Title: Comparative assessment of the vertical distribution of soil organic matter (SOM) in cultivated and non-cultivated land and its impact on soil pH

Cultivation can lead to declines in soil organic matter (SOM) which is central to soil quality. The aim of this study was to assess the impact of cultivation on SOM and how it impacts soil pH, another vital indicator of soil health. The study was carried out at the Korpa Experimental Station in Iceland, in a cultivated field (CL) and a non-cultivated (NC) field. Data were collected from two soil depths, the upper (0 – 15 cm) and bottom (15 – 25 cm) depths, and from one soil profile in each field. SOM was measured using the loss-on-ignition method, whereas pH was measured in water (1:5 soil-water ratio). The results showed a significant effect of treatment (cultivation/noncultivation) (F = 8.0; p = 0.008) and soil depth (F = 7.15; p = 0.012) on SOM. SOM was significantly highest in the upper layer of the NC field. The effect of treatment on soil pH was not significant (F= 2.23; p=0.147) whilst the effect of soil depth was significant (F = 18.61; p = 0.0002). There was a significantly strong negative correlation between SOM and pH in the CL (r = -0.771, p = 0.0005) and NC fields (r = -0.785, p = 0.0003). There was a significantly strong negative correlation between SOM and pH in the bottom depth (r = -0.824, p = 0.0001) but not in the upper depth (r = -0.291, p = 0.275). There was a strong relationship between SOM and soil structure and soil colour. The low BD values observed in the CL profile (0.46 – 0.63 g/cm3 ) and NC profile (0.34 – 0.60 g/cm3) were indicative of richness in SOM. This study showed the importance of good agronomic practices to maintain SOM. In the poorer countries, there is a need to promote cheap options such as mulching, compost manure addition, fallowing and crop rotations that enhance SOM and ensure good quality of soils.