Each green manure is different
6 September 2021 - Fertilization Manager
Green manures can make a good contribution to water retention and soil structure. Crop residues of a green manure crop that is high in carbon and low in nitrogen remain in the soil longer than crop residues high in nitrogen. The amount of effective organic matter (EOM) that a green manure crop leaves behind on the field is decisive.
Green manures offer more benefits than just increasing water holding capacity. The benefits involve chemical and physical soil fertility as well as biological soil health:
- Nutrients left over after the main crop is grown are not washed out but are fixed. After the green manure crop dies, those nutrients become available again for the following crop.
- Green manure crops contribute to the organic matter of the soil.
- The roots make a positive contribution to the structure of the soil and stimultate soil life.
- When sowing a hardy green manure crop on clay soils, the soil can be worked sooner after a wet period. In case of excess precipitation, the green manure will extract moisture from the soil.
- The soil cover provided by green manure crops protects the soil from erosion, compaction and/or drifting.
- Succesful cultivation of a green manure crop suppresses weed growth.
- Some green manures can suppress the development of nematodes.
Effective organic matter
Effectively increasing the organic matter content with a green manure crop works best with a crop with a low decomposition rate and a high carbon/nitrogen ratio (C/N ratio). The content of effective organic matter (EOM) determines how long the positive effects on water retention capacity and soil structure will be noticeable.
Effective organic matter (EOM) is the fraction of organic matter that remains in the soil one year after the application of crop residues, manure or compost. Organic matter consists of both a dynamic (labile) and a stable fraction. How dynamic or how stable the organic matter is depends on the ratio of the different elements in it.
Choice of green manure and EOM
The degradation rate of green manures in the soil and the contribution to EOM, is represented by the humification coefficient (HC). At an HC of 0.7, 70% of the organic matter is degraded within a year, leaving 30% as effective organic matter. Thus, some green manures are more suitable for increasing stable organic matter than others.
Table 1 shows the contribution to EOM of the most commonly used green manures. If the soil test shows that the organic matter balance is on the dynamic side, it is worthwhile to use a green manure with a low HC and a high EOM.
Table 1: Contribution green manures to the EOM in the soil. Source: Masterplan Mineralenmanagement
(sown in the stubble)
|Japanse oats (Avena strigosa)
|Clover, red (under cover crop)
|Clover, white (under cover crop)
|Artificial pasture (autumn grass)
|Ryegrass, English, in stubble
|Ryegrass, English, under cover crop
|Ryegrass, Italian, in stubble
|Ryegrass, Italian, under cover crop
Organic matter consists mainly of carbon (C), nitrogen (N), phosphorus (P) and sulfur (S). Dynamic organic matter contains relatively high amounts of N and S and is easily decomposed by soil life. In the process, nutrients are mineralized which become available to the crop. Stable organic matter contains relatively high amounts of C and is less readily broken down by soil life.
Organic matter quality
Thus, both dynamic and stable organic matter contribute to soil fertility. Dynamic organic matter increases chemical soil fertility and is food for bacteria; stable organic matter increases physical soil fertility and is food for soil fungi.
The report of Fertilization Manager shows the organic matter quality of the sampled soil (see Figure 1).
Figure 1: Fertilization Manager shows that soil organic matter is in balance. Source: Fertilization Manager Eurofins Agro
Soil testing provides insight into the quality of the organic matter in the soil. Based on this, the choice of one or the other green manure can be made.