Soil organic matter, surface crusting and erosion in Sahelian farming systems based on manuring or fallowing

Abstract

The cultivated soils of the Sahel are sandy, very poor in organic matter, and are prone to surface crusting and erosion. Farmers sustain pearl millet production through permanent cultivation with manure input or long-term fallowing. Due to population increase fallows are reduced bringing on low yields. This on-farm study (Banizoumbou, semi-arid Niger) examines the impact of clearing and weeding on soil surface processes, the decline in soil organic matter under prolonged cultivation, and whether manure inputs are sufficient to sustain millet production. Short-term effects were examined in a 3-year experiment with dung application (5 Mg ha−1) and weeding frequency (no, two and four weedings). Long-term effects were studied a 4-year experiment comparing three farming systems: (i) permanent cultivation with manure; (ii) cultivation after long fallows >15 years; (iii) cultivation after short fallows 3–5 years. Due to the difference in scale, different processes of erosion were studied, saltation and creep in the first experiment, suspension in the second. Dung deposits provided mechanical protection of the soil surface by reducing crusting and by trapping aeolian sand. A very low cover of dung, 0.5–5%, is enough to protect the soil effectively by forming barriers against creep and saltation. Suspension occurs during tillage. Manual cultivation, while disrupting aggregates and surface crusts, enhances wind erosion and this leads to loss of clay and silt from the top soil. Changing from long to short fallow periods and even to permanent cultivation, constitutes a trend towards more intensive land use and this, eventually, leads to extremely sandy top soils (from 890 up to 934 g kg−1) and coincidences with a soil surface becoming less crusted (from 49% of field surface affected down to 7%). Manure input cannot prevent the erosion of fine earth which seems to be an irreversible feature of long-term cultivation. Dung inputs, 0.5–1 Mg ha−1 per year, are insufficient to increase substantially millet grain yield (350 kg ha−1 in manured; 300 kg ha−1 in long fallow fields), or elevated soil organic matter (2.9 g kg−1 soil, in manured fields; 4.0 g kg−1 in long fallow fields, 0–20 cm). Yet grain yield remains stable under permanent cropping, while fields without manure need fallowing for long periods to restore fertility. Under short fallows without dung input soil organic matter drops to 2.4 g kg−1, and grain yields to 125 kg ha−1.