Avocado trees have comparatively few known mineral deficiencies in commercial groves in California. Generally, only nitrogen and zinc need to he applied extensively. Iron chlorosis occurs occasionally. Recent studies indicate the possibility that there may be phosphorus starvation in some situations.

Nitrogen is the most widely used fertilizer in California avocado production. Young trees need only a few ounces of nitrogen per tree annually. Older trees need somewhat larger annual applications to maintain normal yields.

Lack of vegetative growth. Pale green, small leaves Reduced yields Premature defoliation Leaves with yellow veins (severe deficiency)

Different varieties have their own nitrogen needs, and their responses to nitrogen dosages will differ (Table 1). For varieties other than MacArthur, no advantage appears to come from nitrogen levels in the leaf above 2.0 percent. In MacArthur, the leaf level usually will not reach 2.0 even when fertilizer is wastefully applied. In Fuerte, high levels of nitrogen actually reduce yield. For varieties not listed, a leaf nitrogen level of about, but not over, 2.0 percent is suggested.

* Levels In 5- to 7-month-old spring-cycle leaves from mature trees.

Leaf samples for nitrogen measurement should be the youngest fully expanded and mature leaves available in the August- October period. These would normally be spring cycle leaves, five to seven months old. The results should govern the amount of nitrogen to be applied in the following year. If the nitrogen level is higher than desired, less nitrogen should be applied than before; if the level is lower than desired, increase the nitrogen dosage; if at the desired level, stay with previous dosage (assuming there has been no change in other cultural practices).

Agricultural laboratories are available throughout California's avocado growing districts to perform leaf analysis. It is wise to let trained laboratory personnel collect and analyze leaf samples. The analysis is no better than the sample collected, and training is required to select the correct leaves for analysis.

Where leaf analysis is not used, it is suggested that annual applications per acre in mature orchards be in the range of 100 to 200 pounds of elemental nitrogen from all sources for most varieties. For Hass, 150 to 200 pounds is suggested. Because some soils retain nitrogen more efficiently, and because irrigation water may contain some nitrogen, the observant grower will eventually be guided to the best dosage for his trees by tree appearance and fruit yield.

A planted or volunteer cover crop under or near the tree will compete with it for nitrogen. Additional fertilizer should be applied to overcome this competition.

The amount of nitrogen that should be applied to achieve the desired leaf level and yield varies from grove to grove depending upon past applications, soil type, irrigation and tillage practices, and materials used. This is a situation where "the footprints of the owner" is the best fertilizer; observation and careful records will ultimately define the best nitrogen regimen for each grove.

In groves with mixed varieties and production patterns, this rule of thumb is useful. On trees with heavy fruit set, little new growth, and pale leaf color, use extra nitrogen - up to twice the grove average. (Caution: be sure these symptoms are not from some other cause, such as root rot or under irrigation.) On mature trees with few or no fruits, strong vegetative growth, and dark green leaf color, use less or no nitrogen.

Use fertilizers with caution. Application of nitrogen at rates higher than necessary to maintain yield is not only costly but can contribute unnecessarily to nitrate pollution, salinity, or soil acidity problems. Too much fertilizer applied at one time to any size tree can cause root damage, leaf burn, and defoliation. In severe cases, trees may be killed.

The nitrogen requirement of the avocado tree is greatest during the time of flowering and fruit setting. However, timing of nitrogen application has had no practical influence upon fruit yield or quality.

Chemical forms of nitrogen, applied in January or February over the area occupied by the roots (roots concentrate where irrigation water is applied), will be moved into the root zone by rains. If nitrogen is applied in the irrigation water, application of most of the annual dosage of nitrogen should be made early in the irrigation season.

Alter the trees come into commercial bearing, the timing of nitrogen application does not influence fruit production. The critical factor is the amount of nitrogen applied per year. Therefore, the scheduling of nitrogen application can be at the grower's convenience. Although experimental evidence does not give statistically significant differences, there have been small but statistically non-significant increases in fruit production in favor of applying an appreciable amount of nitrogen prior to bloom.

Materials In selecting a nitrogen source material, consider economy, residual effects on soil reaction, salinity and tilth, and convenience. Refer to Table 2 for the nitrogen content of some commonly-used fertilizers.

Cost per pound of nitrogen can be determined by this formula:

(Cost per ton x 100) / (Percent nitrogen x 2,000 pounds) equals Cost per pound of elemental nitrogen

Example: urea, $250 per ton, 46 percent nitrogen

($250 x 100) / (46 x 2,000) equals 27.2 cents per pound of elemental nitrogen

 Continuous use of ammonium sulfate for many years may make the soil so acid that the penetration rate of irrigation water is reduced and the availability of other nutrients is affected. Continuous use of ammonium nitrate, anhydrous ammonia, or urea will also acidify the soil, but less rapidly than will ammonium sulfate. Many alkaline irrigation waters tend to offset this effect.

Usually, one does not need to be concerned about the contribution of fertilizer salts to soil salinity problems. However, under certain conditions, such contribution must be taken into account. Data in Table 2 show that, in general, the higher the nitrogen content in the fertilizer, the lower the salt index per unit of applied nitrogen.

If water penetration is a problem, consider applying part of the nitrogen in the fall in the form of bulky organic matter, such as manure, which is also a good source of phosphorus and potassium. When manure is used on non-tilled orchards, part of its nitrogen is lost to the air.

If the manure is cultivated into the soil, most of its nitrogen will be available to the tree. Use of manure fertilizers frequently produces tip burn of the leaves because of the high chloride content of many manures in southern California.

* The bigger the value, the greater the salinity hazard. ** Residually basic material. *** 1 gallon UAN-32 = 3.55 lbs, elemental nitrogen at 60 F.

Fertilize young trees cautiously so as not to harm them, yet sufficiently to ensure maximum growth. Table 3 gives suggested amounts of nitrogen to inject into a drip or irrigation system. Dry fertilizer should be applied evenly within the irrigation pattern for each tree irrigated by a low-volume system. This will occur naturally if fertilizer is applied through the irrigation system. If irrigation is sprinkler-applied over a large portion of the orchard floor, broadcast the fertilizer just before a rain or irrigation, The suggested amounts in Table 3. may require adjustments determined by leaf analysis after coming into commercial bearing.

* Ounces or fluid ounces of fertilizer material per tree per month during a typical 8 month irrigation season.

**Some avocado varieties come into commercial bearing at a younger age. The amount shown in the table is an approximation; leaf analysis preferably should be used as a guide to the amount of nitrogen to apply annually.

Zinc deficiency, commonly known as "mottle leaf," occurs in many southern California avocado groves. The tree may decline and even die without a small, but essential, amount of zinc. Phosphorus fertilizer application, as discussed in the next section, may induce or aggravate zinc deficiency. Poultry manure applications, since it is high in phosphorus, may also interfere with zinc nutrition.

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 duster" appearance. Yield is reduced, and some of the fruit may be smaller and rounder than is normal for the variety.

Control can be achieved by application of zinc as a foliar spray or, in acid conditions (soil pH < 7.0), to the soil.

In most areas, best results are from foliar sprays, using conventional pest control equipment to wet the foliage thoroughly. Amounts of spray requited per acre depend upon tree size. For young bearing groves, 300 to 400 gallons are suggested; whereas, for mature groves of large trees, apply up to 600 to 800 gallons. Either of the following two formulas is satisfactory.

Formula No.1 1 pound zinc sulfate (36% metallic zinc) in 100 gallons water 

Formula No.2 2 pounds zinc oxide m 100 gallons water

Proprietary zinc compounds have also worked well. When using proprietary materials, follow the manufacturer's dosage recommendations.

Zinc sprays have been successfully applied from airplanes and helicopters, particularly where groves are inaccessible to ground rigs. Air applications require about as much zinc per acre as do ground sprayers in mature groves, but in 10 to 20 gallons of water per acre. The low volume/high concentration method also applies to concentrate ground sprayers.

Timing normally is not critical, but applications are most effective in June and July, when spring-cycle leaves are expanded. Because zinc does not translocate readily from sprayed leaves to young growth occurring after spraying, severely affected trees may require re-spraying.

Soil applications of zinc have been successful on acid soil (pH< 7.0) in San Diego and Santa Barbara Counties. This method of treatment is particularly useful where crowded trees or steep terrain limit use of spray rigs. Responses to banded soil applications of dry zinc sulfate have lasted as long as five years, whereas foliar sprays and probably zinc applied through the irrigation system must be repeated annually. Another use for soil treatment is on those trees that are so deficient in zinc that an annual foliar spray does not correct the deficiency. Zinc (as zinc sulfate or zinc chelate) may also be applied through sprinklers or drip irrigation systems.

The methods of banded soil application are either to place the zinc on the soil surface in a 3 to 5 inch wide band or to pour a handful into each of 6-inch deep holes made by a shovel at 15 to 20 places - by either method, in the irrigation zone.

The effectiveness of zinc application to the soil depends upon the soil type, the amount used, and the method of application. Some trees have been injured, so it is wise to try an application on a few trees first.

As a guideline, Table 4 shows suggested amounts of zinc sulfate (36% metallic zinc). Chelated forms of zinc may correct the deficiency, but appear to have no advantage over zinc sulfate. In normal maintenance, adjust rates according to leaf analysis values.

Phosphorus occasionally needs to be applied to avocado trees in California. Favorable responses have been observed in San Diego County and in Rancho California.

There are no specific symptoms for the initial stages of phosphorus deficiency. A deficient tree generally tends to be stunted and totally lacks vigor. Twig die-back is common in moderate to extreme cases. The tree canopy is open and sparsely foliated with little leaves. The presence of little leaves could lead one to believe it is zinc deficiency, except that the characteristic leaf mottling is normally absent. In extreme deficiency, randomly distributed necrotic areas may occur on the leaf surface (see figures).

Phosphorus deficiency may or may not affect all trees in a grove; rather, it may occur in isolated, random groups of trees. These groups of trees most likely indicate that the entire grove is borderline for phosphorus and would benefit from phosphorus application.

Before the decision is made to treat with phosphorus, confirmation of phosphorus deficiency by leaf analysis is strongly recommended. If the leaf analysis shows leaf concentrations of < 0.07% (most varieties other than Fuerte) or < 0.05% (Fuerte), the grove definitely should be treated with phosphorus. If leaf analysis indicates levels greater than 0.10% (varieties other than Fuerte) or 0.08% (Fuerte), it is likely that phosphorus treatment will not be beneficial.

Control can be achieved by application of a liquid source of phosphoric acid in the irrigation system or as a soil band of dry fertilizer applied in the irrigation wetting pattern. A positive response has been observed with a corrective application of five gallons/acre in a mature grove through the irrigation system. A maintenance program of three gallons/acre annually has been observed to be sufficient.

In a mature grove, 2.5 pounds of P205 per tree as a band in the wetting pattern should be adequate to correct phosphorus deficiency. This treatment should be effective 3-5 years. 

Iron deficiency is difficult to correct. It is most commonly found in Ventura and Santa Barbara Counties and does not occur on acid soils.

In mild forms of iron deficiency, leaves show a network of green veins and veinlets against a lighter green background. As the deficiency worsens, the inter-veinal area becomes yellowish white, and the veins lose their green color. In severe cases, leaves are smaller, completely chlorotic (yellow), and show tip and marginal burn. This is accompanied by defoliation and twig die-back. Iron chlorosis may occur on a portion of the tree, on individual limbs, or may affect the entire tree; and yield may be materially reduced.

Iron deficiency usually occurs in soils containing lime (calcium carbonate), which limits the tree's utilization of iron. The deficiency is accentuated by excess soil moisture and low oxygen content in the soil. The use of less water, by shifting to the alternate middle procedure of irrigation or replacing furrows with sprinklers, is sometimes helpful.

The use of Mexican race rootstocks (such as Topa Topa, Duke, and Ganter) rather than rootstocks of the Guatemalan race has minimized the amount of iron chlorosis.

Chelate forms of iron have given variable results. Treatment is expensive, and results are short-lived, The most effective application methods have been to inject the chelates in solution into the root zone; or to rake back the leaves, broadcast the material, work it into the surface soil, push the leaves back, and irrigate immediately. The suggested amounts of Sequestrene® 138-iron are 1/4 to 1/2 pound per mature tree applied in May or June. Annual applications appear necessary. 

Often, salt accumulations are confused with a nutritional difficulty. 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 burn. Excess sodium shows up as an interveinal leaf burn along with twig dieback. A rapid burn of the leaves at the tips or at the base of the leaf followed by defoliation suggests either an excessive fertilizer application or inadequate irrigation. 

The ranges of elements in avocado leaves have been established tentatively, as shown in Table 5. These are useful as guides to fertilization and orchard management, but should not be taken as the absolute values for all varieties under all conditions. Leaf analysis from trees prior to coming into commercial production are more difficult to interpret, particularly for nitrogen. In the absence of other deficiencies or excesses, zinc and iron deficiencies may be diagnosed from visual symptoms.