Avocados and Water
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). Leaf blight (Figure 1) is often confused with salt or tip burn (Fig. 2), but is actually a fungal disease that forms an irregular dead pattern on leaves and leads to defoliation. Blight is associated with lack of water, while salt burn is due to poor quality water and poor irrigation habits. Leaf blight often shows up after Santa Ana conditions, when growers get behind on their irrigations and the root zone dries out suddenly. There has been a high incidence of this disease the last two years. In both cases, defoliation leads to sunburned trees and fruit which can be severe economic losses. The only way to prevent these conditions is to keep up with your irrigation schedule.
To get your water to go further, it is important that the system is tuned in order to get the best distribution uniformity (DU). Many of our systems were installed 40 years ago and old age can lead problems, such as clogging, broken emitters, mixed emitters that put out different amounts and leaks. With poor DU, some trees get too much water and others do not get enough. Even fairly new irrigation systems can have poor DU, especially after a harvest. Poor water pressure on our step slopes is probably our main problem. A DU of 80% means 10% of the emitters are putting out more than the average and 10% are putting out less. The irrigator to compensate for the under irrigated 10% will run the system 10% longer to make sure the under irrigated trees get enough and over irrigating 10% of the trees with 20% more water than they need. A call to the local Resource Conservation District office can get a free DU evaluation and recommendations that are usually pretty reasonable to follow.
Aside from improving DU, it is important to know when and how much water to apply. When to apply can be evaluated by the hand or feel method (https://nutrientmanagement.tamu.edu/content/tools/estimatingsoilmoisture.pdf) which is fast and cheap. Or it can be done by tensiometer, Watermark or some of the more expensive electronic sensors. But these tools only tell you when to irrigate, not how much. This can be done by turning the system on (once you have made sure you have a good DU) and over the period of the irrigation insert a piece of rebar into the soil to determine the depth of infiltration. The rod will go down as far as the soil is moist and stop when it hits dry dirt. When you have about two feet of infiltration you will know how long to run the system to get an appropriate amount of water. A typical loam usually takes about 150 gallons per tree to two feet. Another way to get an approximation of the amount to apply is to use the Irrigation Calculator at http://www.avocadosource.com/tools/IrrigationCalculator.asp.
Managing the Tree Canopy
Significantly pruning trees can reduce the amount of water transpired by the tree. Trees that are about 15 feet in height, can be pruned by half and they will use half the water. Massive 30 foot trees would need to have a major pruning to significantly reduce water use. In extreme drought conditions and for the long term welfare of the grove, large trees should be stumped (Figure 3) or scaffolded (Figure 4) and paint white to prevent sunburn. Scaffolding usually produces fruit much sooner than stumping, because retaining a significant part of the trunk and branches the tree does not exert as much energy to regrow and retains buds that have been under apical dominance for less time. When new shoots appear they should be headed back to force lateral branches which is where the flowers will form.
All the prunings should be chipped and left in the field. This will help conserve water and help control Phytophthora root rot. Root rot or crown rot trees should not be pruned until they have been brought to health with one of the phosphorous acid formulations. They all are effective. Pruning a sick redirects the trees energy to fighting off the disease when it starts pushing new growth and then does not have the energy to fight off the disease. Or if you do have areas that are diseased (sunblotch, root rot, crown rot, etc.), windblown, in shallow soils or areas of recurrent frost, you might just remove the trees completely to save water.
White kaolin (Surround) 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 it if it is applied to fruit, it is very difficult to remove.
These are some steps that a grower can take to improve water management and create a more efficient use of water to help survive this period of not knowing how long this drought will last.
Figure 1. Leaf blight is a disease that occurs with lack of water of any quality.
Figure 2. Salt damage from poor quality water and poor irrigation habits.
Figure 3. Stumped avocados for lack of water.
Figure 4. Scaffolded avocado that should produce fruit sooner than a stumped avocado.
I was just speaking to a group of Certified Crop Advisors and there was some confusion about the units used by different labs to report their results, so I put together this sheet to help understand the relationship between the different terms. They are usually interchangeable, but one needs to know how they convert between each other. So here is a cheat sheet.
Common ions in water: calcium (Ca2+), magnesium (Mg2+), sodium (Na1+)
sulfate (SO42-), chloride (Cl-), carbonate (CO32-), bicarbonate (HCO3-), boron (H3BO3)
Measured as parts per million (ppm) or milligrams per liter (mg/l), which are interchangeable , or milliequivalents per liter (meq/l). A milliequivalent is the ppm of that ion divided by its atomic weight per charge.
Example: Ca2+ with atomic weight of 40 and a solution concentration of possibly 200 ppm. Ca2+ has two charges per atom, so it has a weight of 20 per charge. 200 ppm divided by 20 = 10 meq of calcium for a liter of water.
Total Dissolved Solids (TDS): measure of total salts in solution in ppm or mg/L
Electrical Conductivity (EC): similar to TDS but analyzed differently.
Units: deciSiemens/meter(dS/m)=millimhos/centimeter (mmhos/cm)=
1000 micromhos/cm (umhos/cm).
ConversionTDSEC: 640 ppm=1 dS/m= 1 mmhos/cm=1000 umhos/cm
Hardness: measure of calcium and magnesium in water expressed as ppm CaCO3
pH: measure of how acid or base the solution
Alkalinity: measure of the amount of carbonate and bicarbonate controlling the pH, expressed as ppm CaCO3.
Sodium Adsorption Ratio (SAR): describes the relative sodium hazard of water
SAR= (Na)/((Ca+Mg)/2)1/2, all units in meq/l
1.5 feet of water with EC of 1.6 dS/m adds 10,000 # of salt per acre
and that same water with 20 mg/l of nutrient will supply 80# of that nutrient/acre
Sea water has ~ 50 dS/m, 20,000 ppm Cl, 10,000 ppm
Irrigation water WATCH OUT- 1,000 ppm TDS, 100 ppm Na/Cl, 1 ppm B
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