Abstract

Soil Organic Matter (SOM), also known as humus, is the prerequisite for soil formation and one of the basics for human and animal nutrition. As a result of climate change, a reduction in the CO2 - concentration in the atmosphere is expected due to “Carbon Sequestration”. Determination and evaluation of SOM still involves many problems. There are no scientifically based guide values for the (site-specific) humus content of the soil. Total levels say nothing about the supply status of the soil with organic matter. An assessment of the supply status is therefore carried out using the so-called humus balance method according to VDLUFA. It is necessary to differentiate between permanent humus and nutrient humus. Permanent humus cannot be determined analytically, but its delimitation can be proven experimentally in long-term field experiments. The nutrient humus is subject to constant conversion by soil organisms. Its average content is 0.3% organic carbon (C_org). This and Nitrogen (N) in the soil have a high spatial and temporal variability. In one field, the differences in C_org - content are often more than 0.5 percentage points; however, with 0.2 percentage points C_org already makes the difference between “well cared for” and “impoverished”. On the other hand, there is an upper limit for the humus content, exceeding which would otherwise cause environmental pollution. With an accumulation rate of no more than 10% and a carbon content of 40% in the dry matter of the plant, carbon sequestration is already arithmetically excluded. Due to the lower yields and thus lower harvest and root residues, the humus supply of the soil in organic farming can only be secured with very high animal stocking densities.