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he ability of organisms to detect and avoid contaminated soil indicates the stress potential of a particular soil and it is ecologically relevant due to the direct relationships with soil biodiversity and its quality as habitat for those organisms. Avoidance and effect of contaminants has been studied for some time in aquatic toxicity but only in recent years have avoidance tests been developed for soils with different chemical substances. Most of these bioassays were performed using laboratory artificial soil recommended by OECD (Organization for Economic co-operation and Development) and ISO (International Organization for Standardization) using collembolans, earthworms, and enchytraeids as test organisms. In this paper, the avoidance behaviour of earthworms was studied, for the commonly used fertilizer, potash under temperate condition (i.e. 20ºC) in OECD artificial soil. The range of concentrations of potash for analysis was determined from pre-conducted acute toxicity tests and was chosen as 140.62, 281.25, 562.5, 1125 and 2250mg/kg. Net Response results of the avoidance behavioural tests indicated a varied distribution from 0 to 57.50% from lower to higher concentration.The net response of the worms was significant at the highest concentration (i.e. at 2250mg/kg of potash). Free movementof worms covering the whole test area without showing a preference to a specific area was observed in the lowest concentration of potash (i.e. 140.62mg/kg). Thus from the experiment it is inferred that usage of chemical fertilizer, potash in low concentrations will not affect the survival of earthworm and when the dosage of potash is 281.25mg/kg and above, it is affecting the earthworms’ population. When the dosage exceeds the avoidable limit, it may result in lethal effects also. The results of EC50, LOEC and NOEC values were determined as 109.3 [95% CL – n.d.], 44.9 and 20.3 respectively. Thus potash as a fertilizer must be used in the limited dosage only which will otherwise affect the earthworm population adversely.