Fertilizing with digested slurry
The practice of regular organic fertilizing is still extensively unknown in most tropical and subtropical countries. Due, however, to steady intensification of agricultural methods, e.g. abbreviated fallow intervals, some form of purposeful organic fertilizing, naturally including the use of digested slurry as fertilizer, would be particularly useful as a means of maintaining tropical soil fertility. Since Third World farmers have little knowledge of or experience in organic fertilizing methods, particularly with regard to the use of digested slurry, the scope of the following discussion is limited to the general plantgrowth efficiency factors of digested slurry.
Fermentation-induced modification of substrate
- Anaerobic digestion draws carbon, hydrogen and oxygen out of the substrate. The essential plant nutrients (N, P, K) remain, at least in principle, in place. The composition of fertilizing agents in digested slurry depends on the source material and therefore can be manipulated within certain limits.
- For all practical purposes, the volume of the source material remains unchanged, since only some 35 - 50% of the organic substances (corresponding to 5 - 10% of the total volume) is converted to gas.
- Fermentation reduces the C/N-ratio by removing some of the carbon, which has the advantage of increasing the fertilizing effect. Another favorable effect is that organically fixed nitrogen and other plant nutrients become mineralized and, hence, more readily available to the plants.
- Well-digested slurry is practically oderless and does not attract flies.
- Anaerobic digestion kills off or at least deactivates pathogens and worm ova, though the effect cannot necessarily be referred to as hygienization (cf. Table 3.8). Ninety-five percent of the ova and pathogens accumulate in the scum and sediment. Plant seeds normally remain more or less unaffected.
- Compared to the source material, digested slurry has a finer, more homogeneous structure, which makes it easier to spread.
Table 3.8: Survival time of pathogens in biogas plants (Source: Anaerobic Digestion 1985)
| Bacteria | Thermophilic fermentation | Mesophilic fermentation | Psycrophilic fermentation | |||||
| | 53-55 °C | | 35-37 °C | | 8-25 °C | | ||
| | Fatality | Fatality | Fatality | |||||
| | Days | Rate | Days | Rate | Days | Rate | ||
| | | (%) | | (%) | | (%) | ||
| Salmonella | 1-2 | 100.0 | 7 | 100.0 | 44 | 100.0 | ||
| Shigella | 1 | 100.0 | 5 | 100.0 | 30 | 100.0 | ||
| Poliviruses | | | 9 | 100.0 | | | ||
| Schistosoma ova | hours | 100.0 | 7 | 100.0 | 7-22 | 100.0 | ||
| Hookworm ova | 1 | 100.0 | 10 | 100.0 | 30 | 90.0 | ||
| Ascaris ova | 2 | 100.0 | 36 | 98.8 | 100 | 53.0 | ||
| Colititre | 2 | 10-1 - 10-2 | 21 | 10-4 | 40-60 | 10-5 -10-4 | ||
Table 3.9: Concentration of nutrients in the digested slurry of various substrates!
(Source: OEKOTOP, compiled from various sources)
(Source: OEKOTOP, compiled from various sources)
| Type of substrate | N | P2O5 | K2O | CaO | MgO |
| | —% TS— | ||||
| Cattle dung | 2.3 - 4.7 | 0.9 - 2.1 | 4.2 - 7.6 | 1.0 - 4.2 | 0.6 - 1.1 |
| Pig dung | 4.1 - 8.4 | 2.6 - 6.9 | 1.6 - 5.1 | 2.5 - 5.7 | 0.8 - 1.1 |
| Chicken manure | 4.3 - 9.5 | 2.8 - 8.1 | 2.1 - 5.3 | 7.3 - 13.2 | 1.1 - 1.6 |
Fertilizing properties
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 liquid manure.
- 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.
Table 3.10: Effects of digested slurry on crop yields (Source: Chengdu 1980)
| Plants tested | Quantity of digested slurry | Yield | Increase | | |
| | | with digested slurry | with liquid manure | | |
| | (m³ /ha) | (kg/ha) | (kg/ha) | (%) | |
| Sweet potatoes | 17 | 24000 | 21500 | 21500 | 12 |
| Rice | 15 | 6500 | 6000 | 500 | 8 |
| corn (maize) | 22.5 | 5000 | 4600 | 400 | 9 |
| Cotton | 22.5 | 1300 | 1200 | 100 | 8 |
The importance of digested slurry as a fertilizer is underlined by the answers to the following questions:
- How much chemical fertilizer cap be saved with no drop in yield?
- Which yield levels can be achieved by fertilizing with digested slurry, as compared to the same amount of undigested material, e.g. stored or fermented liquid manure?
- By how much can yields be increased over those from previously unfertilized soil? Depending on those answers, a certain monetary value can be attached to digested slurry, whereas the labor involved in preparing and applying the fertilizer must be given due consideration.
Storing and application of digested slurry
With a view to retaining the fertilizing quality of digested slurry, it should be stored only briefly in liquid form in a closed pit or tank and then applied to the fields. Liquid storage involves a certain loss of nitrogen due to the evaporation of ammonia. For that reason, and in order to limit the size of the required storage vessels (a 30-day supply corresponds to about 50% of the biogas plant volume), the storage period should be limited to 2-4 weeks. The resultant quasi-continuous mode of field fertilization (each 2-4 weeks), however, is in opposition to the standing criteria of optimum application, according to which fertilizer should only be applied 2-4 times per year, and then only during the plants' growth phase, when they are able to best exploit the additional nutrient supply.
 |
| Fig. 3.7: Slurry storage and composting. 1 Biogas plant, 2 Slurry composting pit with green cover 3 Masonry storage pit (V = 10 Sd), 31 Sturdy wooden cover, 32 Overflow (Source: OEKOTOP) |
If, for reasons of economy and efficiency, liquid fertilizing should appear impractical' the digested slurry can be mixed with green material and composted. This would involve nitrogen losses amounting to 30 - 70%. On the other hand, the finished compost would be soil-moist, compact (spade able) and much easier to transport.
If irrigated fields are located nearby, the digested slurry could be introduced into the irrigating system so that it is distributed periodically along with the irrigating water.
