Humus: Facts and myths

Everything flows! - So the philosophy goes. A parallel to agriculture can be found in the humus content of the soil. This follows a "flow" equilibrium of supply and decomposition of organic matter. Flow equilibrium means that there is a feedback between supply and decomposition. The final resulting humus content over the years does not fluctuate as much as the supply of organic matter in everyday management. Different assessments of the potential for a significant increase in humus content in arable soils result from the difference between understanding this complex process via cascades of soil microbes as flowing or seeing it as static.

Until 1960, the regular supply of organic matter from root-rich forage crops and organic fertilisers was part of all arable thinking. Today, these management elements are hardly ever found on cash crop farms. Has the humus content thus completely disappeared? No. The flow equilibrium of build-up and decomposition has merely shifted slightly downwards. Buffering factors here are very high yields of cash crops with corresponding root excretions and their harvest residues. In addition, there has been a gradual reduction in tillage, which has emerged with the development of modern tillage systems.

The crucial point, however, is that "flow" equilibrium means that the decomposition rate decreases in a regulating manner with a lower supply of organic matter. One percent more or less humus is just as big a difference as 1 degree temperature difference in the global climate. It is not so easy to change this in the short term.

And yet: ten years ago, the Thünen Institute in Braunschweig drew the sobering conclusion: "We lose relevant amounts of carbon every year on all sites used for agriculture." A new equilibrium in humus content has thus obviously not been and still has not been achieved. This is a challenge for each individual arable farm to counteract the predominant decomposition of organic matter by cultivating catch crops and using organic fertilisers in an additional stabilising way.

If you are a root crop farmer and farm on sand, you should compensate for the increased turnover with fertiliser, soil cultivation and catch crops - and where appropriate, also with surface rotting and microbial carbonisation. On the other hand, if you have enough forage crops in the rotation, you can mobilise their harvest and root residues for the cash crops (e.g. by shelling them twice). And those who conventionally cultivate mainly threshed crops on heavy, wet marshes tend to have other problems than the humus content of their soils (e.g. field foxtail grass).

The decomposition of organic matter in the soil - from previous crop, fertiliser or humus - is a process that is relevant to the success of arable and crop farming. This is because it is also associated with many important positive effects:

• Nutrient supply (nitrogen, phosphorus, sulphur),
• CO2 release as a "yield turbo" during assimilation,
• Hygienisation (decomposition of fungal pathogens and weed seeds),
• fermentation (root crops are better suited for this than cereals).

In an arable farming book from 100 years ago, the last point reads like this: "The parched soil is friable and crumbly, so that it is both well aerated and favourably placed in its moisture state. It contains a greater quantity of absorbable plant nutrients. The best tilth is achieved when the soil is subjected to a thorough fallow treatment. The farmer who has not understood this and does not take care of the fermentation remains a bungler in his profession all his life." This is comparatively drastic language for a textbook. In those days, when there was a good supply of organic matter, the targeted mobilisation of soil potential was considered to be correspondingly important. Why not revive the "fermentation economy" as an agronomic instrument, at least partially in crop rotation (e.g. with rape), in the same way as the "humus economy"? Both sides of the humus coin are valuable and worthy of attention.