To assure the sustainability of land management systems, farm managers need reference guidelines and identification of relationships between soil measures and soil function, which can allow valid comparisons across variations in climate, soils, land use, topography and management systems. SOC and its evolution into humic carbon (HC) is arguably the best single indicator of soil quality because of its impact on the other physical, chemical and biological indicators. In a long-term experiment, the effects of 40 years of organic, mixed and mineral fertilizations on SOC and its evolution into humic carbon were evaluated in three types of soil . The experiment has been underway since 1964 on the University of Padova Experimental Farm (Northern Italy). The treatments derived from the factorial combination of 3 types of soil (clay, sandy and peaty) with 6 types of mineral, organic or mixed fertilization, organized in two randomized blocks. A total of 36 lysimeters (surface of 4 m2 and 80 cm deep) were cultivated. Fertilization rates were as follows: 0 no fertilization; F1 manure (20 t ha-1 y-1); M1 mineral fertilization (100 kg ha-1y-1 N); F1M1 manure (20 t ha-1 y-1) + mineral fertilization (100 kg ha-1 y-1 N); F2 manure (40 t ha-1 y-1); M2 mineral fertilization (200 kg ha-1 y-1 N - 100 P2O5 - 280 K2O). Soil samples were collected from the upper 20 cm layer and analysed for SOC, HC and four humic fractions: 1) molecular weight of less than 25 kDa, 2) molecular weight between 25 and 55 kDa, 3) molecular weight between 55 and 75 kDa and 4) molecular weight above 75 kDa. The effect of fertilization on SOC varied according to soil type. In mineral soils (clay and sandy), manure applications significantly increased SOC content (p<0.01). In the organic soil (peaty) the SOC didn't appear to be related to fertilization type. The HC/SOC ratio was 16% on average, without significant differences between treatments. Humic carbon yields varied according to soil type: there was a significantly higher ratio in the sandy soil (31.9%) (p<0.05) than in the clay (9.4%) and peaty soils (7.1%). The sandy soils had a higher percentage of fractions with high and medium-high molecular weight, denoting the existence of soil-crop conditions favourable for completion of the humification process. On the contrary, the fraction with medium-high molecular weight was not extracted from the peaty and clay soils, where humic substances with medium-low molecular weight, less than 55 kDa, prevailed. As regards the effects of different fertilizations on the humic fractions, significant differences emerged in the comparisons of the fraction with low molecular weight: the M1 treatment had higher levels (22.2%) than those in treatment L2 (11.8%) (p<0.05); the other treatments had intermediate values of around 14%. In general, there was a prevalence of the fraction with high molecular weight in the manured treatments. This confirms the better humus evolution with manure.