Boron

Boron cross-links pectin components in cell walls and helps maintain membrane integrity. It is also required for pollen germination and pollen tube growth, making it particularly important for fruit set, seed set, and grain filling.

Boron reaches roots mainly by mass flow with soil water, so its supply depends on adequate soil moisture as much as on total soil boron. It is prone to leaching on sandy, well-drained soils, particularly under high rainfall, while soil organic matter is an important reservoir that releases boron as it decomposes.

• Sandy, low-organic-matter soils are prone to boron leaching and deficiency
• High soil pH reduces boron availability
• Dry soil conditions can also limit boron uptake by restricting mass flow to roots

Because boron does not move within the plant once deposited, deficiency symptoms concentrate at growing points: death of shoot and root tips, hollow or cracked stems, and poor pollen viability leading to reduced fruit or seed set. Sugar beet (heart rot) and brassica crops (hollow stem) are classic examples of boron-deficiency disorders.

Boron has an unusually narrow safe range compared with most nutrients: the soil or tissue level associated with toxicity is not far above the level associated with deficiency. Excess boron causes leaf-margin scorch and necrosis, and can result from over-application of boron fertilizers or from boron-rich irrigation water in some arid regions.

Boron and calcium are functionally linked, since both contribute to cell wall structure, and boron nutrition is often considered alongside calcium status. Because boron reaches roots mainly through mass flow, its availability is also closely tied to soil moisture and can be reduced under drought.