The fertilizing properties of digested slurry are determined by how much mineral substances and
trace elements it contains; in tropical soil, the nitrogen content is not necessarily of prime
importance—lateritic soils, for example, are more likely to suffer from a lack of phosphorus. The
organic content of digested slurry improves the soil's texture, stabilizes its humic content, intensifies
its rate of nutrient-depot formation and increases its water-holding capacity. It should be noted that
a good water balance is very important in organically fertilized soil, i.e. a shortage of water can wipe
out the fertilizing effect.
Very few data on yields and doses are presently available with regard to fertilizing with digested
slurry, mainly because sound scientific knowledge and information on practical experience are
lacking in this very broad domain. Table 3.10 lists some yield data on digested-slurry fertilizing in
the People's Republic of China.
For a practician faced with the task of putting digested slurry to good use, the following tendential
observations may be helpful:
- While the nitrogen content of digested slurry is made more readily available to the plants
through the mineralization process, the yield effect of digested slurry differs only slightly from
that of fresh substrate (liquid manure). This is chiefly attributable to nitrogen losses occurring at
the time of distribution.
- Digested slurry is most effective when it is spread on the fields just prior to the beginning of the
vegetation period. Additional doses can be given periodically during the growth phase, with the
amounts and timing depending on the crop in question. For reasons of hygiene, however,
lettuce and vegetables should not be top-dressed.
- The recommended quantities of application are roughly equal for digested slurry and stored
- The requisite amount of digested-slurry fertilizer per unit area can be determined as a mineral
equivalent, e.g. N-equivalent fertilization. The N, P and K doses depend on specific crop
requirements as listed in the appropriate regional fertilizing tables.
With a view to improving the overall effect of slurry fertilizer under the prevailing local boundary
conditions, the implementation of a biogas project should include demonstration trials aimed at
developing a regionally appropriate mode of digested-slurry application. For information on
experimental systems, please refer to chapter 10.6 - Selected Literature.
Proceeding on the assumption that the soil should receive as much fertilizer as needed to replace
the nutrients that were extracted at harvesting time, each hectare will require an average dose of
about 33 kg N, 11 kg P2O5 and 48 kg K2O to compensate for an annual yield of 1 - 1.2 tons of, say,
sorghum or peanuts. Depending on the nutritive content of the digested slurry, 3-6 t of solid
substance per hectare will be required to cover the deficit. For slurry with a moisture content of
90%, the required quantity comes to 30-60 t per hectare and year. That roughly corresponds to the
annual capacity of a 6-8 m³ biogas plant.
Like all other forms of organic fertilizing, digested slurry increases the humic content of the soil, and
that is especially important in low-humus tropical soils. Humus improves the soil's physical
properties, e.g. its aeration, water retention capacity, permeability, cation-exchange capacity, etc.
Moreover, digested slurry is a source of energy and nutrients for soil-inhabiting microorganisms,
which in turn make essential nutrients more available to the plants. Organic fertilizers are
indispensable for maintaining soil fertility, most particularly in tropical areas.