Boron in Plants

Boron is one of the seven micronutrients required for normal growth of plants.

In nature, boron is usually present at an average concentration of 10 ppm. However, the range of Boron concentrations in the soil solution, in which plants suffer neither toxicities nor deficiencies, is very narrow (0.3-1 ppm).

Boron is essential for normal growth of plants because it promotes proper cell division, cell elongation, cell wall strength, flowering pollination, seed set and sugar translocation. Boron is also essential for the plant hormone system. Plants that suffer boron deficiency may suffer growth defects.

Boron uptake by plants

Boron uptake by plants is controlled by the boron level in soil solution rather than the total boron content in soil. Boron uptake is a passive (non-metabolic) process. It moves with water in plant's tissues and accumulates in the leaves; therefore, Boron uptake and accumulation are directly dependent on the rate of transpiration.

Boron mobility in the phloem is now known to be plant-specie dependent.

Boron Toxicities and Deficiencies

Boron deficiency symptoms:
Limited budding, bud break, distorted shoot growth, short internodes, increased branching, flower buds falling and inhibition of fruit and seeds development

 Symptoms of boron deficiency in pepper

Boron toxicity symptoms include:

Chloritic leaf tips, leaf necrosis, and later leaves falling and even plant death.

Boron in the soil

Boron is soils can be categorized into 3 groups:

1. Boron as a mineral component
2. Boron adsorbed to soil particles, such as clay minerals, iron or aluminum oxides and organic matter. Adsorption of Boron to clay particles is reversible so the adsorbed boron can be released to the soil solution. It also has high affinity to iron and aluminum oxides.
3. Boron in soil solution as boric acid (H₃BO₃) and borate ions (BO₃^-). The boron in soil solution is available to plants (mainly as H₃BO₃).

The ratio between the Boron concentration in the soil solution and the Boron adsorbed to soil particles is affected by the components of the soil (clay minerals, free oxides and organic matter) and also by other factors such as type and concentration of salts in the soil, pH and temperature.

Actually, most of the boron in soil is adsorbed to organic matter, acting as a pool of boron from which the boron can be readily released into the soil solution.

Preventing boron from accumulating in the root zone

Since toxic levels of Boron are only slightly higher than deficiency levels, it is important to keep a non-toxic level of Boron in soil solution. In order to achieve that, root zone should be flushed either periodically or continuously.

The water amount and the irrigation intervals should be determined in the same way done for treating salinity buildup in soil.