Abstract. The restorative ability of herbaceous (Psophocarpus palustris, Pueraria phaseoloides) and woody (Leucaena leucocephala, Senna siamea, Acacia leptocarpa, Acacia auriculiformis) legume species and of natural regrowth was studied on an eroded and compacted Oxic Paleustalf in southwestern Nigeria. Compared to the control treatment that was continuously cropped for 15 years, four years of fallowing significantly improved test crop yields. However, fallowing with the above species did not substantially improve soil properties, particularly soil bulk density. A longer fallow period may be needed to amend soil physical conditions of this degraded Alfisol. Soil chemical properties were greatly improved following land clearing and plant biomass burning in 1993. However, the residual effect of burning on soil fertility was insignificant in the second cropping year. Among the fallow species, P. palustris and natural fallow showed the best residual effect on test crop performance. Despite the high biomass and nutrient yields of S. siamea and A. auriculiformis, test crop yields on these plots were low due to the border effects from the uncleared and fallowed subplots.
Downward movement of applied Ca3+, Mg2+, K+, and NO3‐ in a coarse‐textured, kaolinitic Ultisol profile was studied in the laboratory using undisturbed soil columns. The soil columns were leached with an amount of water equivalent to the annual rainfall of the sampling site (2,420 mm) through a rainfall simulator over a period of 72 days. Added nitrate in the forms of Ca, Mg, or K salts leached readily through the profile. Nitrate leached mainly with Al3+ ions. Cations leached differentially, depending upon the exchange selectivity of the soil and upon the surface loading of the cation in question. The leachability of the cations under the experimental conditions follows the order K < Al ≈ Ca ≈ Mg < Na.The vertical distribution of added Ca in the leached profile may be predicted by Walter's formulation of ion exchange chromatography.The limed soil columns retained more Ca in the surface layer than the columns treated with Ca(NO3)2 when an equal amount of Ca was applied.
Cation exchange equilibria in an acid Ultisol profile involving K‐Al, Ca‐Al, K‐Ca, Na‐K, and Na‐Ca exchanges were studied. Selectivity coefficients, surface activity coefficients and free energy changes were computed using the thermodynamic formulations.In exchange equilibria involving Al, the magnitude of selectivity coefficients Kc, was affected by pH, surface composition, and hydrolysis of Al. In equilibria not involving Al, good reversibility for K‐Ca, Na‐K and Na‐Ca exchanges was obtained. Potassium was selectively adsorbed in all horizons of the profile. The over‐all selectively sequence follows the order of K>Al>Ca≈Na.
The pH dependent cation exchange capacity (CEC) and pH buffering capacity (pH BC) are two crucial properties in soil fertility management of variable charge soils. However, they are rarely measured, since most of the existing methods are cumbersome and time‐consuming. We propose a new method, based on the silver‐thiourea (AgTU) method, for the simultaneous measurement of pH dependent CEC and the pH BC. In general, this method consists of first increasing the pH of the soil to about pH 7 and then gradually acidifying the soil by titrating with HNO3, while measuring pH and CEC after each equilibration. This way, both CEC at different pH values and the pH BC are measured on the same sample, without the multiple washing steps needed in other procedures. Various aspects of the proposed method were tested and the results obtained were compared with the ion adsorption method. The modified AgTU method obtains field relevant results and is suitable for the routine analysis of large numbers of samples.
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