Because organic farmers have relatively few control options when insect pest populations reach problem levels, a preventive approach to pest management is essential in organic systems. However, given the limited research base regarding relationships between soil fertility, plant health, and insect growth and reproduction, it’s unclear in many situations exactly what this should mean to farmers in terms of inputs and practices.
What do we know so far? In general, mineral nutrition status is known to influence factors such as growth and yield of crop plants by affecting changes in growth pattern, plant morphology and anatomy, and particularly chemical composition. For example, thickness of epidermal cells, degree of lignification, sugar concentrations, amino acid content in phloem sap, and levels of defensive compounds are all influenced by nutritional status of the plant, and in turn either affect or are presumed to affect resistance to insects (Marschner, 1995, Patriquin et al., 1995). Much of the work done to explore plant-insect relationships has involved aphids and nitrogen. For example, there is substantial evidence that aphid reproduction is increased by high levels of soluble N (e.g., amines, amides, amino acids) in host plant leaves (McKee 1962; Auclair, 1963, 1965; van Emden et al., 1969).