Standard and modified methods for soil organic carbon determination in agricultural soils

The presented study demonstrates the benefits of using modified vertical photometry methods for rapid, reliable and accurate soil organic carbon determination in agricultural soils. For tests we selected and analysed 25 arable (Ap horizon) topsoil (Cambisol) samples, so that the determined values cover as wide a range as possible. In the proposed method soil organic carbon is determined using 0.2 M potassium dichromate solution in diluted (1:1) sulphuric acid. The data obtained using the proposed method correlated strongly with the data obtained by Vario EL and titration methods. The new modification safeguards the objectivity of results, since it gives strong correlation coefficients (r = 0.989–0.990, P < 0.001, n = 25) with the dry combustion method data. The findings suggest that different photometers did not significantly affect results. However, comparison of conventional photometry and photometric determination using Multiskan MS equipped with the Genesis Lite microplate software revealed the following advantages of the latter technique: higher measurement accuracy, the facility to edit and save calibration graphs, the possibility of optimized and multiple replications, higher labour efficiency; improved safety for research personnel and lower reagent consumption. Strong and highly significant (P < 0.001) correlations were determined between the data obtained using different methods, and respective linear regression equations for the re-calculation of SOC values were developed.