Potato-Legume Intercrop Effects on Water and Nutrients Use Efficiency, Crop Productivity and Soil Fertility in a Humic Nitisol, Kenya

Abstract

Integration of legumes into potato-based cropping systems is a common practice in developing countries presumably as a water and nutrient management strategy. This study was conducted to assess the effect of incorporating legumes into the potato production system on soil water conservation, economic returns, nutrients (N and P) use efficiency, and soil physical and chemical properties. Field-based trials were conducted for four consecutive seasons between 2014 short rains and 2016 long rains at the Field Station, University of Nairobi. The treatments comprised of; a pure stand of potato (PS), potato-dolichos (Lablab purpureus) (PD), potato-bean (Phaseolus vulgaris) (PB) and potato-garden pea (Pisum sativum) (PG) intercrops, which were laid out in a randomized complete block design with each treatment replicated four times. The generated data on ground cover, soil moisture content, yield, economic returns, nutrient uptake and use efficiency, and soil physical and chemical properties were analysed using generalized linear models (GLM) in R software version 2.2.3, while means were separated using Tukey’s Honest Significant Difference (HSD) at (p ≤ 0.05). Pearson correlation analyses were applied to determine the relationship between the measured variables.The highest ground cover of 69% was recorded at tuber initiation stage (56 days after planting) in PD compared to 66% in PG and PB and 56% in PS. Similarly, the highest soil moisture content (SMC) values were recorded at tuber initiation stage: 230, 207, 201 and 188 mm m−1 for PD, PG, PB and PS, respectively with significant (p ≤ 0.05) differences between treatments. Nutrient uptake by potato was significantly higher in PS and PD than in PB and PG, which was a reflection of tuber yield that followed the trend of PS (36 t ha−1) = PD (35) < PB (30) = PG (29). When tuber and legume grain yield were converted into potato equivalent yield (PEY), the intercropping systems outperformed the potato pure stand. For instance, PD had significantly (p ≤ 0.05) the highest economic returns of US$ 9,174 ha−1, nitrogen use efficiency of 43 kg PEY kg−1 N supply and phosphorous use efficiency of 169 kg PEY kg−1 P supply. After two years of potato-legume cultivation and incorporation of crop residues into the soil, soil physical and chemical properties at 0.3 m depth such as sand, silt and clay were only influenced by slope position with sand decreasing down the slope whereas, the opposite was observed for silt and clay. Bulk density (BD) was significantly (p ≤ 0.05) above the initial value (1.03 g cm−3) recorded before the start of the experiment and increased down the slope in all treatments with averages of 1.21, 1.14, 1.13 and 1.07 g cm–3 in PS, PB, PG and PD, respectively. PD recorded significantly (p ≤ 0.05) higher pH (5.9), total nitrogen (3.7 g kg–1 N), organic carbon (37 g kg–1), available P (26 mg kg–1) and cation exchange capacity (34 cmolc kg−1) compared to all other intercropping systems. Sand and BD were inversely (r = 0.33–0.84; p ≤ 0.05) correlated with the soil chemical properties such as pH, P, N OC and CEC which in turn were directly (r = 0.38–0.49; p ≤ 0.001) correlated with clay. The data in this study indicate that intercropping potato with L. purpureus is a sustainable production practice that would ensure high tuber yield and improved soil fertility.