Effects of soil amendments on growth and biomass yield of early generation seeds of sweet potato (Ipomoea batatas (L.) Lam) grown in net tunnels

Early generation sweet potato vines are multiplied on nursery beds with high densities using soil medium within insect-proof net tunnels to inhibit the entrance of virus-transmitting insects (aphids and white flies). However, the rapid multiplication beds require suitable soil amendments to support vigorous growth of vines. To this end, farmyard manure, wood ash, sawdust, compost, coffee husk and control (soil only) were evaluated using Randomized Complete Block Design with three replications during the belg and meher seasons of 2016 to 2017 in the Chefe testing site of Hawassa Agriculture Research Center. Results also showed that the soil pH was higher by 8.9% and 1.4% due to the application of wood ash and sawdust whereas there was 9.3, 5.9 and 0.9% decrease in soil pH due to the use of compost, coffee husk and farmyard manure, respectively. All soil amendments enhanced soil moisture conservation compared to the control. A 47.0%, 31.2%, 30.3% and 26.5% increase in cation exchange capacity was observed by the end of second year due to use of coffee husk, wood ash, sawdust and farmyard manure, respectively. The potassium content was increased by 47.0%, 45.5% and 35.9% due to the use of wood ash, farmyard manure and coffee husk, respectively. The pencil root length, pencil root width and below-ground biomass were not affected significantly (P < 0.05) by differences in soil amendments (P < 0.05) in this experiment. The percentage increment in above-ground biomass due to the farmyard manure, coffee husk and compost was 62.01%, 53.56% and 49.87%, respectively higher than the control. Sawdust suppressed vine growth compared to the control. Coffee husk and farmyard manure medium produced higher number of branches (23 branches / plant) during meher whereas coffee husk produced the same during belg (14.4 branches / plant). Significantly (P < 0.05) higher number of vines was obtained due to farmyard manure (4686 vines/m(2)) and coffee husk (4602 vines/m(2)) compared to the control (2683 vines/m(2)). Significantly longer internodes were recorded due to the farmyard (100.6% greater) compared to the control. Thus, 50% farmyard manure, 50% coffee husk or 50% compost are recommended for better growth of pre-basic sweet potato vines in net tunnels. This study proclaimed possibility of nursery bed vine multiplication through use of local organic residues at the level of small holders and research sites during short and long rain seasons.