Avocado June Jobs - 1999

Irrigation should be in full swing this time of year.  I often get asked how frequently irrigation lines should be checked for clogging and breakage.  The answer is every time an irrigation is applied.  That does not sound like fun, but during the season when young coyote pups are roaming and chewing on lines or after pickers have gone through or ants have suddenly discovered that the irrigation line makes a nice hideout, it is not uncommon to have damage occurring on a weekly basis.  When water is expensive or hard to come by, a regular walk can save some money. 

Especially with drip emitters, with their greater frequency of application, missing a few irrigations can severely stress a tree.  Even the partial clogging of a dripper can lead to a stressed tree.  It is often difficult to determine whether a dripper is fully or partially clogged.  A simple test that can be done on occasion is to take a 35 mm film can and capture water from the emitter for 35 seconds.  If full after that time, that represents a rate of about one gallon an hour. If the can fills in 15 seconds, it is a two gallon per hour emitter.  This can test on drippers or a similar one done on microsprinklers with larger containers is invaluable for determining emission uniformity and making sure that all the emitters that you think are putting out a certain amount, actually are doing so. 

On the topic of irrigation systems, are you fully using its capabilities?  With a fertilizer injector there should be no need for tromping around making dry urea applications.  There are a large number of growers who still have not adapted their systems.  Putting fertilizer through the irrigation system puts the material where the roots are, and the roots are found where the water is.

Another material that should be considered being injected through the irrigation system is zinc.  In the past, an air application  was considered the only possible means of supplying zinc.  Zinc deficiencies have been reported on calcareous soils, such as found  along the coast north of Los Angeles, and also in acid, sandy soils low in zinc, such as San Diego.  With an air application it is necessary to time the application to the spring leaf flush, before it has hardened off. 

In general, foliar zinc is considered immobile, meaning that the zinc stays where it is applied.  If the roots require zinc, and a foliar application is made, there is some debate whether the zinc will ever arrive below ground.  In fact, there are cases where a foliar zinc may not go beyond the surface of the leaf to the interior where plant cells can use it.  However, zinc taken up by roots is carried to the leaves. 

To evaluate whether irrigation-applied zinc sulfate was effective on a calcareous soil (pH 7.8), we have made quarterly applications of 2 pounds of zinc sulfate per tree  for the last two years through microsprinklers.  The application effectively doubled the leaf zinc within a year, indicating that uptake was effective.  Using this labor saving technique, of course requires that the liquid zinc sulfate is compatible with your water, and that clogging precipitates do not occur.  It may also be necessary to acidify the water, if the pH is too high, in order to keep the zinc in solution.  We are still learning about the technique and are not sure whether the rate is too high or whether a little bit, applied more frequently would be the trick.  But we are learning how to better use our irrigation system.