Effects of organic mulch on yield and selected pests of organically grown tomato (Solanum lycopersicum L.) and sweet pepper (Capsicum annuum L.)

The objectives of this study were (i) to determine the effects of Mexican weeping pine (Pinus patula Schelde. ex Schltdl and Cham) and wild lemon grass (Cymbopogon spp.) mulches and time of their application on yield and quality of organically grown tomato (Solanum lycopersicum L.) and sweet pepper (Capsicum annuum L.), (ii) to investigate the effect of pine and wild lemon grass mulch on pests and beneficial arthropods under organic production of tomato and sweet pepper, and (iii) to examine the decomposition rate and chemical products of pine and wild lemon grass. Tomato cv. ‘Tanya’ and sweet pepper cv. ‘California Wonder’ were grown organically at Lushoto and Ubiri wards in Lushoto district, Tanga region, North-Eastern Tanzania. The experiment was laid out in a Randomised Complete Block Design (RCBD) with three replications and repeated for three seasons. The treatments were pine mulch applied 3 days after transplanting (PI), and 21 days after transplanting (PA), grass mulch applied 3 days after transplanting (GI), and 21 days after transplanting (GA) and two unmulched controls weeded and unweeded. Dry pine and wild lemon grass materials were used in compost making, each mixed with forest soil, cow manure, green grass and water. Laboratory analysis was done later on at Sokoine University of Agriculture (SUA) Soil Science laboratories to determine the chemical composition of the composts. Data collected included the number of leaves per plant, plant height, fruit yield, number of fruits per plant, fruit size, marketable and unmarketable fruit weight. Beneficial arthropods were also collected using pitfall traps for the three seasons. In each treatment and location, data on numbers and dry weight of weeds, disease incidences and insect pests were also collected. The time taken for compost to mature and their chemical composition in the period of three and twelve months of composting were recorded. The analysed parameters included organic carbon (OC), total Nitrogen (TN), phosphorus (P), potassium (K), copper (Cu), zinc (Zn), iron (Fe), and manganese (Mn). Data were subjected to Analysis of Variance (ANOVA) at P≤ 0.05 and where applicable Post Hoc. Tukey’s (HSD) test was used to compare means. Data analysis was done using R Statistical Package (AGRICOLAE Version 1.2.1). Results showed that both types of mulch had positive influence on yield and quality components regardless of the time of mulch application. Among the three seasons, the highest yields were obtained during the wet and cold season (April – August, 2013). Mulching with either pine or wild lemon grass did not affect the number of trapped arthropods while across the seasons, numbers of arthropods increased significantly (P ≤0.05). In weed control, results revealed that pine and wild lemon grass mulch had similar (P≤0.05) effect as weeding. Laboratory analysis of compost containing pine and wild lemon grass containing compost showed significant changes in OC, TN, Na, Fe and Mn between three and twelve months of composting. In conclusion, application of both pine and wild lemon grass mulch brings higher yields of tomato and sweet pepper than hand weeded practice. Both types of mulch lasted throughout the production period without need replenishment. Types of mulch and time of application did not have a significant influence on yield, weed control, insect and disease attacks instead the presence of mulch reduced numbers of weeds, increased numbers of beneficial arthropods, yield and quality of tomato and sweet pepper. Pine and wild lemon grass containing compost released nutrients as early as three months after heaping. It is therefore recommended, that pine and wild lemon grass can be used as organic mulch three to 21 days after transplanting tomato and sweet pepper seedlings but using pine and wild lemon grass containing compost requires further investigation. It is further recommended for future scholars to also focus on studying beneficial arthropods and their relationship with insect pests where pine and wild lemon grass are used as mulch