The California blueberry industry has experienced substantial growth over the past decade. During this period, many growers have learned that blueberry production is often challenging.
Blueberry, Vaccinium corymbosum is an ericaceous (member of the heath family), acid soil loving plant.
It is characterized by a very narrow range of environmental adaptation. Within this range, blueberry is capable of quite exuberant growth and production. Where soil and irrigation water and other environmental conditions lie outside this narrow range of adaptation, commercial blueberry growers must modify soil chemistry, soil physical properties and irrigation water chemistry. Modifications are necessary to insure the continued production of high yielding plantings of quality fruit.
Despite a grower’s best efforts, blueberry plantings can display moderate to severe leaf chlorosis, which often inhibits optimum production. Such plants usually suffer from stunting or lack of vigor. Production of fruit-wood is limited. Even flowering can be reduced.
When confronted with such problems, the grower should recognize that “iron chlorosis” is often not the specific problem. The plants are often not precisely deficient in iron. The term “iron chlorosis,” as used in the context of blueberry culture, is something of a misnomer. The normal range for iron concentration in blueberry leaf tissue is: 80 – 200 ppm Fe. Chlorotic blueberry leaf tissue can often have leaf iron concentrations that are equal to or even higher than the tissue levels found in green blueberry leaves. Thus, it is important to determine the precise cause of the chlorosis. Think of the presence of visual chlorosis symptoms as a “stress indicator” for the blueberry plant. The fall and winter period is an opportune time to address these problems, since root zone changes are not desirable during the flowering, fruit set and harvest periods of the year.
Possible causes of iron chlorosis
Possible causes for blueberry “iron chlorosis” include:
--Excessively high soil pH. Most blueberry growers are aware that blueberries thrive in an acid soil pH regimen. The ideal soil pH range is 4.8 to 5.2, though exceptions exist, for a variety of reasons. Appropriate preplant acidification of soils is an important cultural practice. Ideally, the soil should be tested for Soil Acid Requirement Determination, well in advance of the planting date. This will allow sufficient time to determine the soil acid requirement, apply soil sulfur or sulfuric acid and allow time for the soil to react sufficiently, prior to planting. Six months to a year is not unreasonable to sufficiently modify the soil. For San Joaquin Valley growers, it is advantageous to plant during the fall (October-November) to permit good root development prior to the arrival of summer heat.
--Presence of Free Lime in the Soil: The presence of free lime (calcium carbonate) in the soil, while often related to soil pH, can have specific effects of its own. The lime can occur as finely divided particles or less favorably, can be present as massive layers of “caliche” or similar of varying thickness and at variable depth. Ideally, such planting sites are identified during the planting site selection process prior to site development work. Where finely divided lime particles are present, estimates of soil sulfur requirement based upon soil texture alone can underestimate the amount of soil sulfur or sulfuric acid necessary to obtain the desired soil pH adjustment to pH 5.0. Again, the use of the Soil Acid Requirement analytical procedure is critical.
--Excessively High Bicarbonate Concentration in the Irrigation Water: Blueberry is quite sensitive to the toxic effects of excess bicarbonate concentration in the irrigation water. Most growers are aware of the need for irrigation water acidification. Care should be exercised to acidify the irrigation water sufficiently to neutralize most, but not all of the bicarbonate present in the irrigation water to allow for a modest amount of buffering and to minimize the risk of over acidification of the soils irrigated with the acid treated water. The bicarbonate concentration in the irrigation water should be determined. Over acidified irrigation water can rapidly result in adverse over acidification of blueberry soils, where the soil pH drops to below 4.0. Ideally, a residual bicarbonate concentration of 0.5 meq./l in the treated acidified water is desirable. Once the bicarbonate level of the water is determined, calculate the amount of acid necessary to neutralize sufficient bicarbonate to obtain the desired residual bicarbonate. Consult your advisor if uncertain.
-- Adverse Soil Physical Properties: Soil texture is an important feature of planting site evaluation. Blueberries should not be planted on soils where the silt + clay fraction exceeds 40 percent (USDA Soil Texture Triangle). Ideally, textures of sandy loam, loam and loamy sand are acceptable. Sandy clay loam and sandy clay should be avoided since fine clay particles insinuate themselves into the pore space formed by the coarser sand particles and reduce air filled porosity to an unacceptable level. Addition of ample amount of conifer based (lignin rich) wood residual organic amendment during the preplant phase of development is a useful tool for ameliorating adverse soil structure and texture. Use of such organic amendment can render otherwise unsuitable soils, to be quite productive blueberry soils.
--Restricted Soil Aeration: Ample soil aeration is key to successful, high yield, blueberry production. Typically, the greatest impediment to use of ample amount of organic amendment is the cost. However, the benefits of ample organic amendment addition are well compensated over the years, particularly where soil texture is less than ideal.
---Soil Compaction: Tillage, the construction of raised berms and the use of ample wood residual organic amendment are important. All soil compaction issues should be eliminated prior to planting.
-- Specific Element Toxicity: Excessive boron produces a bronzing and yellowing of youngest terminal growth in the initial stages. More severe toxicity symptoms include leaf margin necrosis. Measure your irrigation water boron content. Blueberries are susceptible to rather low concentrations of boron. Boron should not exceed 0.3 ppm B in the irrigation water.
--Chloride: Blueberry is intolerant of chloride ion. Do not use muriate of potash fertilizers in blueberry fertilization programs.
--Sodium: Blueberry can accumulate ample concentrations of sodium where irrigation water sodium concentration is high. Visual appearance may be lighter green than usual or yellow. Growth is restricted.
The above conditions can all cause visual symptoms of yellowing or “iron chlorosis” in blueberry plantings. The first step in alleviating the chlorosis is to identify the cause of the stress. Often, multiple causes are present, simultaneously in the field. Consider these features when evaluating your situation.