The influence of liming with oil-shale ash on the soluble P fraction in soil

The acidification of agriculturally used soils is a widely known phenomenon. Acidification may be caused by various factors. One part of the acidification results from natural processes, including acid plant root exudates and the oxidation of flora and fauna residues in the soil. Another contributing factor is acidic rain, which may have a natural origin or be caused by human activities such as burning coal, sulfur-containing materials, and emissions from metallurgy and chemical factories.

For sustainable agriculture, it is essential to maintain the optimum soil pH to ensure high yields. Typically, liming is employed to establish the suitable pH for plant growth, using carbonaceous materials. Commonly used materials include milled limestone or dolomite, and sometimes ashes from biomaterial. In Estonia, for over 60 years, fly ash from the burning of oil shale in power plants has been used for liming agriculturally used fields. However, the bottom ash from power plants remains unused and is stored in ash hills, accumulating to more than 600 million tons. This ash is an alkaline material with a high content of calcium, potassium, and magnesium. In 2025, the company RagnSells plans to start an experimental factory producing high-quality CaCO3 from this ash. The residue of this process will be a white solid alkaline material with an increased content of magnesium, making it suitable for liming agriculturally used fields. As liming can lock some phosphorus in the soil into an insoluble phase, there may be a decrease in soluble (plant-available) phosphorus. Therefore, we conducted an incubation experiment with this experimental liming agent on three different soils and used two fertilizer norms.  Our research aimed to monitor changes in water-extractable and plant-available P (by the AL method) content in different soils during a 24-week incubation period.