Source. It appears in exchangeable soluble and non-exchangeable forms in the soil BO₃3- and B₄O₇^2-. It is present in the forms of boric acid (H₃BO₃), calcium and magnesium borates and constituent of silicates. Its calcium and magnesium salts are soluble. Its availability to plant decreases with increase in pH.

Regions. Plants require boron in leaves and seeds.

Functions. Boron is required for uptake and utilization of Ca²⁺, pollen germination and cell differentiation and carbohydrate translocation. It plays role in nitrogen metabolism, hormones metabolism and fat metabolism. It regulates cellular differentiation and development.

Deficiency symptoms

(i) It causes brown heart disease in beet roots.
(ii) In apple internal tissues become corky.
(iii) Darkening of tissues.
(iv) Hypertrophy followed by death of shoot apices.
(v) Leaves curl and become brittle.
(vi) Roots turn short and stubby
(vii) Deficiency of boron may lead to early fall of flowers and fruits.