Layered double hydroxides intercalated with borate: effect of fertilization on boron leaching and successive sunflower cultivations

Boron (B) can easily leach into the soil when added in the form of a soluble fertilizer such as borax or boric acid (H3BO3); however, large quantities of B can cause toxicity in plants. The use of a slow release source of B such as layered double hydroxides intercalated with borate may minimize leaching/toxicity of B. The objectives of this study are to evaluate the availability and leaching of B from magnesium–aluminium layered double hydroxides intercalated with borate anions (Mg2Al-B-LDH) and H3BO3 in a sandy soil, and the B bioavailability of these two sources in consecutive cultivations of sunflower. Mg2Al-B-LDH and H3BO3 at two rates (10 and 20 mg dm−3 of B) were incubated for 1, 2, 3, 5, 10, 20 and 40 days. After incubation, the soil B availability was evaluated using the hot-water extraction method. For the leaching study, 20 mg dm−3 of B from the two sources was incorporated into the top 0.5 cm of soil and incubated for 1, 5, 10, 15, 20, 25 and 30 days before leaching with water volume equivalent to the total soil-pore space. Sunflower plants were cultivated for two consecutive seasons in greenhouse using Mg2Al-B-LDH and H3BO3 and six rates (0, 0.5, 1, 2, 3 and 5 mg dm−3) in a factorial 2 × 6, with four replicates. Plants received a single application of B before sowing at the first cultivation. H3BO3 had greater availability of B in the soil than Mg2Al-B-LDH regardless of the applied rate or time of incubation. The slow release of B by Mg2Al-B-LDH resulted in a significant reduction in B leached compared to H3BO3. There was a quadratic and linear response of sunflower aboveground biomass and B accumulation with increasing rates of H3BO3 and Mg2Al-B-LDH, respectively, in the second cultivation only. A boron rate of 5 mg dm−3 applied as H3BO3 resulted in plant toxicity, while Mg2Al-B-LDH promoted a slow release of B to the plants throughout the cultivation period.