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2. The essentiality of certain elements in minute quantity for plants with special reference to copper;Arnon;Plant Physiology,1939
3. Farm Animal Manures: an overview of their role in the agricultural environment. Manual 44;Azevedo,1974
4. R.P. Bosshardt, Potash and Phosphate Institute, Singapore, personal communication, 1986.
5. Effects of Humic Substances on Plants;Chen,1986