Field carbon sequestration assessment involves soil organic content mapping which relies heavily on destructive soil sampling and expensive laboratory analysis methods. These inconveniences may be alleviated by the use of electromagnetic sensing techniques that measure apparent soil electrical conductivity (ECa) instantaneously without any need for physical contact with the soil. The availability of exhaustive ECa data may reduce drastically sampling and analysis efforts for characterizing the spatial variability of physical or chemical soil properties. In this study an electromagnetic dual dipole device (EM38DD, Geonics, Inc.), mounted on a PVC sled and pulled by an ATV vehicle, has been used to measure ECa on a long-term conservation tillage split-block experiment in Southern Spain, where crop residue decomposition is being monitored. Traditionally twelve soil samples were taken on a regular grid within each block. Using geostatistical techniques and ECa maps optimal soil sampling schemes were designed for mapping soil organic matter content at the plot scale. Results show that the initial soil sampling effort required for representing accurately the spatial variability of the soil organic matter content in the plots can be reduced using ECa data. These methods can also be used to assess the spatial variability of other soil chemical and physical properties. Key words: apparent soil electrical conductivity, soil sampling, organic matter content.