EFFECTS OF FALLOW DURATION ON SOIL PHOSPHORUS FRACTIONS AND CROP P UPTAKE UNDER NATURAL REGROWTH AND PLANTED PUERARIA PHASEOLOIDES FALLOW SYSTEMS IN SOUTHWESTERN NIGERIA

Abstract

Soil phosphorus fractions, maize and cassava P uptakes in natural regrowth fallow and Pueraria phaseoloides fallow systems, as affected by the duration (1–3 years) of fallow, were assessed in 1998 and 1999 in a long-term fallow management trial, established in 1989 at the International Institute of Tropical Agriculture, Ibadan, Nigeria. After one and three years fallow respectively, the mean P accumulation in fallow vegetation was 4.2 and 9.4 kg ha⁻¹ in 1998, and 8.9 and 12.1 kg ha⁻¹ in 1999 for natural fallow. For Pueraria, the corresponding values were 6.3 and 7.4 kg ha⁻¹ in 1998 and 6.1 and 13.5 kg ha⁻¹ in 1999. The positive effects of fallow lengths on dry matter and P accumulation were not as pronounced under Pueraria. Continuous cropping reduced soil pH and depleted Olsen P more than crop-fallow rotations under both fallow systems. An increase in fallow duration enhanced the availability of resin P, NaHCO₃-, and NaOH- extractable P fractions, especially under natural fallow. Continuous cropping in a natural fallow system resulted in lower resin inorganic P (P_i) and NaHCO₃-extractable inorganic and organic P (P_o) fractions, and higher refractory P fractions (concentrated HCl-extractable P_i and P_o and residual P) as compared to crop-fallow rotations. Although continuous cropping in the Pueraria system resulted in higher refractory P fractions than the crop-fallow rotations, the effects of fallow length on P fractions were not consistent or pronounced. Maize and cassava P uptakes increased with fallow duration. Phosphorous in maize grain was positively correlated with fallow vegetation P accumulation. Total P uptake by maize was positively but weakly correlated with NaHCO₃-P_o, Olsen P and fallow vegetation P accumulation.