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(or Thomas slag), an alkaline phosphorus fertilizer. It is a by-product of the production of steel from phosphorusrich pig iron (see). Basic slag is a dark powder that is insoluble in water and does not cake. It contains not less than 14 percent P205 in citrate-soluble form. Basic slag is most suitable for use on acid soils as a base fertilizer. It is used in the Federal Republic of Germany, France, and Great Britain. It is not produced in the USSR.