1. Bergmann, W. 1992. Nutritional disorder of plants. Gustar Fischer, Stuttgart.
2. Britto, D. T. and H. J. Kronzucker. 2002. NH4+ toxicity in higher plants: A critical review. J. Plant Physiol. 159: 567–584.
3. Cao, W. and T. W. Tibbitts. 1993. Study of various NH4+/NO3− mixtures for enhancing growth of potatoes. J. Plant Nutr. 16: 1691–1704.
4. Cresswell, G. C. and R. Q. Weir. 1997. Plant nutrient disorders: Ornamental plants and shrubs. INKATA press, Singapore.
5. De Hertogh, A. A. 1994. Home Forcing of Potted Amaryllis (Hippeastrum). Leaflet No. 8529. North Carolina Cooperative Extension Service, Raleigh.