BORON AVAILABILITY TO PLANTS FROM COAL COMBUSTION BY-PRODUCTS

Agronomic use of coal combustion by-products is often associated with boron (B) excess in amended soils and subsequently in plants. A greenh ouse study with corn (Zea mays L.) as test plant was conducted to dete rmine safe application rates of five fly ashes and one hue gas desulfu rization gypsum (FDG). All by-products increased soil and corn tissue B concentration, in some cases above toxicity levels which are 5 mg ho t water soluble B (hwsB) kg(-1) soil and 100 mg B kg(-1) in corn tissu e. Acceptable application rates varied from 4 to 100 Mg ha(-1) for dif ferent by-products. Leaching and weathering of a high B fly ash under pending conditions decreased its B content and that of corn grown in f ly ash amended soil, while leaching of the same fly ash under laborato ry conditions increased fly ash B availability to corn in comparison t o the fresh fly ash. Hot water soluble B in fly ash or FDG amended soi l correlated very well with corn tissue B. Hot water soluble B in fly ash amended soil could be predicted based on soil pH and B solubility in ash at different pH values but not so in the case of FDG. Another g reenhouse study was conducted to compare the influence of FDG and Ca(O H)(2) on B concentration in spinach (Spinacia oleracea L.) leaves grow n in soil amended with the high B fly ash. The Ca(OH)(2) significantly decreased tissue B content, while FDG did not affect B uptake from fl y ash amended soil.