Boron adsorption–desorption characteristics of irrigated soils in the Jordan Valley

•Boron solubility, adsorption/desorption from Jordan Valley soil samples

Increasing replacement of fresh water by saline or reclaimed wastewater in the predominantly drip-irrigated Jordan Valley (JV) soils poses a threat of boron (B) accumulation in these soils. Twenty nine composite soil samples in each of the two layers, surface (0–25 cm) and subsurface (25–50 cm), were collected from sites located along the entire JV. The samples were analyzed for major chemical and physical properties. Water soluble B was determined in both hot water and saturated paste extracts. Boron adsorption was carried out from 50 molc/m3 NaCl/CaCl2 background electrolyte solutions of SAR 5, but of increasing B concentration (0.25–4.00 μg/mL). These solutions were arranged to percolate duplicate columns of three soil samples representing Northern JV of Shuna (Fluventic Haplustolls), Middle JV of Deir-Alla (Fluventic Haplustepts) and Southern JV of Sweimeh (Typic Torrifluvents). Regeneration of B from soil columns was determined. First, duplicate soil columns were permeated with solutions of constant B concentration of 1.0, 1.5, 2.0 or 4.0 μg/mL. Following soil–liquid B equilibrium, the columns were incubated at room temperature for one week then permeated with the background solution of 0 μg B/mL. The permeation continued until three successive effluent fractions showed the same B concentration. The process of incubation/permeation was repeated two more times. The results showed that, with the exception of a few outlier values from the Southern end of the JV, B in saturated paste or hot water extracts wasn't linearly correlated with soil salinity of the surface and subsurface samples. The reason for that was argued to be due to the anthropogenic factor of intensive agriculture that generated high demand on B as contrasted by high rate of salt accumulation in surface soil layers caused by the use of conservative irrigation systems with increasingly poor quality water. The low B levels in most samples marked the JV soils to be marginal to deficient in that nutrient. Soluble B in paste or hot water extracts followed a 1:1 relation between either fraction in surface and subsurface layers. However, the hot water extractable B was almost 4 times the corresponding levels of paste extract for both surface and subsurface samples. This implied that crop uptake of B takes place mainly from the paste extract fraction. Adsorption of B onto the three soil samples conformed to the Langmuir adsorption isotherm. These soil samples exhibited a similar affinity constant for B (0.12–0.21 mL/μg), but their maximum adsorption capacity decreased in the southward direction geographically; being the highest for the Northern Shuna (39.7 μg B/g soil) and the lowest for the Sweimeh (8.7 μg B/g soil). The similarity of the Langmuir affinity constant reflected the similarity of clay mineralogy. In addition, the decreasing adsorption capacity was consistent with the decreasing clay content and CEC of these samples. The three soil samples showed a tendency for B regeneration while this tendency decreased with the advancement of incubation/leaching cycles. Almost half of the adsorbed B was leached from the Shuna-North soil columns as contrasted by half of that quantity for the other two samples.