soon after harvest without the problem of coating fruit making its removal difficult at the packing house.