Digestate as Fertilizer

Possible applications
4 Possible applications
Organic fertilization is an important source of plant nutrients and organic
matter for humification. The nutrients that are present in digestate are
partly mineralised in various forms and are partly in organically bound
form. This leads to differences in availability over time compared to mineral
fertilizers.
Thousands of farmers use the many advantages of organic
fertilization for agricultural crops. Since using
nutrients also depends on the application technique,
the weather, and the fertilized crop, there may be
greater fluctuations in the effect of organic fertilizers.
The nutrient content of digestate varies depending on
the feedstocks used. In addition, there are different
compositions depending on the treatment (separation,
drying, etc.) digestate has undergone. Therefore, when
separating the liquid and solid fractions of digestate,
the liquid fraction is more likely to contain ammonium
(NH 4
) and potassium (K 2
O) while the solid fraction is
more likely to contain phosphate (P 2
O 5
) and organic
material.
For plant-appropriate nitrogen fertilization, regular examination
of the digestate is, therefore, indispensable.
In principle, such analyses show that liquid digestate
often has a dry matter content of 4-6%, and 60-80%
of the nitrogen is present as directly available NH 4
due to anaerobic digestion. This has an effect on the
pH value of the digestate, which is higher than that of
liquid manure (about 8), which increases the risk of
gaseous ammonia losses. This must be counteracted
with technical measures accordingly, (see Chapter 5:
“Application techniques”).
The nutrient composition of the digestate and the effectiveness
of the nutrients are crucial for fertilization
planning. This is, for example, prescribed in the German
fertilizing regulation on the basis of a fertilizer requirement
determination. The availability of nitrogen
depends directly on the NH 4
content and the ratio of
carbon to nitrogen (C/N ratio). Fertilizers with a narrow
C/N ratio (slurry, manure, liquid digestate) have a much
faster nitrogen availability than fertilizers with a wide
C/N ratio (compost, manure, solid digestate). The table
“Ingredients in typical digestate” provides an indication
of the possible nutrient compositions.
The ammonium present in the digestate can be regarded
by the farmer directly in the year of application as
mineral available nitrogen. This value is around 60%
for liquid digestate. In addition, there is a proportion
of nitrogen, which is organically bound, but which is
mineralised (available) during the vegetation period.
The availability of nitrogen bound in organic substance
varies. A small proportion is mineralised relatively
quickly and can be absorbed by the crops in the year
of application. Nitrogen, which is more strongly bound
in the organic substance, is mineralised very slowly.
Depending on weather conditions and soil tillage intensity,
release rates of 1-3% of total nitrogen per year are
to be expected.
The German Fertilization Ordinance exemplifies how
this continuous release of nutrients is incorporated into
legal requirements. In the case of digestate, an effectiveness
of at least 50% of the total nitrogen content
applied is specified; for solid digestate only 30%. In
addition, subsequent delivery from the previous year
in the amount of 10% of the total nitrogen applied in
the previous year must be added. The background to
this requirement is that a continuous supply of organic
fertilizers leads to an accumulation of humus in the soil
with the consequence of a slowly increasing release of
nitrogen. The following example illustrates this:
Ingredients in typical digestate
Form of digestate DM [%] N total
[kg/m 3 ] NH 4
[kg/m 3 ] NH 4
share [% of N total
] P 2
O 5
[kg/m 3 ] K 2
O[kg/m 3 ]
Liquid digestate 6.5 5.1 3.2 62.7 2.3 5.5
Liquid separated fraction 5.7 4.9 3.1 63.3 2.0 5.4
Solid separated fraction 24.3 5.8 2.7 46.5 5.0 5.8
Data from Bayerische Landesanstalt für Landwirtschaft
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