Use of Boron-Laden Magnesia as an Amendment for Acid Soils

Abstract

Magnesia, after it has been used to remove excess boron (B) from irrigation waters, could be amended to acid soils as a liming material. However, B phytotoxicity is a potential hazard. The objective of the study was to evaluate the use of B-laden magnesia as an amendment for acid soils. Three inorganic, loamy, acid soils were used. Magnesia, after saturation with 2 mg B g À1 , was added to the three acid soils as follows: 0.5 and 1.5 g kg À1 in the two less acidic soils (pH 5.9 and 5.3) plus an additional rate of 2.5 g kg À1 for the most acidic soil (pH 4.7). Control soil samples received no magnesia. The treated and control soils were then used in a greenhouse study for cultivation of corn (Zea mays L.). Although the original soils contained low levels of available B, the potential risk of causing B phytotoxicity because of magnesia addition was high because more than 43% of the total B desorbed from the magnesia was released instantly in the treated soils. With the application rates of 0.5 and 1.5 g kg À1 , the corn total biomass yield significantly increased (P ≤ 0.01) by 9 to 33% and 7 to 18%, respectively. In addition, soil pH was improved (e.g., it increased by 2.2 to 2.7 units with the application rate of 1.5 g kg À1 ), and B and Mg concentrations in the soil and corn significantly increased (P ≤ 0.01) to approximate sufficiency ranges. In contrast, the 2.5-g kg À1 addition to the most acidic soil resulted in a significant decrease of corn yield, which was accompanied by a significant increase of soil-available B at levels greater than 2 mg kg À1 . The B in the corn tissues was at phytotoxic levels (123 mg kg À1 ). It was concluded that, for acid soils similar to those used in this study, addition of B-laden magnesia at a rate of 0.5 g kg À1 (equivalent to 2 Mg ha À1 ) could be recommended for liming.