Nutrient availability and pearl millet production in Sahelian farming systems based on manuring or fallowing

Abstract

Pearl millet production in the sandy soils of the Sahel is maintained by fallows and manures are the principal added source of fertility. Due to rapid population growth fallow periods are reduced, yields and nutrient availability decline and manure seems to be the only reasonable alternative for fallowing. This on-farm study (Banizoumbou, semi-arid Niger) investigates crop response to low and very low nutrient availability as well as within-season shortages and within field variations. Two sources of data were combined: a 3-year field experiment with dung application (5 Mg ha −1 ) and weeding frequency (no, two and four weedings); and a comparison of farming systems, n = 32, field × years (i) permanent cultivation with manure; (ii) cultivation after long fallows >15 years; (iii) cultivation after short fallows 3–5 years. The permanent cultivation of soils in the study area is achieved with extremely low levels of manure input 0.5–1 Mg ha −1 per year, adding about 10 kg N, 1.2 kg P and 5 kg K to the soil. Dung inputs allow grain yield (350 kg ha −1 ) and nutrient supply (ECEC 0.81 mmolc kg −1 , total N 151 mg kg −1 , 0–20 cm depth) to remain stable under prolonged cropping (10–17 years), while field without manure need fallowing for long periods (>15 years) to restore fertility. The trend from long to short fallow periods accords with decline in grain yield from 300 kg ha −1 yields down to 125 kg ha −1 and of a ECEC reduction from 1.10 to 0.72 mmolc kg −1 . The positive effect of dung application on crop growth is restricted to the first half of the season resulting in extra panicles and 50% extra crop residues. Other yield components remained unaffected and grain yield is but slightly improved because the initial good conditions do not continue. Very small variations in fertility—ECEC, NPK—induced by dung application, excessive weeding or due to soil surface conditions, had yet an impact on pearl millet growth and grain yield. The field experiment showed that nutrient levels and grain yield decline with excessive weeding but less so if dung is applied. Manuring is an alternative for fallowing rather than replacing it because additional constrains crop up with prolonged cultivation: the population of coprophage insects was reduced so some dung stayed unconsumed, and extremely sandy soils developed because wind erosion had carried off the most fertile particles from these rather bare soils making manure input no longer effective. © 2004 Elsevier B.V. All rights reserved.