[Arid_gardener] Question from Home-Hort WWW page

[Dick]

Avocados will absolutely take full sun provided that you irrigate at least three feet deep at the drip line as soon as you detect dry soil three inches deep. When you can sink a soil probe at least three feet deep at several places around the drip line, you can shut the water off. Irrigate again when the soil in the basin splitting the baseline is again  dry about three inches deep. The species  I fertilize only with 21-0-0 at the drip line with 1/3rd the annual nitrogen requirements based on trunk diameter applied in March, June and September. Avocados also require abundant phosphorous but our soil is loaded with it already. All desert soils are deficient of nitrogen and I rarely apply anything but 21-0-0.

Avocados cannot tolerate wet feet so your first consideration is for perfect drainage or irrigation practices consistant with the soil percolation rate. Irrigation water must flow all the way through the feeder root zone pulling oxygen in behind it. Root-rot, occurring in the absense of oxygen, is sudden, complete and irreversal.  I have seen four year old avocado trees flat on the ground almost over night but inexplainable wilt is usually the first symptom.
Drainage is critical because as the water table travels through the root zone, vital oxygen (air) is drawn in behind it. 

Avocado roots are relatively shallow. Therefore, a three or four inch thick surface-layer of organic mulch is an advantage in keeping feeder roots cool but avocado foliage naturally shrouds the trunk all the way to the ground and should be encouraged to do so unless you must park your car beneath it. Micro-organisms feed off the bottom of the decaying mulch and their frass is an important food source mfor avocados.

Avocado bark cannot tolerate direct sun, not even in San Diego. Extended direct sun will always kill the bark and that kind of damage can be seen on every tree I have ever seen in the valley except one in South Phoenix growing in mostly shade provided by an overhang. A coat of white latex paint will, however, totally protect bark exposed to full sun by reflecting rather than adsorbing the rays.

Like most other trees, feeder roots are in a zone around the drip line, To get proper irrigation, it is important to maintain a shallow trench roughly splitting the drip line. The trench center line should coincide with the drip line where a shallow 1\2 inch head of water should be maintained until you can easily sink a soil probe 36 inches deep several places around the drip line, deep irrigation is critical, then you can shut off the water. With irrigation in the right place, one never has to irrigate inside of a shallow trench. I have several healthy heavy bearing citrus that have never been irrigated anyplace but the drip line in 40 years--unless it rained. Fat chance.  Don't irrigate again until the soil is quite dry at least three inches deep.

Avocados cannot tolerate wet feet and are not salt tolerant, both requirements for deep flush irrigation and excellant drainage. 

I had once identified six seedling avocado trees bearing edible fruit  in the Salt River Basin even though legend tells us the seed never runs true. Avocados are always picked green and hard and ripened off the tree but I had one tree in San Diego bearing abundant fruit that would never ripen except for an eighth of an inch beneath the skin regardless of what I did to make it happen. But it was a beautiful landscape speciman to which I later grafted several other avocado varieties.

Avocado trees do well here but one has to deal with their peculiar sexuality and poor tolerance to salt existing abundantly in Arizona water and soil. Most plants can tolerate fairly well the natural water salt level but it is the concentration of salt in the shallow root zone by evaporation that tends to concentrate saline compounds to toxic levels. That is why deep irrigation is very important to flush salt below the feeder roots. A four inch layer of mulch covering the basin of avocados will help keep salt levels down.

Further questions?

Regards.

Dick Gross, Sec/Ed.
Arizona Rare Fruit Growers

----- Original Message ----- 
From: <CHEROKEE991 at cox.net>
To: <arid_gardener at Ag.arizona.edu>
Sent: Sunday, April 13, 2008 3:39 PM
Subject: [Arid_gardener] Question from Home-Hort WWW page


> JIM CAVINESS
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> CHEROKEE991 at COX.NET
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> WILL AVACADO TREES TAKE FULL AZ SUN
> 
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