University of Pannonia, Georgikon Faculty of Agriculture, Department of Crop Production and Soil Sciences, Keszthely, Hungary Introduction The most important function of soil is that it is the basic media of crop production (Varallyay, 2002). Consequently, sustaining and ameliorating of soil structure is crucial. Decline of soil structure and degradation of its stability are frequently caused by tillage, crop management and getting moist (Chan et al., 2003). Soil structure is built up by groups of primary particles that cohere to each other, and which are called aggregates (Kemper and Rosneau, 1986). (Di Gleria et al., 1957). Water-stability is often investigated by wet sieving. Earlier reported methodological problems were solved and circumstances affecting soil structure were standardized with the new modified wet sieving method proposed by Six (2000). The Normalised Stability Index (NSI) characterises aggregate stability by comparing the aggregate distribution after two differently disrupting wetting methods. The two different wetting methods are: (1) fast wetting, namely rapid immersion in water (Slaking (S)), which disrupts the aggregates to the highest extent, therefore produce the lowest aggregate amount; and (2) slow wetting, namely capillary wetting (Capillary (C)) to field capacity, which disrupts the aggregates to the lowest extent and therefore achieves the highest aggregate amount. The capillary wetting method was earlier tried by us to get different size fractions to evaluate the distribution of soil organic matter in soils (Huisz et al., 2006). In this paper our aim was to test the standardised wet sieving method proposed by Six (2000). We investigated the effect of farmyard manure addition on soil structure and its quality and on the stability of aggregates. Material and methods Site description The soil samples originate from the “Comparing experiment of organic and mineral fertilisation” long-term field experiment in Keszthely, Hungary. The investigated soil type is a sandy loam textured Eutric Cambisol (soil type FAO), Alfisol (soil type USDA). In this paper addition of farmyard manure (0kg N + 0kg P + 0kg K + 6 x 17,3ha t