Conducting Your Humic Assessment

The humus balance aims to compare the gains and losses of humus, at the plot scale, associated with a technical itinerary. Humus gains come from the input or return of organic matter (OM) to the soil, while losses result from the mineralization of humus. The humus balance allows modeling the effects of different cropping techniques on the long-term evolution of the humus stock. An Excel spreadsheet provided by Market gardening Living Soil facilitates performing the humus balance.

Establishing your humus balance

Calculating humus losses

Humus losses occur during secondary mineralization characterized by the mineralization coefficient K2. Expressed as a %, it reflects the portion of humus degraded by the soil fauna. It varies depending on the soil texture and the agricultural practices chosen. In MSV one will therefore choose SD (no-till) if there is no soil tillage or TCS (simplified cropping techniques) if superficial tillage is performed.

Calculating humus gains

Humus gains are associated with the OM or crop residues brought to the plot. The humus gain is calculated by multiplying the returned biomass by the coefficient K1 (humification coefficient) of the residues : expressed as a %. It corresponds to the efficiency of the transformation of OM into stable humus. It varies depending on the nature and composition of the plant.

Interpreting your humus balance

The humus balance is the difference between humus gains and losses. In our example, it equals 6.45t/ha, i.e. an addition of 0.19 points of OM.


POSITIVE BALANCE	NEGATIVE BALANCE
Humification > mineralization. OM rate increases.	Humification < mineralization. OM rate decreases.
Result obtained thanks to the input of organic matter and the cessation of tillage which allows the arrival of soil fauna. Soil with good structural resilience.	Result caused by too low return of organic matter and the intensity of tillage. In the long term, there is a risk of loss of soil structural stability and thus a threat to fertility.

The humus balance should primarily be used to understand the impact of our practices. So do not hesitate to “play” with the spreadsheet to see in which cases practices have a positive or negative humus balance. To do this, simply download it to your computer.

Caution

The humus balance is only a modeling of agricultural practices. It is not reality. Significant work measuring the evolution of the OM rate over several years by laboratory analyses must consolidate these humus balances. It should also be noted that the calculation of the humus balance is based on criteria whose theoretical values can be questioned (the soil depth considered, the values of the coefficients K1 and K2 which vary according to soil properties and climatic conditions, the returns due to roots and extra-radicular products); all these variations can influence the final result of the balance. Nevertheless, it remains a good tool for evaluating the soil organic carbon stock, practical for managing the return and input of OM in the plot.