This experiment was conducted to evaluate the impact of Albizia lebbeck Benth (its green manure and N2 fixing ability of its tree rows) on soil nutrient status (physical and chemical properties) under alley cropping (an agro-forestry system) with Irish potato (Solanum tuberosum L). The experimental design was a randomized complete block design (RCBD) comprising five treatments and three replicates. Soil sample analyses (pre and post experiment) were done. Tending operations and data collection on apparent growth and yield parameters throughout the five cropping seasons were carried out. Results on the soil physical properties indicated that the nine samples from the three soil depths of T4 (with the highest rate of green manure application at 10 ton ha-1 and A. lebbeck tree rows) were predominantly sandy loam probably because of the rate of green manure application which had increased the ratio of smooth to coarse fragment in favor of smooth. Three of the samples of T1, T2¬ and T3 were sandy clay loam as opposed to T4 with only two. Though T0 also had only two of its nine samples being sandy clay loam which could be attributed to the initial textural class of T0 (T0: control experiment). Thus, a non-significant difference between the pre and post planting soil texture was observed (sandy clay loam, clay loam and sandy loam constituted the dominant textural classes). Regarding the soil chemical properties, the organic matter and nitrogen were very low and the values decreased from pre planting to after planting (no significant difference between treatments/blocks) due to crop removal, leaching and high rates of decomposition/mineralization but the treatments applied (A. lebbeck green manure and its tree rows) significantly influenced the available P at 0 to 10 cm depth and K at three soil depths. Block effects were observed at significant level on the pH at 10 to 25 cm and 25 to 40 cm depths and on Mg at 0 to 10 cm depth. The available P decreased generally from the surface soil (down wards) to the subsoil in the study site owing to absorption by the plants and crop removal. The values of exchangeable acidity and effective cation exchange capacity increased generally from surface soil to the sub-soil probably due to leaching or infiltration of the exchangeable cations. The morphological growth parameters (plant height, leaf count and collar girth) except stem count were highly influenced by the different levels of A. lebbeck green manure application and its tree rows (sequel to N2-fixation) at P≤0.1 which subsequently brought about significant differences in yield, tuber count (P≤0.5) and tuber weight (P≤0.01) of the crop (Irish potato) in all the 5 treatments and throughout the 5 cropping seasons due to improvement on the nutrient status in the agro-forestry farm.
Key words: Albizia lebbeck, Alley cropping, soil physical and chemical properties, nutrient status, crop yield.
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