Digestate as Fertilizer

Application techniques
5 Application techniques
In contrast to mineral fertilizers, organic fertilization requires greater technical effort
in order to actually supply the plant with nutrients. Losing them into groundwater and air
must be minimised.
The high NH 4
content in conjunction
with higher pH values carries
the risk of losses in the form of
ammonia (NH 3
) during storage and application
of the digestate. The risks can be
minimised by low-loss application and immediate
incorporation, which then result in
an increase in the nutrient effect. The best
possible use of nutrients is as the basis of
the fertilizer value of digestate. If technical
and organisational measures are observed,
the NH 4
effect can be approximated by digestate
to that of the effect of mineral fertilization.
Techniques for spreading liquid digestate
that best meet the requirements mentioned
above are processes with drag hose, drag or
slit-shoe distributors as well as flat injection
processes. Although a wide distribution allows
for low-emission application, it is very
weather-dependent. Good results can only
be expected at cool temperatures in combination
with subsequent rainfall. When
choosing the technique, a crucial aspect is
whether it is transferred to the stock or not.
The wide distribution by means of an upward
deflecting baffle plate (see illustration:
“Wide distribution”) was common for
a long time and very susceptible to loss. It
Wide distribution
Wide distribution
has been banned in Germany since 2016.
As a cost-effective spreading technique, an
impact element is still permitted, which
guarantees a horizontal or, better, downward-directed
spreading of the digestate.
This makes it possible to keep NH 3
losses
low in comparison. Nevertheless, the distribution
accuracy suffers from external
weather conditions, especially wind. Immediate
incorporation into the soil is essential,
especially at warm temperatures.
The advantage of this technology is – in addition
to the affordable purchase price – the
potential application in growing stock and
on grassland as well.
A significantly better lateral distribution
and lower NH 3
losses during spreading
are achieved by attaching drag hoses to a
linkage, which means that the digestate is
fed directly to the soil at regular intervals.
Thanks to the simple technology and the
light-weight, large working widths, highpower
output can be achieved. Since no
pressure is exerted on the hoses, the digestate
is partly deposited on plants in growing
crops or on grassland, which can lead
to damage to the plants in unfavourable
weather conditions.
In addition to spreading with drag hoses,
the use of drag or slit-shoe distributors is
particularly suitable for existing crops.
Pressure is exerted on the ground here by
means of the attached steel springs. However,
this should not be confused with an
injection that requires much more ground
pressure. The soil skids on the underside of
the tractors open the crop and easily break
up the soil (depending on the type and density
of the soil), which favours infiltration
of the digestate into the soil. At the same
time, damage to the plants and the grass
cover is largely prevented. Direct ground
contact also reduces the potential for loss.
However, the technical effort is higher and
is noticeable in terms of weight and capital
expenditure.
The injection with slitters, in which the digestate
is introduced directly into the soil,
is optimal with regard to losses and thus
also plant availability. This procedure is
also possible in the growing stock. In grassland
in particular, the danger of excessive
damage to the grass cover can be seen here.
A method frequently used in biogas plants
is underfoot fertilization by injection prior
Drag hose
Injection with slitter
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