EFFECT OF EXCESS BORON FERTILIZATION ON STATUS AND AVAILABILITY OF BORON IN CALCAREOUS SOILS

Field experiments were conducted at Al-Qatif area in the eastern region of Saudi Arabia to study the status and availability of B under B fertilization regime in three types of calcareous soils and to evaluate the response of two alfalfa varieties, Hassawi (local variety) and Hyden (american variety), to increasing levels of added B. Boron was applied at 7 rates as Na2B4O7.10H2O. Four cuttings were taken from each site at different intervals. Data showed that extractable B by hot-water and NH4HCO3-DTPA (8 days after borax application) was significantly (p < 0.001) affected by soil type and B applications. The amount of B recovered by hot-water from the three soils, 200 days after borax application, was in the following order: sandy loam > sandy clay loam > clay loam. Total dry matter of alfalfa (4 cuttings) was significantly (p < 0.05) affected by soil type, borax application rates and alfalfa variety. The critical level of B in plant as determined by Cate and Nelson analysis, ranged from 148 to 652 mg kg-1 dry matter for Hassawi cultivar and 138 to 521 mg kg-1 for Hyden cultivar in the first harvest. However, the upper critical levels for the 2nd, 3rd and 4th harvests were 800, 875 and 935 mg B kg-1 dry matter for Hassawi and 603, 723 and 812 mg B kg-1 for Hyden varieties, respectively. Nevertheless, the lower critical levels for 2nd, 3rd and 4th harvests ranged from 148 to 153 mg B kg-1 dry matter for Hassawi and 138 to 142 mg B kg-1 for Hyden.