Determination of TOC in Agricultural Soil, Dried Manure and Sediments Application Note · TOC Solid Analyzer

Introduction

The carbon contained in soil has different sources and effects on the soil‘s quality. Total inorganic carbon (TIC), mainly carbonate and hydrogen carbonate, is a natural part of soil but can also derive from artificial addition e.g. fertilizers. Total organic carbon (TOC) is generated by organic matter that is present from natural sources (plants, animals, microorganisms, degradation processes) and is a precondition for soil formation and for the preservation of its fertility. On the other hand, contaminations caused by humans, as by industrial and agricultural processes (oil residues, waste, overfertilization, etc.) can also be an origin of elevated TOC levels.

The TOC content in soil is also mainly influenced by erosion, biological decomposition and farming and affects soil properties (e.g., color, fertility, but also toxicity). For example, topsoil rich in humus is subject to special protection. The humus content in such soils typically ranges from 1 – 8%, which equals a TOC range from 0,5 – 4%, meaning the humus content is typically twice the TOC concentration.

This makes TOC an important parameter in agricultural soil classification and environmental protection, needing to be regularly monitored. For this purpose, elemental analysis techniques based on high-temperature combustion of all sample components have proven best suited, followed by NDIR detection of the formed CO2 (e.g., EN 13137 or DIN EN 15936, ISO 10694). According to these norms the differential or direct TOC method can be applied to measure the TOC concentrations in solids. The direct method is of advantage especially with samples of high TIC and low TOC content because here, the error affected subtraction of two big numbers (TC - TIC = TOC) is avoided and a higher sample throughput can be achieved, since only one measurement for one parameter has to be conducted instead of two measurements in TOC difference method.