A study of the uncertainty sources in the assessment of organic carbon stocks in a layer of 0–30 cm at the scale of the sampling area (100 × 100 m) laid on soddy-podzolic cultivated soil (Albic Glossic Retisol (Aric, Loamic, Ochric)) was carried out. In the experiment, two sampling methods were used – the classic 10-cm layers from profiles and with an auger to the depth of 0–30 cm. The soil bulk density was determined by the Kachinsky method, the carbon content was determined by the Tyurin method. Some of the samples were additionally analyzed at the Bryansk State Agrarian University. The uncertainties associated with natural variation, sample preparation and the proper analytical process are estimated. The analytical uncertainty of the bulk density under the conditions of the experiment did not depend on the sampling depth and amounted to about 6%. The analytical uncertainty of Tyurin’s method did not differ in two laboratories. Its contribution was 5–9% of the total variation of the soil organic carbon content in the area. The uncertainty of sample preparation determined from 11 to 26%, natural variation – from 49 to 68% of the total variance, respectively. Determination of the carbon content in the samples taken by the auger, when the sample is taken immediately at 0–30 cm, wins in reducing intermediate operations and gives comparable results compared to layer-by-layer sampling. The uncertainty of sample preparation determined from 11 to 26%, natural variation – from 49 to 68% of the total variance, respectively. Determination of carbon content in samples taken by auger, when the sample is taken immediately at 0–30 cm, wins in reducing intermediate operations and gives comparable results compared to layer-by-layer soil sampling.