Distribution and dynamics of boron in Tunisia

Boron is a soil microelement with a narrow margin between deficiency and toxicity. The results show a decrease in available boron with depth for calcareous soils (Luvisol-1 Vertisol Fluvisol and Cambisol) while they increase in non-calcareous soils (Luvisol-2 and Luvisol-3). Calcareous soils contain more total boron than non-calcareous ones. Chemical fractionation shows that readily soluble boron represents less than 1.7% of the total while the residual fraction exceeds 97.5%. Granulometric fractionation reveals that the majority of available boron is associated with the silty fraction (51%) followed by the sandy (34%) and clayey (15%) fractions. In terms of humic substances humic acids account for 70.69% of total boron compared with 27.88% for fulvic acids and 1.43% for humin. Adsorption and desorption follow the Freundlich isotherm with high boron retention during the desorption process. In hydroponics the optimum boron dose for lettuce (Lactuca sativa L.) is 0.5-5 mg/kg to avoid toxicity. However for soil-grown lettuce this concentration may exceed 5 mg/kg.