Acidification of a kaolinitic alfisol under cropping and nitrogen fertilization in West Africa

This study was conducted near Ibadan, Nigeria to determine the extent of soil acidification due to N fertilization and leaching of Ca, Mg and K from the surface layers of an Oxic Paleustalf (Egbeda series). The N sources included two inorganic N fertilizers, urea (UA) and ammonium sulfate (AS), and a non-leguminous green manure, Alchornea cordifolia (Alc).; Applying UA instead of AS as a N source reduced the surface soil acidification rate by 32.5%. The Alc organic N source reduced the acidification rate by 18.5 and 45.1% compared with UA and AS treatments, respectively. Combined use of organic and inorganic N inputs retarded soil acidification, and should enhance sustainability. Decreased acidification by combining organic N with inorganic N sources was 8.1 and 17.3% for Alc+UA and Alc+AS, respectively, compared to UA and AS treatments. The decline in surface soil pH after one year of cropping (2 seasons) followed the order: AS > (AS + Alc) > UA ≃ (UA + Alc) > Alc > Control. The pH (H₂O) of the surface (0–2.5 cm) layer in the AS treatment declined from 6.2 to 4.5 after one year (two seasons) of cropping.; Soil acidification led to significant leaching losses of Mg and K from the surface soil, but leaching of Ca was less pronounced. Application of Alchornea mulch retarded the downward movement of Ca, Mg and NO ₃, but did not affect leaching of K.; Both organic and inorganic N inputs gave significantly higher grain yields than the control for both seasons indicating that the soil was deficient in N for maize growth. Because of limited rainfall during the second season, maize yields in the second season were lower than that of the first season. The total yield increase due to N addition was 86, 88, 65, 71, and 110% for UA, AS, Alc, Alc+UA, and Alc+AS, respectively. Soil acidification in the AS treatment did not affect maize yield even though soil pH (H₂O) was lowered to 4.5. Among the four N treatments, Alc+AS resulted in the highest grain, indicating a potential beneficial effect of combining organic and inorganic N inputs.