Excerpted from University of CaliforniaA griculture Sciences Bulletin
Recent research has helped us diagnose mineral deficiencies in avocado more accurately and to apply the appropriate amounts or fertilizers for best tree growth and avocado production. The following recommendations are based on limited experiments coupled with successful grove practices conducted in most of the avocado growing areas of Southern California. Avocado trees have comparatively few mineral deficiencies in commercial orchards in Callfornia. Only nitrogen and zinc need to be applied extensively; iron chiorosis occurs occasionally.
NITROGEN: is the most widely used fertilizer element for avocado production in California. To maintain normal yields most orchards need annual nitrogen fertilization. Young trees need only a few ounces of nitrogen per tree annually. BUT USE FERTILIZERS WITH CAUTION: Excessive amounts on any size tree applied at any one time can cause root damage, leaf burn, and defoliation. In severe cases trees may be killed.
Symptoms of nitrogen starvation include: lack of vegetative growth; pale green, small leaves; lower yields; premature defoliation; and in severe cases, leaves may show yellow veins. LEVELS. Field research data show that there is a best nitrogen level for highest yield, but how much material you need to apply for best results varies within and among varieties.
For the Fuerte variety, too high or too low a nitrogen content in the leaves will lower fruit production. The most productive range as indicated in the graph is between 1.6 and 2.0 per cent nitrogen in five- to seven-month-old spring cycle leaves.
This same range is applicable to the MacArthur variety. Experimentally, values above 2.0 per cent have not been obtained even with an annual application of 400 pounds of elemental nitrogen per acre.
Limited experimental data for the Jalna variety indicate that highest yields are obtained at about 2.0 per cent lear nitrogen, and that 1.6 per cent is inadequate.
The best range for other varieties, including Hass, should be assumed to be the same as with Fuerte until aiditional information is obtained. The amount or nitrogen that should be applied to achieve the desired leaf level and yield will vary also from orchard to orchard, depending on past applications, soil type, irrigation and tillage practices, and material used.
TIMING. The nitrogen requirement of the tree is greatest during the period of flowering and fruit setting, but too much nitrogen may reduce fruit set. If an adequate supply of nitrogen is available for this period there will be normally enough carry-over in the soil to take care of the tree until the following spring. Chem-ical forms of nitrogen, applied over the area occupied by the roots in January or February will be moved into the root zone by the spring rains. If nitrogen is applied in the irrigation water, apply most of it early in the irrigation seasone
If water penetration is a problem, you may consider supplying part of the nitrogen in the fall in the form of a bulkly organic matter such as manure. When manure is used on nontilled orchards, part of its nitrogen is lost to the air. If the manure is cultivated into the soil, most of the nitrogen will be available to the tree. Its use frequently results in more tip-burn on the leaves because of the high chloride content of many manures in southern California.
ZINC: deficiency or "mottle leaf" as it is commonly called, occurs in many avocado orchards in southern California. The avocado tree may decline and even die without the small but essential amounts of zinc.
The earliest symptoms are mottled leaves developing on a few of the terminals. The areas between the veins are light green to pale yellow. As the deficiency progresses the yellow areas get larger and the new leaves produced are smaller. In advanced stages a marginal burn develops on these stunted leaves, twig dieback occurs, and the distance between the leaves on the stem is shortened, giving a crowded "feather-dustert9 appearance. Yield is reduced and some of the fruits may be more round than is normal for the varietye CONTROL: can be achieved by applying zinc as a spray to the foliage or, in some soils, by application of zinc to the soil. In most areas, best results are obtained by thoroughly wetting the foliage, using conventional pest control equipment.
Proprietary compounds containing zinc have also worked well, including those containing manganese and phosphorus, although there is no evidence that these latter two elements are deficient in Cal-ifornia avocado trees. When using proprietary materials, follow the manufacturer9 5 recommendations on dosage.
Zinc sprays have been successfully applied by aircraft partic-ularly where groves are inaccessible to ground rigs. Airplanes usually apply about as much zinc per acre as ground sprayers but in 10-20 gallons of water per acre.
Timing is not critical, but applications will be most effective in June and July, when spring-cycle leaves are expanded. Severely affected trees may require respraying several ~onths later.
IRON: deficiency is difficult to correct. Fortunately, it is not generally found in California orchards.
In mild form, iron-deficient leaves show a network of green veins and veinlets against a lighter green background. As the deficiency increases, the interveinal area becomes yellowish white and the veins lose their green color. In severe cases leaves are smaller, completely chiorotic and show tip and marginal burn. This is accom-panied by defoliation and twig dieback. Iron chlorosis may occur on individual limbs or affect the entire tree and yield may be materi-ally reduced.
CONTROL. This deficiency occurs usually on soils containing lime which limits the utilization of iron by the tree. The deficiency is accentuated by excess soil moisture and low oxygen content in the soil. The use of less water is sometimes helpful. This can be accomplished by shifting to the alternate middle procedure of irri-gation or to replace furrows with sprinklers.
EXCESS OF SALTS. Often salt accumulations are confused with a nutritional difficultye The avocado is particularly sensitive to salts. It accumulates chlorides and sodium more readily than most other tree crops. An accumulation of chloride manifests itself as a tip and marginal burn of the older leaves, premature defoliation, and sometimes a progressive mottled yellowing behind the bud. Excess sodium shows up as an interveinal leaf burn along with twig die-back. A rapid burn of the leaves at the tips or at the base of the lear followed by defoliation suggests either an excessive fertilizer application or inadequate irrigation.
Through experience gained thus far in leaf analysis, the ranges of elements in avocado leaves have been established tentatively. These are useful as a guide to fertilization and orchard management but should not be taken as the absolute values for all varieties under all conditions.
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