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 the various tissues of plants. He taught that the carbonic acid of the atmosphere was the principal source of the carbon in plants; that their hydrogen came from water, their nitrogen from ammonia present in the air and soil. The sulphur, which is an ingredient in the protoplasm ("the basis of life") of living plants, comes solely from the sulphates and sulphur compounds contained in the soil; and, finally, that the mineral ingredients found in the ashes of plants come from the soil in which they grow. Liebig further demonstrated that plants cannot live without these mineral ingredients, and that the humus of the soil is incapable of forming the sole nutrient material for plants.

Liebig says in his book (p. 174):—

Liebig's theory was a revolutionary movement on the accepted ideas of scientific agriculture which dawned at the commencement of the nineteenth century.

He supported his theory by well-conducted experiments, showing that fields containing an excess of humus