TRACE A. OF AVOCADO

David E. Crowley1, Woody Smith1, Ben Faber2, A. deficiency 3 Mary Lu Arpaia Typical leaf zinc deficiency 1 Dept. of Environmental Science, University of California, Riverside, CA symptoms. Note the chlorosis 2UC Cooperative Extension, Ventura County, CA (yellowing) between veins and 3UC Kearney Agricultural Center, Parlier, CA the reduced leaf size (top).

B. deficiency SUMMARY Iron deficiency symptoms on It is easy to confuse zinc and iron deficiency symptoms, new growth. Iron deficiency so growers should rely on annual leaf analysis. Also, it is and zinc deficiency are easily essential for avocado growers to know their soil pH, confused. and soil iron and zinc levels before embarking on a fertilization program. For zinc, best results are with soil B. application through fertigation or banding, and there is no apparent advantage to using chelated zinc. For iron fertilization, the best solution is to use soil applied Sequestrene 138, an iron chelate. It is highly stable and can last for several years recycling many times to deliver more iron to the roots. It is important not to mix trace metal with phosphorus fertilizers.

INTRODUCTION Avocados require many different nutrients for growth that include both macro and micronutrients. Macronutrients such as nitrogen, potassium and phosphorus are generally provided as fertilizers. Micronutrients are those nutrients that are essential for plant growth and reproduction, but are only required development of small round fruit. The first step in treating at very low concentrations. Most micronutrients, or trace trace metal deficiencies is to determine exactly which elements, are involved as constituents of enzyme metal micronutrients are limiting since both iron and zinc molecules and other organic structures. Zinc and iron deficiencies can cause similar foliar symptoms. Although are important trace elements required for plant health. zinc deficiency is often considered to be the most common Zinc is an important component of a number of key trace metal deficiency in Southern California, iron metabolic enzymes as well as influencing protein synthesis, deficiencies are actually much more common. In both carbohydrate and auxin metabolism and membrane cases, the leaves show chlorosis (yellowing) that is caused integrity. Iron plays a key role in the process of respiration by problems with chlorophyll synthesis (Figures A and B). and the manufacture of chlorophyll for photosynthesis. Another possibility is that trees showing leaf chlorosis may simultaneously have both zinc and iron deficiencies, in Most trace elements are normally available in soil in which case both problems need to be corrected at the sufficient quantities. When trace element deficiencies do same time. For zinc, normal leaf levels should range occur, it is usually the result of chemical conditions in the between 20 to 40 ppm. Leaf deficiency symptoms will soil that make metal elements insoluble and unavailable to occur when the foliar zinc concentrations fall below the plant. This is especially true with zinc and iron that 15 - 20 ppm. Iron deficiencies on the other hand occur at have very limited solubility at pH 6 or above. Trace metal leaf concentrations below 35 - 50 ppm, although in some deficiencies may also be caused by certain chemical cases trees may show iron deficiency symptoms at much reactions that occur in soils containing lime or that are higher leaf concentrations. This latter problem is due to irrigated with water containing high amounts of bicarbonate. the uptake of bicarbonate from the irrigation water, which Poor soil drainage and root disease can also be contributing inactivates the transport of iron in the leaf tissue. In factors that will limit the growth of feeder roots that are addition to problems caused by irrigation water, bicarbonate responsible for metal uptake. An understanding of which ions can also be produced in the soil when calcium factors are causing a trace metal deficiency can be helpful carbonate (lime) dissolves and generates HCO ions. In in determining the best method to correct the problem, 3 situations where soils contain high levels of lime, it may be and whether or not the trees should be treated with a trace nearly impossible to correct trace metal deficiencies since metal . the problem is not due to metal availability, but is instead Very often trace metal deficiencies are indicated by the a physiological problem in the plant tissue (Crowley and appearance of leaf yellowing or, in the case of zinc, by the continued on page 6 (o-hydroxyphenylacetic acid). All of growth (Soil Testing Plant Analysis TRACE METAL these materials are water soluble, but Handbook, 1990) and plants such as NUTRITION it is important to choose the right one peanut may even be poisoned when continued from page 5 since they have different abilities to zinc concentrations exceed 36 ppm Smith 1996). Fortunately, this problem form stable metal complexes depending (Borkert et al., 1998). After reviewing usually occurs only in “hotspots” in on the soil pH. For example, EDTA soil analysis reports from commercial the orchard, which attract attention prefers to chelate calcium rather than labs that were generated for avocado but are not indicative of conditions iron or zinc at neutral to alkaline pH orchards in San Diego County, we throughout the orchard. In some (pH > 7.0). In higher pH soils, iron found that many avocado orchards cases, iron deficiency symptoms may and zinc chelated with EDTA will now contain DTPA extractable zinc even occur on one side of a tree but eventually be displaced by calcium concentrations that range between 100 not on the other, which is due to root and the metal ions will no longer be to 200 ppm, or more than 100 times growth into lime pockets in the soil. available to the tree. On the other the normal levels that are needed by hand, the metal chelator DTPA is plants. Not only is this excessive, but relatively stable with zinc at high if more zinc is applied, it may eventually METHODS FOR CORRECTION OF TRACE pH and may also be used to supply accumulate to levels that exceed the METAL DEFICIENCIES and iron. For iron fertilization, legally allowable quantities for soil the best chemical is EDDHA contamination under U.S. EPA Under normal conditions, avocado (Sequestrene 138). This metal chelator regulations (presently 2800 ppm in trees will use naturally occurring is highly stable and can last for several U.S., 200 ppm in Europe). There is organic matter-metal complexes that years in the soil since it recycles many also the possibility that avocado tree will solubilize zinc and iron and other times to dissolve more iron in the roots might be temporarily damaged and hold them in a form that soil and deliver it to the roots. The by excessive fertilization, although can be taken up by the roots. These applied amount of a trace metal this has not yet been investigated. naturally occurring metal complexes fertilizer depends on the type of Growers should, therefore, augment are very effective in low pH soils. fertilizer material as well as the severity their annual leaf analysis for zinc with However, in situations where avocados of the deficiency. Chelates are generally soil analysis to minimize an excessive do not obtain sufficient quantities of provided in amounts of 1.4 to 1.8 oz. build-up of zinc in the soil. metals by natural processes, or where per tree, although in some cases much absolute quantities of the metals are higher quantities may be required if limiting, such as in sandy soils, a variety the trees are located on a calcareous FIELD EVALUATION OF FOLIAR AND SOIL of methods have been developed to soil or a lime “hotspot.” APPLIED FERTILIZERS correct trace metal deficiencies. These To evaluate the currently recommended methods include foliar applications of With metal salts such as zinc sulfate or methods of zinc fertilization and better zinc sulfate and zinc chelates (Goodall zinc oxide, it is much easier to apply determine how much zinc is needed, et al., 1979), trunk injections of trace too much fertilizer, as these materials we conducted a three-year experiment metals (Whiley et al., 1991), or soil are relatively inexpensive and have in Ventura County to compare different applications of zinc and iron fertilizers traditionally been recommended in zinc formulations and application (Wallihan et al., 1958). When trace excessive quantities. Available zinc methods (Crowley et al., 1996). metal fertilizers are applied to the soil, is measured as the amount that can Results of these experiments showed they can be applied as metal chelates be extracted using DTPA under that for soil applied fertilizers, zinc or as various metal salts such as zinc standardized laboratory test conditions, sulfate was the most effective while sulfate, zinc oxide, iron sulfate, and and represents only a fraction of the zinc chelates were the least effective and iron chloride. All of these materials total zinc that is present in the soil. most expensive. Trees fertilized with can be applied directly to the soil or Embelton and Wallihan (1966) the zinc chelates at the manufacturer’s may be injected into the irrigation recommended the use of zinc sulfate recommended rate were no different water. In general, chelated metal at a rate of 7 lbs. per mature tree. than unfertilized control trees. fertilizers are more expensive than This application was to be repeated In comparison, zinc sulfate (7 lbs. metal salts but can be used in smaller every three years. However, if the ZnSO 36% per tree) applied either quantities since they retain their standard recommendation is followed, 4 as a quarterly simulated irrigation or solubility after they are added to soils. the total zinc levels in the soil are annually as a single soil application, In contrast, inorganic metal salts will instantly elevated to greater than 150 increased foliar zinc contents to 75 remain in solution only for a short ppm under the tree after a single and 90 ppm, respectively. However, time and will then precipitate into soil application of the fertilizer. These levels as reviewed above, this concentration minerals that are no longer available will quickly decline as the fertilizer is excessive and much lower quantities to the tree. Metal chelate fertilizers precipitates out as new soil minerals, could probably be used to adjust the can include many different chemicals but will still maintain a very high level leaf tissue contents to levels between that are usually referred to by their for one or two years. In contrast to 20 and 40 ppm. In this experiment, abbreviated names and include EDTA the 150-ppm levels reached after we also examined foliar applications (ethylene-diamine-tetra-acetic acid), fertilization, most plants have been of zinc sulfate or zinc oxide and an DTPA (diethylene-triamine-pentaacetic found to require only 0.5 to 1 ppm organic complexed zinc fertilizer, zinc acid), or EDDHA (ethylene-diaminedi of DTPA extractable zinc for normal metalosate. In general, all the foliar RECOMMENDATIONS density of 110 trees per acre. applied materials appeared to be Following application of the fertilizer effective based strictly on foliar tissue Leaf tissue analysis provide the in the spring, leaf analysis should be analysis. However, with foliar applied most valuable tool for diagnosing taken in the period between late fertilizers there is considerable difficulty trace metal deficiencies. It may be August and early September. The in determining how much of the zinc worthwhile to test different areas in amount of fertilizer to apply the actually penetrates the leaf tissue the orchard before deciding to treat following spring can then be adjusted versus how much remains on the leaf the entire orchard with a fertilizer. according to the tree response that was surface where it can be detected by Many groves located on calcareous obtained the previous year. In our tissue analysis but is of no use to the soils may contain “hot spots” where experience, trees that contain greater plant. In our study, this problem was trees will show iron and zinc deficiencies than 50 ppm Zn will show little or no particularly evident with zinc oxide, that are not easily corrected using response to additional fertilizer even which was easily washed from the leaf fertilizers or foliar applications. If when it is applied in high quantities. surface using a diluted hydrochloric there are only a few affected trees, it acid. Previous research also has may be best to either spot treat these Commercially formulated liquid zinc shown that there may be problems trees, or accept the fact that these sulfate fertilizers are sold by the gallon with translocation of foliar applied trees will have metal deficiencies. and contain 12% zinc by weight. The zinc (Kadman and Lahav 1978). If this liquid is kept at pH 4 - 5 to keep it in is true, then when the tree is sprayed, An orchard that is producing 10,000 solution and to permit better availability the leaves that come into direct contact lbs. of fruit per acre will remove 220 to the trees. In addition to the premixed with the spray may have sufficient grams (~ 1/2 lb.) of actual zinc per liquid fertilizers, liquid zinc fertilizers zinc, but the rest of the canopy as well acre per year. At the present time, also can be prepared using the powder as roots and fruit may still have zinc fertilizers tend to be excessively form of zinc sulfate, which is very deficiencies that could affect their used. For example, it is a common soluble. However, this material is growth and physiology. practice in San Diego County to exothermic and can be mixed in apply 6 lbs. of zinc sulfate (36%) small amounts only. Solid zinc sulfate To further investigate this question, every five years to mature trees. This is 36% zinc by weight and has low we carried out experiments similar to is equal to 47 pounds of actual zinc solubility in its granular form. This is those that were conducted in Israel by per acre per year or nearly 100 times the reason it is used for soil banding. Kadman and Lahav (1978), in which more than that which is removed. radioactive zinc was used as a tracer Consequently, many avocado orchard A variety of methods can be used to to follow zinc uptake and translocation. soils are probably being over-fertilized. apply the fertilizers. Banding fertilizers In our research, zinc was applied as a If leaf tissue analysis reveal that zinc are effective for spot treating areas in 1-cm spot to either the top or bottom deficiencies are occurring in the the orchard, but are also expensive of newly expanded leaves. Using orchard, the grower should keep good since they must be hand applied. photographic film and another records to monitor how the orchard There is also a greater tendency to method (liquid scintillation counting) has responded to the zinc fertilizer apply excessive quantities that result to quantify the movement of the and thereby determine the amounts in very high immediate concentrations radioactive zinc, we showed that that should be applied annually as a that diminish over the next five to six virtually all of the applied zinc sulfate standard maintenance program. The years as the zinc or iron precipitates remained in the spot where it was exact response to zinc or iron fertilizer into insoluble minerals that cannot applied and that only a small amount will vary from orchard to orchard be extracted by the DTPA soil test. (around 5%) moved out into the depending on the soil pH, organic Although not yet tested, there is leaves above and below the treated matter levels, soil texture, and concern that very high levels of zinc leaf (Crowley et al., 1996). Similar salinity. Although zinc fertilizers have that occur in the soil immediately results were obtained with both zinc traditionally been applied during the after banding may be inhibitory to metalosate and zinc EDTA. We also winter months when no other fertilizers root growth. Repeated soil application examined a number of surfactants are applied, our research clearly treatments may also lead to accumulation with a range of chemical properties shows that soil applied fertilizers are of total zinc to hazardous levels. Thus, to determine if these materials would taken up best during the period of the best way to apply zinc and obtain increase leaf zinc penetration. In new root growth in the spring and the greatest fertilizer use efficiency general, surfactants were beneficial early summer. is to apply it in smaller amounts at for spreading the zinc over the leaf more frequent intervals. This can be surface, but resulted in only a small If fertilization is necessary, a good accomplished by injecting the trace increase in uptake. Altogether, our starting level is 3.5 ounces of actual metal fertilizer into the irrigation results suggest that all of the tested zinc per tree. Many growers apply water. However, if fertigation is used, materials are only effective in treating liquid zinc sulfate in one or two zinc and iron fertilizers should never the outer leaf canopy that comes in applications per year at a rate be mixed with phosphorus fertilizers direct contact with the foliar spray between 7 to 30 gallons per acre per as this will result in precipitation of and are probably of little benefit to year. The recommended rate given the rest of the tree. above(3.5 oz./tree) translates to 17 gallons of liquid Zn sulfate (12% Zn) per acre for orchards planted at a continued on page 12 A Phytophthora rootrotofavocadobytrunkinjection. C Whiley, A.W. K.G. Pegg, Zinc deficiencyintheavocado.Calif.Agric.12: 4-5. Wa A G Analysis andFertilizer Problems. Auckland, New Zealand. In:Proc. 8thInt.Colloq.to avocadotrees.P. Plant 225-230. Kadman, A.andE.Lahav. 1978. Experimentswithzincsupply avocados. Calif.Avocado Soc. Yrbk.50:87-93. Embleton, T.W. andE.F. Allihan.1966. Soil applicationsofZnfor 7 zinc deficiencyinavocado.CaliforniaAvocado Soc. Yrbk. Crowley, D.E.andW. Smith.1996. Soil factorsassociatedwith fertilization ofavocadotrees.Hort Science. 31:224-229. Crowley, D.E.,W. Smith,B.Faber, andJ.A.Manthey. 1996. Zinc 2 C in Borkert, C.M.,F.R Cox, andM.R.Tu into thedevelopingfruit. absorbed bytherootsandtranslocatedthroughouttree are preferredtofoliarapplicationsincethetracemetalscanbe In efficacy ascomparedtoothermethods. to newleaftissue,butso far they plugging oftheirrigationemitters.Foliar fertilizerscanbeapplied zinc orironphosphateintheirri A NUTRITION OF TRACE METAL continued from page7 Santa Ana,CA92705-5655 1251 E.DyerRoad, Suite210 California Avocado Commission A Subscription queriesshouldbeaddressed to: or totheaddressbelow. Guy Witney: Editorial commentsshouldbeaddressedto 9: 171-183. 9: 2991-3005. vocado fertilization.Univ. Calif.Coop. Ext. Bull.2024. ustral. J.Expt.Agr. 31:575-578. orrection ofzincandborondeficienciescontrol ommunications inSoil Science andPlantAnalysis. oodall, G.E.T.W. Embleton,andR.G.Platt.1979. voResearch Subscriptions llihan, E.F., T.W. Embleton,andW. Printy. 1958. VOCADO [email protected] peanut, soybean,rice,andcorninsoilmixtures.In: J.B. Saranah, and P. and J.B. Saranah, REFERENCES ainline,thatwillcause gation appear tohaveverylimited c ker. 1998. Zincandcopper general, soilapplications W.

Langdon. 1991.

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