Each green manure is different

6 September 2021

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:

 

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

Green manure
(sown in the stubble)
EOM
(kg/ha)
N-delivery
(kg/ha)
Leafy cabbage 840 30
Fodder radish 850 30
Yellow  850 30
Hoppenrupsklaver 790 60
Japanse oats (Avena strigosa) 850 30
Clover, red (under cover crop) 1165 60
Clover, white (under cover crop) 850 60
Artificial pasture (autumn grass) 450 30
Phacelia 850 30
Ryegrass, English, in stubble 980 30
Ryegrass, English, under cover crop 1155 30
Ryegrass, Italian, in stubble 1080 30
Ryegrass, Italian, under cover crop 1255 30
Stubble tuber 830 30
Tagetes 865 30
Primrose 700 60
Vetch 645 30
Winter rye 850 30
Spring turnip 770 30

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

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.

Fertilization Manager