Fly ash application on two acid soils and its effect on soil salinity, pH, B, P and on ryegrass growth and composition

Application of alkaline fly ash to acid soils is related to beneficial effects, such as increase of pH to a desired level and nutrient supply to plants, and to possible adverse effects, such as enrichment of soils with substances toxic to plants and animals (e.g. B, Mo, Se) and increase of salinity to undesirable levels. Therefore, use of alkaline fly ash as a beneficial amendment of acid soils needs to be evaluated with respect to phytotoxic and environmental impacts. Samples of alkaline fly ash, from two different sources, were added to two Red Mediterranean acid soils at rates equal to 5, 20 and 50 g kg−1 soil, and changes, relative to the untreated soil, of soil pH, salinity, B and P levels were measured. Ryegrass (Lolium perenne L.) was grown in pots containing fly ash–soil mixtures for 300 days, and dry biomass yield and cumulative plant uptake of B and P were calculated. Soil application of fly ash at these rates increased the pH, up to about 8, and the electrical conductivity of the saturation extract, up to about 2.5 dS m−1, in both soils. Available soil P (0.5 M NaHCO3 extractable) was unaffected by fly ash application. Water soluble B remained less than 1 mg litre−1 in the saturation extract, and hot water extractable B was less than 1 mg kg−1 soil. Dry biomass yield of ryegrass and cumulative plant uptake of B and P increased significantly with fly ash application. Therefore, fly ashes with low B and salt content can be used as liming agents in acid soils at rates not exceeding the 40 Mg/ha. Potential environmental impacts must also considered.