Agronomic Sustainability of ASB Cameroon Land Use Systems
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Source file: D:\Projects\ASB\ASB Country and Thematic reports\Cameroom Final Report\Final Report&Synthesis of PhaseII-Cameroon.xml
Authors: J. Kotto-Same, A. Moukam, R. Njomgang, T. Tiki-Manga, J. Tonye, C. Diaw, J. Gockowski, S. Hauser, S. Weise, D. Nwaga, L. Zapfack, C. Palm, P. Woomer, , Andy Gillison, D. Bignell, J. Tondoh
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The Agronomic Sustainability Working Group officially started work in January 1998, following a decision made at the Bogor Global Steering Group meeting in August 1997. As researchers were making headway along the original Phase II themes of global environmental benefits and systems adoptability, it became clear that agronomic sustainability is an important interface for linking global environmental issues with local farmer concerns. Increases in productivity on already cleared land could very well directly and indirectly contribute to global environmental benefits: directly through the development and enhancement of, for example, complex agroforests and planted short fallows, and indirectly through a reduction in farmers need to clear forest land for agriculture. However, it is key that these increases in productivity are achieved in a sustainable manner. To this end, the working group developed indicators of sustainability as a tool for the preliminary assessment of the longer-term field-level agronomic constraints in each of the land use systems.
No specific field
measurements were made by the agronomic sustainability working group. The indicators are based on measurements made
by the climate change, biodiversity and synthesis and linkages working
groups. Some of this information was
used directly, e.g. for soil compaction assessments; some data was combined
with information derived from literature to create new parameters, e.g. the
nutrient balance calculations; and some critical assessments were based on the
field experience of relevant researchers, e.g. the crop protection constraints. Many of the indicators thus derived and
assessed for
The indicators employed for the Phase II evaluation fall into three main categories of measures: soil structure, nutrient balance, and crop protection. The methodological details follow.
Good soil structure is critical for maintaining the long-term capacity of agricultural land to produce crops. Soil compaction, carbon saturation deficit, active soil carbon, and soil exposure were used to assess the status of land use systems (hereafter referred to as land cover types).
As nutrients are removed from a piece of land through the harvested product, it is important to assess if these are adequately replenished through internal processes and/or external inputs such as fertilizers. Internal processes making nutrients available to plants include soil organic matter mineralization, release from the soil matrix, and biological fixation of atmospheric nitrogen. At the same time, nutrients can be lost from the system through processes such as leaching, lateral flow, soil erosion, and denitrification. It is not easy to rapidly measure many of these processes. Simplified nutrient balances are, therefore, often used as a first indication of the nutrient dynamics of a system. We limited our calculations to the 3 major plant macronutrients: nitrogen (N), phosphorus (P), and potassium (K).
Nutrient
Export, Balance, and Depletion
Nutrient exports are easily determined if the quantity of harvested products, including any crop residues removed from the field, and their nutrient content are known.
Nutrient Export = (nutrient content x harvest off-take) summed across all products over system cycle / length of system cycle; (kg/ha/yr)
A more precise measure is:
Simple Nutrient Balance = nutrient import - nutrient export; (kg/ha/yr)
Nutrient imports include fertilizers and N fixation through legumes in the system. Fertilizer inputs are corrected for use efficiencies, i.e. 25% of N, 20% of P, and 30% of K fertilizers are assumed to be effectively taken up by crops and thus helping to compensate for nutrient exports. Negative balances indicate greater exports than imports.
It is often desirable to calculate an NPK index that combines the 3 macronutrients to investigate trade-offs between nutrient balance and other parameters of a system (e.g. biodiversity, profitability etc.):
NPK Index = Sum of N, P, and K ranks / 3;
Where land use systems are ranked in terms of the simple nutrient balance with the highest value receiving a 1, the second receiving a 2, the third a 3 etc. The NPK Index is valid for within country comparisons only.
Nutrient exports are high in the intensively managed, high-yielding cocoa systems and in the oil palm systems (Table 12). The potassium export in oil palm systems, through harvesting of oil palm bunches, is particularly high. Farmers use fertilizer to off-set this potassium export quite successfully, as the simple nutrient balance values indicate. In contrast, little to no fertilizers are used in cocoa systems, resulting in negative nitrogen, phosphorus and potassium balances. In the extensive cocoa systems, less nutrients are exported due to the lower yield levels. Exports by annual crops from short fallow systems are only slightly lower. The long fallow food crop systems have the lowest nutrient exports together with community forests. The NPK-index closely reflects the trends in the simple nutrient balance.
The key to understanding the implications of these nutrient
exports is relating them to nutrient stocks in the soil as well as in dead and
living plant biomass. Since this
information was not available for the sampled sites in
Table
12. Nutrient export, simple
nutrient balance, and the NPK-index in different land
use systems in the
|
Land use systems |
Nutrient Export (kg ha-1
yr-1) |
Simple Nutrient Balance (kg ha-1 yr-1) N P K |
Index |
||||
|
SF Food intercrop LF Food intercrop SF - Intensive SF - Intensive FOR Extensive cocoa with fruit FOR Extensive SF Oil Palm FOR Oil Palm Community-based |
8.5 1.3 18.6 18.3 10.0 9.7 17.0 17.2 (1.0) |
1.3 0.2 3.9 3.9 2.1 2.1 3.0 3.0 (0.2) |
4.9 1.0 13.3 13.0 7.3 7.0 16.6 17.6 (1.0) |
- 8.5 - 1.3 - 18.6 - 18.3 - 10.0 - 9.7 - 16.9 - 17.1 (- 1.0) |
- 1.3 - 0.2 - 3.9 - 3.9 - 2.1 - 2.1 - 3.0 - 3.0 (- 0.2) |
- 4.9 - 1.0 -13.3 -13.0 - 7.3 - 7.0 0.7 - 0.3 (-1.0) |
3.7 2.0 8.7 8.0 5.3 4.7 4.3 5.0 (1.7) |
Note: Community-based
forest values are based on estimates only.
Nutrient
Replacement Value (NRV)
Fertilizers play an important role in replacing nutrients exported through harvested products. However, if the cost of the fertilizer required to balance the export is too high relative to the value of the products, farmers will hesitate applying fertilizer even if it is available. The lower the proportion of the fertilizer cost compared to the farm-gate value of the crop, the more likely the farmers will be to consider using fertilizers and thus avoiding nutrient mining (van Noordwijk et al., 1997):
Nutrient Replacement Value =
sum of cost of fertilizers required to replace all exported NPK nutrients / monetary value of all products used throughout the system cycle;
Fertilizer required is corrected for nutrient recovery, ie. only 25% of N, 20% of P, and 30% of
K fertilizers are assumed to actually be recovered by the crops. Nitrogen provided through N-
fixation of legumes is deducted from N export before calculating N fertilizer replacement
requirements. Low NRVs indicate that the crop sequence is of high value relative to the cost
of nutrient replacement through fertilizers. Generally, NRV is only calculated on a specific crop and year basis. This works well for monocrop situations.
The nutrient replacement value is highest in oil palm systems, followed closely by cocoa systems without fruit trees (Table 13). This indicates that the value of exported nutrients relative to the value of the harvested crop is high. The inclusion of fruit trees in the cocoa plantation reduces NRV, i.e., the value of the greater nutrient export is smaller than the value of the crop harvested. Medium levels of NRVs are associated with cocoa systems with fruit trees and short fallow food crop systems. This is the case even though the value of the crop exported from the cocoa systems with fruit trees is 1.5 to 2.5 times higher than the value of food crops planted in short fallow systems. NRV ratios are lowest for the long fallow system.
This NRV approach does not appear to work well for the
Table 13: Nutrient balance to carbon stock ration and the nutrient replacement value in different
land use systems in the
|
Land
use systems |
Nutrient Balance to C
Stock Ratio (g nutrient/yr/ton carbon) N P K |
Nutrient Replacement Value |
||
|
SF
Food intercrop LF
Food intercrop SF
Intensive cocoa with fruit SF
Intensive cocoa without fruit FOR
Extensive cocoa with fruit FOR
Extensive cocoa without fruit SF
Oil Palm FOR
Oil Palm Community-based
|
- 733 -
10 - 210 - 206 - 113 - 109 - 239 - 242 (- 5) |
- 112 -
2 -
44 -
44 -
24 -
24 -
42 -
42 (- 1) |
- 422 -
8 - 150 - 147 -
82 -
79 9 -
5 (- 5) |
0.25 0.12 0.25 0.30 0.21 0.28 0.35 0.32 - |
Note: Community-based forest values are based on
estimates only.
Another important agronomic constraint to sustainable production can be the development of problem weeds and specific pest and diseases. An attempt was made to identify potential crop protection problems; although no field observations were made. Based on the field experience of researchers, it was assessed whether weed problems are or could become a major constraint in different land use systems, unless this was addressed by additional labour and/or technical inputs. A similar assessment was made for pest and disease problems.
Weed Problems
As farmers move through recurrent short fallow cycles in the short fallow annual food crop system, pressure from arable weeds in general is expected to increase significantly (Table 14). There is not only a greater number of weeds, but also a shift from less problematic broadleafs to more difficult grasses. In cocoa and oil palm systems, weed problems are only anticipated during the establishment phase of the perennial crop, after the harvest of the associated annual crops. Weed pressure may be lower in perennial crops planted into newly cleared forest. No major weed problems are expected in the long fallow system.
As with weeds, as farmers move through more short fallow
cycles in the short fallow, annual food crop system, the general pressure from
cassava, groundnut, plantain, and cocoyam pests and diseases is expected to
increase significantly (Table 14). The
pest and disease problems in the cocoa systems are quite serious. Cocoa varieties grown in southern
Table 14. Assessment of the potential for weed, pest and disease problems in different land
use systems in the
|
Land Use Systems |
Weed Problems |
|
|
SF Food intercrop LF Food intercrop SF Intensive SF Intensive FOR Extensive FOR Extensive SF Oil Palm FOR Oil Palm Community-based |
YES NO (YES) (YES) (YES) (YES) (YES) (YES) NO |
YES NO YES YES YES YES (YES) (YES) NO |
Note: Parentheses indicate that assessment refers to a specific phase of the system cycle.
The overall assessment of agronomic sustainability based on the information presented above is provided in Table 15.
Soil Structure: We expect significant decline in soil structure over time in intensively managed, short fallow, annual food crop systems. Alternative biomass management practices associated with planted fallows may reduce this potential problem. A deterioration of soil structure is also expected when perennial crop systems are planted into forest fields. In contrast, perennial crops planted into short fallow land would help to protect the soil better than annual cropping systems. There is greater concern about soil compaction in oil palm systems than cocoa systems because of the slower canopy closure at establishment for the former and the more regular traffic required for harvesting bunches.
Nutrient Balance: The systems that cause most concern in terms of over-exploitation of nutrients are the intensive perennial crop systems, i.e., cocoa and oil palm. The potassium lost in the oil palm systems is compensated for by fertilizer use; however, no fertilizer is applied in the intensive cocoa system. The extensive cocoa system is of somewhat less concern, since the yield levels are significantly lower. Fertilizer use can alleviate most of these concerns, and farmers are willing to use them if the institutional and financial environments are conducive. Although the nutrient exports from the short fallow/food crop system are moderate, we must assume that the nutrient stocks are already low in a system where fallows only grow for 4 years and the above-ground biomass is regularly burned and cleared. Given that short fallows are often planted to subsistence crops with little cash return, the probability of farmers using external inputs is very low. Only the association of higher value annual food and horticultural crops (e.g. tomato) with these systems would enable the use of fertilizers. Nitrogen could be supplied by the planting of nitrogen-fixing fallow species. Finally, we do not expect any nutrient problems in the long fallow and community forest systems.
Crop Protection: We expect that major weed, pest and disease complexes will develop in recurrent short fallow systems. The latter can probably only be addressed through crop breeding. Intensive weed management associated with a prior high value crop (e.g. tomato) may reduce the weed pressure in subsequent subsistence food crops. The cocoa systems also face a major challenge in terms of pest and disease problems that would require a concerted control effort at the community level with major inputs of pesticides. Weeds are a threat in the establishment of all perennial systems.
Overall Agronomic Sustainability: The most sustainable systems appear to be the
long fallow and the community forest systems.
The next most sustainable is the establishment of oil palm systems on
land previously under short fallows. All
other systems have important agronomic constraints associated with them or lead
to possible deterioration of the resource base.
As indicated above, there are potential solutions, but the financial and
institutional environment has to be conducive.
Table 15. Assessment of soil structure, nutrient balance, and crop protection status in
different land use
systems in the
|
Land Use Systems |
Soil Structure |
Nutrient Balance |
Crop Protection |
|
SF Food intercrop LF Food intercrop SF Intensive SF Intensive FOR Extensive FOR Extensive SF Oil Palm FOR Oil Palm Community-based |
-1 -0.5 0 0 -0.5 -0.5 0 -1 0 |
-1 0 -1 -1 -0.5 -0.5 -0.5 -0.5 0 |
-1 0 -1 -1 -1 -1 -0.5 -0.5 0 |
Note: Scores (0, -0.5, -1) indicate relative severity of problem, with -1 most severe.