This study was carried out in soils of province Izmir in the Aegean Region, which are of high importance for Turkish agriculture. In these soils, the amount of native fixed NH4-N, its relationship with other soil properties and their reactions to fix the applying NH4 fertilizer (fixation capacity) were investigated. For this purpose, 23 soil samples were taken from each of two depths (0-20 and 20-40 cm) from the agricultural soils of Izmir (11 from Ege University, Faculty of Agriculture Research, Application and Production Farm, nine from Rural Services Research Institute Fields and three from Ege University Faculty of Agriculture, Department of Soil Science Research Fields). The NH4 fixation capacity with the application of N [100 mg N kg(-1) soil as (NH4)(2)SO4] fertilizer was determined after 4, 8 and 12 days incubation period in all soil samples from 0 to 20 cm depth. In the soil samples of 0-20 cm depth, the amount of native fixed NH4-N ranged from 60 to 231 mg kg(-1) soil and made up 7 to 49% of the total-N. In the soil samples of 20-40 cm depth the related amounts were between 49 and 235 mg kg(-1) soil and 10 to 49% of the total-N respectively. The native fixed NH4-N has a significant negative correlation with the sand (r = -0.591*) and a positive correlation with the silt (r = 0.672*) and plant available K (r = 0.586*) in samples from 20 to 40 cm depth. The results of an incubation experiment indicated a significant negative correlation (r = -0.763**) between the native fixed NH4-N concentration and NH4 fixation capacity of soils. The first group of soils (No.: 2, 6, 9, 10) that have high native fixed NH4-N did not show any fixation from added NH4-N after 12 days of incubation. The second group (No.:1, 4, 5, 7, 22, 23) reached maximum NH4 fixation capacity after 4 days incubation and in some soils (No.:1, 4, 5, 21, 22) the capacity approached to approximately 100% after this time. The third group of soils (No.:8, 11, 13, 16, 17, 18, 19) attained maximum NH4-fixation capacity after 8 days of incubation. Only two soil samples (No.: 3, 14) reached their maximum after 12 days. The last group of soils (No.: 12, 15, 20, 2 1), which were low in native fixed NH4-N due to their high fixation capacity, showed increasing NH4-N fixation (over 100 mg kg(-1) soil added N) after the incubation time, this being evidence that these soils have high mineralization potential and that NH4 from other N sources could be included in the soil NH4 reservoir. (C) 2002 Elsevier Science B.V. All rights reserved.