Incorporation of organic residues into soils can improve P availability to crops in P-fixing soils. Perennial shrubs are commonly found in farmers' fields in Senegal, but little is known about their potential to reduce P sorption. Consequently, the residues of the two dominant shrub species, Guiera senegalensis and Piliostigma reticulatum were amended to soils where each is found, the Dior (Rubic Arenosol) and the Deck/Dior (Mixed Haplic Ferric Lixisol), respectively, and compared with beef manure for P sorption characteristics. Amended soils (2% weight/weight) were incubated at 24 °C for 30 or 75 days. Phosphorus isotherms were constructed using the batch technique, and sorption parameters were derived by fitting the Langmuir and Freundlich equations. All organic amendments decreased P adsorption maxima and affinity constants and increased P in the equilibrium solution (equilibrium P concentration) compared with the control. The amount of P desorbed from soils was greater in amended soils than in unamended soils. Soils beneath shrub canopy had lower P sorption capacity than soils outside shrub canopy in P. reticulatum-associated soils, which was reversed in G. senegalensis-associated soils. All organic amendments significantly increased soil pH. The longer incubation period of 75 days, over the 30 days, enhanced the effect of organic amendments in reducing soil P sorption and decreased soil pH. Manure amendments induced the greatest reduction in soil sorption capacity and showed the highest desorption rate. These results indicate that the addition of locally available plant residues to soil at a high rate can reduce P sorption and should be included in nutrient management recommendations of deficient P-fixing Sahelian soils.
1IFDC Ghana-PMB CT 284, Cantonments 3, Orphan Crescent, Labone, Accra, Ghana, West Africa.
2Department of Crop and Soil Sciences, Oregon State University, Corvallis, OR.
3Institut Sénégalais de Recherches Agricoles, Laboratoire National de Recherche sur les Productions Végétales, Dakar Sénégal, West Africa.
4Sciences du Sol, Institut Sénégalais de Recherches Agricoles/CERAAS, Thiès Escale, Thiès, Sénégal, West Africa.
5University of California, Department of Land, Air and Water Resources, Davis, CA.
6Ohio State University, School of Environment and Natural Resources, Columbus, OH. Dr. Dick is corresponding author. E-mail: Richard.Dick@snr.osu.edu
Received Dec. 18, 2007; accepted July 31, 2008.