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  Section: Plant Nutrition » Micronutrients » Boron
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Boron Levels in Plants

Historical Information
  Determination of Essentiality
  Functions in Plants
    - Root Elongation and Nucleic Acid Metabolism
    - Protein, Amino Acid, and Nitrate Metabolism
    - Sugar and Starch Metabolism
    - Auxin and Phenol Metabolism
    - Flower Formation and Seed Production
    - Membrane Function
Forms and Sources of Boron in Soils
  Total Boron
  Available Boron
  Fractionation of Soil Boron
  Soil Solution Boron
  Hydrated Boron Minerals
Diagnosis of Boron Status in Plants
  Deficiency Symptoms
    - Field and Horticultural Crops
    - Other Crops
  Toxicity Symptoms
    - Field and Horticultural Crops
    - Other Crops
Boron Concentration in Crops
  Plant Part and Growth Stage
  Boron Requirement of Some Crops
Boron Levels in Plants
Soil Testing for Boron
  Sampling of Soils for Analysis
  Extraction of Available Boron
    - Hot-Water-Extractable Boron
    - Boron from Saturated Soil Extracts
    - Other Soil Chemical Extractants
  Determination of Extracted Boron
    - Colorimetric Methods
    - Spectrometric Methods
Factors Affecting Plant Accumulation of Boron
  Soil Factors
    - Soil Acidity, Calcium, and Magnesium
    - Macronutrients, Sulfur, and Zinc
    - Soil Texture
    - Soil Organic Matter
    - Soil Adsorption
    - Soil Salinity
  Other Factors
    - Plant Genotypes
    - Environmental Factors
    - Method of Cultivation and Cropping
    - Irrigation Water
Fertilizers for Boron
  Types of Fertilizers
  Methods and Rates of Application

Often when one talks about deficient, sufficient, and toxic levels of nutrients in crops, there is a range in values rather than one definite number that could be considered as critical. Therefore, the term critical level in crops is somewhat misleading. A nutrient content value considered critical by workers in one area may not be considered critical in another area. Likewise, the term optimum level of a nutrient, as used in the literature by some researchers to express a relationship to maximum crop yield, is sometimes not clear. Theoretically, such a level for a given nutrient should be sufficient to produce the best possible growth of a crop. A range of values would be more appropriate to describe the nutrient status of the crop; therefore, the term sufficiency will be used, rather than critical or optimum.

The critical level of a nutrient has been defined as the concentration occurring in a specific plant part at 90% of the maximum yield (117). The concept is equally valid where crop quality is the main concern rather than yield (118). In this respect, rutabaga is an excellent example where deficiency of boron may not affect the mass of roots, but the quality of roots may be seriously impaired.

The ratio of toxic level to adequate level of boron is smaller than that for most other nutrient elements (119). Thus, excessive or deficient levels could be encountered in a crop during a single season. This occurrence emphasizes the fact that a critical value used to indicate the status of boron in crops would be unsuitable. In many cases the values referred to in this section overlap the deficiency and sufficiency ranges. The deficient, sufficient, and toxic boron levels for specific crops as reported by various workers are given in Table 8.3. The deficient and toxic levels of boron as reported in this table are associated with plant disorders and suppressions of crop yields. For some crops, the deficiency and optimum levels seem to differ markedly. Differences in the techniques used and the locations of the various laboratories cannot be ruled out.

Table 8.3

Deficiency, Sufficiency, and Toxicity Levels of Boron in Field and Horticultural Crops

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