UCCE Farm Advisor Gary Bender finally has his 14 chapter book on avocado history, botany and cultural practices on the San Diego County web site. Check it out:
Along the coast, it is very common to see windbreaks protecting the citrus and avocado groves. Invariably the first two rows next to the eucalyptus trees are shorter and less thrifty than the citrus further away from the windbreak. This is due to competition primarily for water, but somewhat due to light, as well. Often by putting emitters on the windbreak, the completion stops. Growers will also root prune between the windbreak and the first row of citrus. Those roots inevitably grow back and pruning must be done again. This also occurs in areas where there are oak trees or other natives that are planted in or around the orchard. Growers will frequently plant right up to the canopy or even under the canopy of the native tree(s), with a similar result seen with windbreaks.
It is important to remember the architecture of roots. Not all trees are exactly alike, but a general rule of thumb is that the active roots go out one and half times the height of the tree. So a 40 foot tree will have competitive roots out 60 feet away from the trunk. That’s why it is best to keep a distance away from a competing tree, because avocados and citrus are just not as competitive as an oak or eucalyptus.
In low rainfall years, this competition is even more intense. Significant defoliation of the crop plant can be seen. The grower then thinks that it is some disease and ponders what to spray, when they should actually be spraying more water.
- Author: Larry Schwankl
Chemical treatment of water for microirrigation systems is required when the water may cause chemical precipitate or biological clogging of the microirrigation drippers or microsprinklers. The chemical treatment varies depending on the clogging source.
Biological clogging problems are most often associated with surface waters—waters that have been stored in reservoirs or ponds, or transported in canals, rivers, etc. While it is often difficult to identify the biological contaminant, algae and biological slimes are often major contributors to biological clogging.
Groundwaters high in iron may also be a biological clogging hazard for microirrigation systems. The dissolved iron in the water provides an energy source for the iron bacteria. The gelatinous slime produced by the iron bacteria can clog emitters, often in conjunction with particulates (silt or clay particles, chemical precipitates, or other contaminants) for which they can provide a “glue” to bind particles together.
Levels of Concern
Certainly any waters that appear “green” prior to use are capable of causing biological clogging but even surface waters which appear clean may be a clogging hazard. Since surface water quality can change drastically across the season, often caused by rising temperatures and falling water levels, it is often not worthwhile to attempt to quantify the biological clogging hazard. It is better to monitor the microirrigation system for any sign of biological clogging and if it appears, treat the water. Often there is a history of biological clogging problems and the manager knows that treatment is required.
Biological treatment methods involve using a biocide that kills the biological contaminant. The two most common biocides used with microirrigation systems are chlorine and copper. Historically, chlorine products have been most frequently injected into microirrigation systems while copper products have been used to control biological growth in ponds and reservoirs. This has changed somewhat with the availability of new copper-based formulations developed for injection into microirrigation systems.
The following are recommended levels of chlorine for biological contaminant control:
Injection Method and chlorine concentration at the end of the last lateral
Continuous injection 1-2 ppm
Periodic injection 10-20 ppm
Contact time between the water with chlorine and biological contaminant is important. Periodic chlorine injections should be at least 4 hours and longer is better. Chlorine injections can continue up to system shutdown, with the chlorinated water left sitting in the lines. This may have limited effect on above-ground lines since they tend to drain out at the lowest point(s), but it may help clean up other parts of the system.
Copper levels to provide effective biocide protection are also quite low, often copper levels less than 5 pm are effective. Follow manufacturer’s recommendation for formulations containing copper.
Chemical Precipitate Clogging
Most chemical precipitate clogging problems are associated with groundwater sources. Elements in solution in the groundwater may precipitate above ground and the precipitates may clog the microirrigation system’s small emitter passageways.
There are many potential chemical precipitates which can cause clogging problems, but calcium carbonate (lime) and iron are two of the more common problems. Lime precipitation is the most common and can occurwhen calcium and bicarbonate levels in the water are 2 meq/l or higher and the water pH is 7.5 or higher.
The most common treatment for lime precipitation clogging is to lower water pH to 7.0 or below. A pH in the range of 6 to 6.5 is effective in removing the calcium carbonate precipitate while not being of risk to system components.
Iron precipitate clogging is not as common as lime precipitation but it is more difficult to deal with. Iron precipitate clogging can occur when the iron levels are 0.2 ppm or higher, although most problems occur when iron levels are 1 ppm or higher. Water pH only needs to be 4.0 or higher for iron precipitation so this pH level includes nearly all waters.
Most people deal with iron precipitation problems by pumping the groundwater into a pond or reservoir where the iron precipitates and settles out. Adequate time is needed for the small precipitates to settle. This dictates an adequately sized pond.
A relatively new way of dealing with iron and calcium carbonate precipitation problems is to continually inject a product containing phosphonate or phosphonic acid. The phosphonate (or phosphonic acid), injected at rates of 5 ppm or less, interferes with the precipitation. There are a number of anti-clogging formulations on the market which contain phosphonate or phosphonic acid as their active ingredient. Phosphonate or phosphonic acid products may be very beneficial for iron clogging problems, for which only aeration/precipitation and settling are currently the only solutions.
Chemicals are often injected through irrigation systems, hence the name chemigation. When applied to fertilizers, it is called fertigation. Surprisingly many growers still apply fertilizers by hand, however the predominant use of micro-irrigation systems in avocados readily lends itself to fertigation. This compatibility is due to the frequency of operation and the control the operator has over the system. If the uniformity of the system is high, applying fertilizers through the irrigation system can improve fertilizer distribution, allow for flexibility in fertilizer timing, reduce labor in spreading materials, allow for less fertilizer to be used and can lower costs relative to hand applications of fertilizers.
Chemigation can still cause environmental damage, particularly when the chemicals injected move readily with the irrigation water. Over-irrigation resulting in deep percolation can contaminate groundwater when a mobile chemical is injected. Contamination can occur if: 1) the irrigation water pumping plant shuts down while the injection equipment continues to operate, causing contamination of the water source or unnecessary amounts of fertilizer to be injected into the irrigation system, or 2) the injection equipment stops while the irrigation system continues to operate, causing the irrigation water to flow into the chemical supply tank and overflow onto the ground. To prevent these problems, backflow devices and check valves can be installed. Local regulations should be followed in selecting these devices.
Many different materials may be injected, including organic fertilizers, dry fertilizers and liquids. The major fertilizer injected is nitrogen, but potassium and micronutrients, as well as water treatments, such as urea-sulfuric acid are also injected. The important point to remember is that the materials must be soluble. Fertilizers delivered as a solution can be injected directly into the system, while those in a dry form must be mixed with water to form a solution.
Fertilizer material differs widely in water solubility, with solubility depending on the physical properties of the fertilizer as well as on irrigation water quality. Agricultural grade fertilizers and amendments are often coated to inhibit moisture absorption and to assist in material flow through machinery. These coatings and other foreign materials can cause clogging problems in the mixing tank, as well as the irrigation system. The foreign material exists in the tank as sediment or as a scum on the surface. To prevent problems, stock tanks should be agitated until the material is dissolved. To further prevent problems, the solution should be filtered between the stock tank and the injection point, and the injection point should be upstream of the irrigation system filters.
Irrigators wishing to inject chemicals have a variety of injection equipment from which to choose, including differential pressure or batch tanks, venturi devices and positive displacement pumps. The most expensive are displacement pumps that are powered by electricity or water. They put out precise amounts of material. Venturi and batch tanks are much less expensive and are relatively simple to operate. Their major drawback is that they require a pressure loss to force the material into the irrigation system. A 10 - 30 psi pressure differential is often required and in some places this differential is not possible. In that case a fractional-horsepower pump can be used to provide the pressure differential. Neither of these devices is as precise as the displacement pumps. However, fertigation does not require a fixed concentration of solution, only a known amount of applied material. And these pressure differential applicators do the job very well.
- Author: Larry Schwankl, Freddie Lamm, Dana Porter
Maintenance of Microirrigation Systems
Predicting Clogging Problems
Solutions to Existing Clogging Problems
System evaluation for emission device clogging
Routine Maintenance Tasks
Microirrigation systems include microsprinklers for tree crops, drip emitters for trees, vines, and some row crops, and drip tape for row and field crops. Microirrigation systems apply water to the soil through emitters that are installed along drip lines and contain very small flow passages. Microirrigation systems can apply water and fertilizers more uniformly than other irrigation methods. This uniformity results in potentially higher yields, higher revenue, and reduced irrigation operating costs.
Uniformity, a performance characteristic of irrigation systems, is a measure of the evenness of the applied water throughout the irrigation system. Distribution uniformity (DU), sometimes called emission uniformity (EU), is an index that describes how evenly or uniformly water is applied throughout the field. A uniformity of 100% means the same amount of water was applied everywhere. Unfortunately, all irrigation systems apply water at a uniformity of less than 100%, and thus some parts of a field receive more water than others. Field evaluations have shown that microirrigation systems have the potential for higher uniformity than other irrigation methods. However, clogging reduces the uniformity of applied water in microirrigation systems, thus increasing the relative differences in applied water throughout a field.
The small flow passages in the emitters and microsprinklers make microirrigation systems highly susceptible to clogging. Clogging reduces the uniformity of the applied water and decreases the amount of applied water. Clogging also decreases the amount of salt leaching around the lateral line in saline soils.
The objective of this web site is to provide information to irrigators about the causes of clogging and the methods for preventing or correcting clogging problems in microirrigation systems. Among the topics covered are the sources of clogging, chlorination, preventing chemical precipitation, filtration, flushing, and monitoring microirrigation systems.
This web site is divided into sections to allow the users to more quickly access the information they want. For example, if you already know you have a clogging problem and you want to solve it, go to the section Solutions to Existing Clogging Problems - "I have a problem and I want to solve it".