Spatial variability of soluble boron in Syrian soils

Abstract

While boron toxicity has been known for a long time, only recently has its widespread significance for field crops, especially barley (Hordeum vulgare L. ssp. vulgare), been recognized. Damage to crops is common in semi-arid areas where levels of hot-water-soluble (hws) B exceeds about 5 ppm. Frequently, much higher levels of B are found in the sub-soil than in the topsoil. This study examined the distribution of B in profiles from agricultural experiment stations across a mean annual rainfall gradient (196-471 mm/yr) in the semi-arid cereal-production zone in north-western Syria. A more intensive surface and depth-wise sampling was conducted at the driest barley-growing station, Boueidar, where plant toxicity symptoms were observed. The wetter stations had low levels of B throughout the profile, while the drier ones tended to have potentially toxic levels, which increased with profile depth. Only the Boueidar site had B accumulations within 20-40 cm of the surface. Profile sampling at Boueidar showed three distinct B distribution patterns, i.e., increasing with depth, uniform with depth, and a zone of B accumulation at 25-75 cm. Surface (0-20 cm) samples at Boueidar had spatially variable B concentrations. Leaching to remove excess B is impractical. Ideally, the problem may be tackled by exploiting genetic variation in B uptake. However, spatial and depth-wise variability shown in this study makes screening for B tolerance difficult in the field; as an alternative, B toxicity tolerance can be evaluated in the greenhouse. This work has heightened awareness of B toxicity in other countries of this agro-ecological zone.