Sustaining soil productivity of cotton- based cropping systems in … · 2015-05-29 · Sustaining soil productivity of cotton-based cropping systems in the Savannahs of west and
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Sustaining soil productivity of cotton- based cropping systems in the Savannahs of west and central Africa: Challenges and Opportunities P. Tittonell and M. Cretenet 30th International Cotton Conference, Bremen (Germany), 24‐27 March 2010
Sustaining soil productivity of cotton-based cropping systems in the Savannahs
of west and central Africa:Challenges and Opportunities
P. Tittonell and M. Cretenet
30th International Cotton Conference, Bremen (Germany), 24‐27 March 2010
Présentateur
Commentaires de présentation
Good Morning Ladies and Gentlemen. Thanks to the Faser Institute for giving me the opportunity to present this paper entitled Sustaining Soil Productivity of Cotton-Based Cropping systems in the Savannahs of West and Central Africa: Challenges and Opportunities.
Cotton production areas in Africa
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The West African Cotton production area is the most important among the different cotton production areas of Africa. It represented in 2004 an annual cotton fiber production of 1 MT, corresponding to 60% of the whole African production.
• Savannah woodlands
• Crops at variable distances from homesteads
• Cotton main cash crop in rotation with food crops
• Smallholder, family agriculture (self-consumption)
Cotton-based systems of West and Central Africa
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The West Africa Cotton production area corresponds to Savannah woodlands ecosystems. The cotton development in West Africa has been a success story in terms of socioeconomic development. That are roads for cotton transport, schools and people's dispensaries built in the villages, new jobs created as blacksmiths, or workers in the ginning and postharvest processing. Cotton is produced by smallholders in the framework of family agriculture. Cotton is the main cash crop grown in rotation with food crops like maïze, sorghum, millet, groundnut, which are consumed on farm. The fields near the homestead are generally used for food crops, while the cotton fields are sometimes far from the homesteads.
Parallel development of Cotton and Cereal productions in West Africa.
• Cotton development has a ‘dragging’ effect on cereal production (in areas and yields) in this part of Africa
• Cotton plays a positive role for food security, contrary to earlier perceptions of competition for resources between cash and food crops (1970s)
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The parallel development of cotton and cereal production corresponds to a dragging effect of cotton cropping systems. More cotton corresponds also to more food crops being produced. The crop management techniques improving yields such as sowing in raws following straight lines, sole cropping instead of mixed with other crops, proper sowing rates, fertiliser application benefit to the different crops in the rotation. The development of oxen draught farming, oxen ploughing, promoted by Cotton Companies resulted in an increase of the average farming area which benefits both cereal and cotton production. Cotton plays a positive role for food security, contrary to earlier perceptions of competition for resources between cash and food crops.
The contribution of West Africa to world cotton production increased from 1 to 5% in 40 years (« le boom cotonnier »)
The 2003 predictions failed in reality: West African cotton production stops growing
The end of the ‘success story’ of African cotton coincides with the WTO summit at Cancun, Mexico
Worldwide and West African production of cotton fibre from 1960/61 to 2008/09.
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Commentaires de présentation
The contribution of West Africa to world cotton production increased from 1 to 5% in 40 years. That's the period we called le boom cotonnier. ↓ But the predictions made in 2003 failed in reality. The west African production collapsed from near 50 %. This marks the end of the success story and coincides with the World Trade Organization summit at Cancun, where the representatives of African cotton producers claimed the end of subsidies to cotton production in Northern countries.
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Cotton crop acreage and yield in North Cameroon
Sharp decrease in cotton production:
1. Less profitability (higher fertiliser costs, lower fibre prices)
2. Poorer yields
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The collapse in cotton production affects in particular the region of North Cameroon. Firstly through the area under cotton dropping from 230 to 150 thousand hectares as a result of less profitability, due to two main reasons: higher fertiliser costs and lower seed-cotton prices. Secondly through a yield decrease in seed-cotton from 1300 to 800 kilograms per hectare. I am going to illustrate some of the main biophysical reasons behind this collapse in cotton yields.
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Evolutions du rendement et des engrais utilisés en culture cotonnière au Nord Cameroun
There was a parallel evolution of cotton yields and fertiliser use
A reduction in fertiliser use has both short (growing season) and long term (cumulative) effects
Long term evolution of yields and fertiliser use on cotton in N Cameroon
Présentateur
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Still in North Cameroon, during the last 25 years, we observe a parallel evolution of cotton yields and fertiliser use. It's well known that a reduction in fertiliser use has both, a short term effect on crop yield during the current growing season, and a long term or cummulative effect involving soil fertility depletion.
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"Temps court": Cotton crop response to fertilisers North Cameroon
Soil
Fert
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+Crop yields without fertilisers as indicator of soil fertility
Crop responses are characterised by 3 parameters:• Crop yields without fertilisers• The production potential with fertilisers• Fertiliser use efficiency
Short term:
Crop responses to fertilisers
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To illustrate the short term effect of fertilisers use on crop yields, here are some response curves of cotton to increasing amounts of fertiliser applied, that were observed on 130 farmers' fields. These response curves have been classified in 4 quartiles according to the yield obtained without fertiliser (as you can see on this axe), considered to be a good indicator of soil fertility. The second and third quartiles have been put together, and are represented by the red curve. The two curves with dotted lines correspond to the 6 fields with the lowest fertility level, in orange, and the 7 fields of highest fertility level here in green. The crop responses are characterised by 3 parameters : the crop yield without fertiliser (on this axe), the production potential obtained with the maximum amount of fertiliser (this point here) and the fertiliser use efficiency as the difference between these two, represented by the slope of the curve.
Soil organic matter is strongly correlated with these three parameters:• Crop yields without fertilisers (control)• The production potential with fertilisers• Fertiliser use efficiency (lower in soils that are too fertile or too poor)
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These 3 parameters are strongly correlated with the content of Organic Matter in the soil, which is represented on this axe. The relationship between the control (crop yields without fertiliser) and the content of SOM is linear, indicating the capacity of SOM to predict or to represent soil fertility. On this sandy soils SOM is the best indicator of soil fertility. The yield potential seems to be maximum as from 0.8 % of SOM. And the fertiliser use efficiency is lower in soils that are either too fertile or too poor, meaning that the maximum returns from applying fertilisers correspond to intermediate soil fertility levels. This figure illustrates how, in the short term, SOM interacts with crop yields, by affecting the fertiliser use efficiency
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residues exported residues returned res.ret. + straw
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Long term experiment on SOM evolutionunder different cropping system
(Aplahoué Benin)
Long term trials allow evaluating cumulative effects of agricultural practices on soil organic matter and productivity
The impacts of fertiliser use and crop residue management are evident after 8 years (16 cropping seasons)
Présentateur
Commentaires de présentation
Now what about the long term effect of cropping systems on soil fertility and SOM? This long term experiment at Aplahoué in Bénin, compared 6 cropping systems corresponding to 3 types of crop residue management : residues that were exported or taken away from the fields, residues that were returned to the soil after harvest, and residues that were returned together with addition of fallow straw as a mulch on the soil surface. Each management type has been evaluated either with or without application of fertilisers. And every year, 2 crops were grown, maize in the first rainy season and cotton in the second one. These are production areas which are bimodal in terms of rainfall. This figure represents the results at the end of a period of 8 years, which we call the "exhaustion phase". Crop residue management is more effective than fertiliser use in terms of maintaining SOM contents in the long term. The fertiliser effect is important for both crops, cotton and maïze, except when straw was applied as a mulch ; meaning in this case that the yield levels obtained are near the potential production of both crops.
Aplahoué BéninYield %
"SOIL MEMORY"
F1 + Residues Returned + Fallow Straw mulch
FM + Rest. + Paillage
858 kg ha-1680 kg ha-1
780 kg ha-1
"Exhaustion phase"
"Regeneration phase"
Years under cultivation
Long term:
Soil regeneration and soil memory
The phase of ‘regeneration’ that follows soil fertility ‘exhaustion’ consists of implementing soil conservation measures
After 8 years of regeneration, the gap between initial and actual productivity is termed soil ‘memory’
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Commentaires de présentation
After the 8 years of the exhaustion phase, all the 6 cropping systems or treatments evaluated received the application of the full package, that is fertiliser, crop residues restitution and mulching. Yields are expressed as percentages of the production potential corresponding to initial full package treatment. After 8 years of this so called regeneration phase, there is still a gap between the initial and actual productivity. This gap is what we call the « soil memory ».
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biomass compartment
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Csoil sequestration through fertiliser use
What to do to improve the soil C compartment ?
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Soil Organic Matter15‐35 TC ha‐1
SOM mineralisation
photosynthesis
Humification 15‐40%
Crop residues
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CultivarsCatch cropsAssociated crops
Agro sylvo pastoralsystems
fertilisation
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General Carbon cycle under cotton-based cropping systems
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Commentaires de présentation
Having in mind such strong relationships between SOM, soil fertility and crop yields, the main scope of our research to improve or sustain soil fertility focuses mainly on improving or maintaining SOM contents. ↓ That supposes to increase the magnitude of carbon flows in this general cycle illustrated here, by improving the primary productivity of the system through the photosynthetic process, and the return and transfer of biomass from the agro-sylvo-pastoral system to the cultivated fields. But this puts us also in a trade-off situation: On the one hand, if we have to feed crops without fertilisers or applied organic manures, the process of SOM mineralisation has to be intensified by soil tillage. By tilling the soil, the SOM decomposes faster and releases the mineral nutrients that it contains. If Fertiliser and manures are available, we could imagine systems based on minimum soil disturbance to be able to preserve organic matter in the soil, which means using the soil to store C and to maintain good soil fertility levels. ↓ By applying fertilisers we promote greater primary productivity and so also a larger C storage in the soil. This graph illustrates the amount of C expressed in CO2 equivalents that could potentially be stored by means of increasing soil organic matter, through increased primary productivity with fertiliser use in a cotton based system. A part of this carbon that may be considered as “sequestered” getting so a value on the carbon market.
• There is no alternative cash crop to replace cotton in West and Central Africa
• Crop production on these soils cannot be sustained without fertilisers
• Cash crops are the ‘gate’ by which fertilisers may enter the system (thus benefiting also food crops)
• Current relative prices of fibre and fertilisers discourage farmers to invest in soil fertility
• Greater productivity means also larger C inputs to increase soil organic matter (a win-win situation ?)
Concluding remarks
Présentateur
Commentaires de présentation
There is no alternative cash crop likely to replace cotton in West and Central Africa Both food and cash crops production on these soils cannot be sustained without fertilisers The cotton crop by way of credit (through contract farming), is the ‘gate’ by which fertilisers may enter the system and benefit also food crops. However, the current relative prices of fiber, seed cotton and fertilisers discourage farmers to invest in soil fertility, and so fertilisers are less and less used Greater productivity means also larger C inputs to increase soil organic matter. So investing in fertilisers may also be seen as a likely win-win situation.
Extra slides
y = 22.2x ‐ 1897r² = 0.69
y = 1.5x + 1016r² = 0.09
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North Cameroon "Soil memory"
1986_1995
1996_2005
"Soil Memory"
• In Cameroon, cotton responds to fertilisers between 1986-1995, but not between 1996-2005, in spite of the application rates
• The ‘memory’ of the first period of soil fertility degradation persists, and translates in poor subsequent responses to applied fertilisers
Organic component of CEC.Parcelles cultivées Tchad¹
