Beltwide Nashville January 2008 Variations in soil fertility

Beltwide Nashville, January 2008 Variations in soil fertility under cotton crops in the Benoue cottongrowing area in northern Cameroon H. Guibert 1, C. Fesneau 2, M. M’Biandoun 3 CIRAD, PERSYST Department, Montpellier, France 2 University of Bourgogne, Dijon, France 3 IRAD, Center of Maroua, Cameroon 1

Contents • Introduction = time-course study • Methods • Results – Soil organic matter patterns – Available mineral element patterns • Conclusions on the results and prospects for the method used

Introduction How long does it take to observe soil fertility changes? Cultivated soil fertility Critical fertility level Years 2004 2010 2017 Cultivated soil fertility observations 2024 Years 2004 2010 2017 2024

Introduction Cultivated soil fertility observations Years 2004 2010 2017 2024 Time-course study Years 19? ? 2004 2010 2017 2024

Methods • 1°) Archived soil analysis data from North Cameroon • 2°) Locating previously analyzed plots • 3°) History of plot soils since first analysis • 4°) Sampling and analyses using the same methods

20 -years cropping sequence history Results Plots fallowed Plots not fallowed Year plots first cultivated Between 1954 and 1964 Between 1965 and 1974 Between 1975 and 1984 Between 1985 and 1989 Djalingo Ngong ( 0 10 kilomèters 20 From GESEP project map

Results: clay content Argiles (%) horizon 0 -20 cm 40 30 20 10 0 Djalingo Study year Ngong 1990 1996 2004

Results: soil C Carbon (mg. g-1) 0 -20 cm horizon Carbon (mg. g-1) 20 -40 cm horizon 6 6 5 5 4 4 3 3 2 2 1 1 Djalingo Ngong Study year Djalingo 1973 1990 1996 Ngong 2004

C/N patterns C/N 20 -40 cm horizon C/N 0 -20 cm horizon 25 25 20 20 15 15 10 10 5 5 Djalingo Study year 1973 1996 2004

Organic residue M CO 2 K 1 M Soil K 2 Soil organic matter Y - . Soil C level (mg/g) horizon 0 -20 cm 6 Values obtained by analysis Values simulated by the Hénin-Dupuis model 4 2 0 0 20 40 60 80 100 Clearing age K 2 = 0. 06 Cequi = 2. 6 mg. g-1

Results : CEC Cations exchange capacity (cmole c. kg-1) 20 -40 cm horizon Cations exchange capacity (cmolec. kg-1) 0 -20 cm horizon 4 4 3 3 2 2 1 1 0 0 Djalingo Ngong Study year 1973 1990 1996 Mean decrease of 1% per year Ngong 2004

Results: saturation by base exchange Exchange complex saturation level (%) 0 -20 cm horizon Exchange complex saturation level (%) 20 -40 cm horizon 130 110 90 90 70 70 50 50 30 30 Djalingo Ngong

Results: available P Available phosphorus (mg/kg) 0 -20 cm horizon Available phosphorus (mg/kg) 20 -40 cm horizon 40 40 30 30 20 20 10 10 0 0 Djalingo Ngong Djalingo Study year 1990 1996 Ngong 2004 Decrease of 2. 73 mg/kg in 14 years, or around 20% in the 0 -20 cm horizon and 5% in the 20 -40 cm horizon

Results: exchangeable K Exchangeable potassium (cmolec. kg-1) 20 -40 cm horizon Exchangeable potassium (cmolec. kg-1) 0 -20 cm horizon 0. 6 0. 4 0. 2 0 0 Djalingo Ngong Study year Djalingo 1973 1990 1996 Ngong 2004 Marked overall decrease: 56% in 14 years in the 0 -20 cm horizon and 36%

Variations in other characteristics and indices • Stable p. H; • Significant decreases in exchangeable Ca and Mg levels; • Other indices (ratios between K, Mg and Ca; Feller's and Piéri's fertility indices) varied negatively.

Mineral budgets and variations in soil mineral contents Mineral balances in each cultivated plot were determined according to the plot histories: These balances revealed: -1°) a equal phosphorus balance; -2°) a negative nitrogen balance; -3°) a highly negative potassium, calcium and magnesium balances. These calculated balances were in line with the soil contents changes of these different minerals.

Conclusion • Interesting method to obtain quick and cost-effective results - But difficulties due to poor data archiving - Indicates that databases should be constructed that facilitate data use • Relatively good compliance between the soil characteristic forecasting model simulations (soil C patterns, mineral budgets) and the field results - N patterns to be confirmed • Problems in studied mineral reserves due to imbalanced mineral budgets - problems confirmed in long-standing cotton-growing areas (mineral deficiency leaf symptoms observed on crops) - a situation that could worsen because of the poor economic situation