Zinc in Almonds

Mario Viveros

UCCE Farm Advisor, Kern County

Deciduous Tree Fruits and Nuts

November 1, 1999

     Zinc is an essential element which is required in small amounts by plants. There is evidence that fruit trees use less than two ounces of zinc per acre. However, when this supply is reduced below adequate levels, the result is dramatic zinc deficiency symptoms.

· Role of Zinc: Mineral elements not only have distinct roles in plants such as electrochemical and structural but also catalytic ones. This is the case of zinc. It has been found to be an enzyme activator for the synthesis of trytophan--a chemical compound from which auxin (indole-acetic acid) is formed in plants. Auxin is a plant hormone essential for cell enlargement in stems and leaves. A reduction of zinc will result in auxin deficiency and, subsequently, in a failure of young shoots and leaves to expand normally during their growth.

     Zinc is also an essential part of certain enzymes such as alcohol dehydrogenase, lactic acid dehydrogenase and others. In addition, its inter-venal chlorosis symptoms suggest that zinc somehow participates in chlorophyll formation.

·  Symptoms: Zinc deficiency symptoms are so dramatic that terms like "little leaf" and "rosetting" have been used to describe them. "Little leaf" as the name implies, refers to small, narrow and pointed leaves. "Rosetting" refers to the failure of internodes to elongate, causing the leaves of several nodes to lie telescoped together in a plane--rosette-fashion.
These characteristic features occur at the tips of new growth and usually are accompanied by a chlorosis which may resemble that of iron deficiency. However, it is readily distinguished by the short internodes at the shoot tips and by small, narrow and pointed leaves. In the summer, one can frequently find normal size leaves that are bent sharply upward on either side from their midribs.

·  Determination of Zinc Sprays: Zinc sprays should be based on leaf analysis as well as on zinc deficiency symptoms. It has been found that the correlation between leaf analysis levels and zinc deficiency symptoms isn't a very good one. On border line cases, one may or may not see zinc deficiency symptoms on the leaves. Furthermore, in almonds, deficiency symptoms are not clear unless the shortage is rather severe. A poor set of nuts will occur before one can see deficiency symptoms on the tree.

     The above information has led growers to make zinc applications a yearly practice. However, other growers have used leaf analysis levels as a guide for zinc sprays. If the zinc level is 18 ppm or less, a zinc application is in order and a zinc application will be required as long as zinc deficiency symptoms are present.

·  Zinc Sprays: There are three different times during the year when zinc sprays can be applied to almond trees: at the beginning of normal leaf fall, during the dormant season and during the spring.
The fall application is commonly done at the end of October to the end of November. Zinc sulfate (36 percent Zn) is used at the rate of 10 pounds per 100 gallons of water. This spray will cause some leaf injury but no damage will be done to the trees.

     The dormant application, as the name implies, is done during the dormant seasons-- when no leaves are present. At this time, the rate is 10 to 15 pounds of ZnSO4 (36 percent Zn) in 100 gallons of water. Warning: do not spray zinc sulfate at the same time as oil sprays; apply it two or more weeks after the oil spray.

     The spring application is done when leaves have nearly attained full size. At this time, basic ZnSO4, or zinc oxide (both at 5 lbs. per 100 gallons of water) are effective foliar sprays. Another effective zinc compound is zinc chelate. Please use table rates.