Sustaining soil productivity of
cotton-based cropping systems in the Savannahs
of west and central Africa:
Challenges and Opportunities
P. Tittonell and M. Cretenet
• 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
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)
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.
0 200 400 600 800 1000 1200 1400 1600 50 000 100 000 150 000 200 000 250 000 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 kg ha-1 ha
Cotton crop acreage and yield in
North Cameroon
Sharp decrease in cotton production:
1. Less profitability (higher fertiliser costs, lower fibre prices)
0 50 100 150 200 250 300 350 400 0 200 400 600 800 1000 1200 1400 1600 1800 1975 1980 1985 1990 1995 2000 2005 2010 fertiliser kg ha -1 Seed-cotton yield (kg ha -1 ) year
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
n=7 n=34 n=66 n=31 n=6 0 500 1000 1500 2000 2500 3000 0 100 200 300 400 500 600 seed-cotton kg ha -1 fertiliser kg ha-1
"Temps court": Cotton crop response to
fertilisers North Cameroon
Soil Fertility
-+
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:
0 500 1000 1500 2000 2500 3000 0,45 0,50 0,55 0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 seed-cotton kg ha-1 SOM %
SOM and Crop response relationship
YIELD POTENTIAL
FERTILISER EFFICIENCY CONTROL
Soil organic matter is strongly correlated with these three parameters:
• Crop yields without fertilisers (control)
• The production potential with fertilisers
0,60 0,70 0,80 0,90 1,00 1,10 1,20 0 500 1000 1500 2000 2500 3000 F0 F1 F0 F1 F0 F1
residues exported residues returned res.ret. + straw
SOM% yield kg ha -1 maize 72-80 cotton 72-80 mean 0-30 cm
Long term experiment on SOM evolution
under 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)
Aplahoué Bénin
Yield %
"SOIL MEMORY"
F1 + Residues Returned + Fallow Straw mulch
FM + Rest. + Paillage 858 kg ha-1 680 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’
0 5 10 15 20 25 30 0 100 200 300 400 500 CO 2 équivalent Tha-1 fertiliser kg ha-1 biomass compartment soil compartment
Csoil sequestration through fertiliser use
What to do to improve the soil C compartment ?
6% Harvest CO2 Soil Organic Matter 15‐35 TC ha‐1 SOM mineralisation photosynthesis Humification 15‐40% Crop residues CO2 Soil tillage Cultivars Catch crops Associated crops Agro sylvo pastoral systems fertilisation 2.5-3.5 TC ha-1
•
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 ?)
y = 22.2x ‐ 1897 r² = 0.69 y = 1.5x + 1016 r² = 0.09 0 500 1000 1500 2000 2500 130 140 150 160 170 180 190 seed-cotton yield (kg ha -1 ) fertliser kg ha-1
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
y = 5.38x + 0.36 R2 = 0.79 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 C g.100g -1 CEC (cmol c .kg -1 ) ¹ H. Guibert0 200 400 600 800 1000 1200 1400 1600 1800 0 0.05 0.1 0.15 0.2 s e e d-c ot ton k g ha -1 K ech. meq/100g
Production potential and exchangeable bases
AL<0.01 Al=0.02 Al=0.03 AL=0.04 Al=0.05
