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Nutrient use efficiency in banana-bean intercropping systems in the Upper Pangani catchment, Tanzania
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Banana is an important food staple and cash crop to millions of people in the humid highlands of Tanzania. Yield ranges between 5.7 and 14 tons per hectare, only attaining 27 % of its potential yield, mainly caused by low soil fertility. The accessible amounts of cattle manure conventionally used to replenish nutrient and manage soil fertility in banana home gardens are insufficient to meet nutritional crop requirements. Other animal manure e.g., poultry, goat and swine could be used to close the deficit, but the quantities produced are negligible. Grasses and crop residues are no longer used to recycle nutrient in banana home gardens due to their competitive use as an important fodder for smallholder zero-grazed dairy cattle system. Importation of additional cattle manure from free-grazed short horned Zebu cattle, which roam the lowlands is too expensive for resourcepoor smallholder farmers who are majority crop producers in the country. On the other hand, mineral fertilizers are not used because of lack of knowledge on their appropriate use, and the perceived high price by farmers. Central to this research is the question of how best to optimize the traditional soil fertility management strategies to enhance banana production in smallholder conditions. As an attempt to address it, we delineated this study in three different areas each involving different role for improved banana production in the north Upper Pangani catchment areas, covering the central-northern highlands of Tanzania. First, an agronomic survey was conducted in 42 farmer managed banana home gardens randomly selected on the eastern slopes of Mount Kilimanjaro and Meru to evaluate soil fertility and investigate its corresponding gradients in foliar nutrition of Mchare banana (Musa spp., AA-East African highland cooking banana “EAHB”). The survey was conducted in two transects (i) Machame transect on the slopes of Mount Kilimanjaro and (ii) Meru transect on the slopes of Mount Meru. During the survey, each banana home garden was delineated in two segments (locations) relative to the distance from the house. The first segment was the area between 15 and 25 m from the house and the second between 35 and 45 m farther. In each segment, sub-samples of 10 cm by 20 cm from both sides of the midrib in the midpoint of the lamina from the third fully open leaf were collected from six Mchare banana plants approximately 6 months old for nutrition analysis. Underneath the sampled banana plant, a topsoil (30 cm surface layer) sample was collected at five points in a 2 m radius circle around a pre-selected mat for fertility evaluation. The results indicated that soil fertility and foliar nutrient gradients between and within farms were not significant. Conversely, soil pH, available phosphorus (available P) and exchangeable (potassium (K), magnesium (Mg), calcium (Ca)) was found to increase significantly from the high towards low altitude areas. The concentrations of P, K and Mg in Mchare banana leaves decreased with elevation, whereas those of nitrogen (N), Ca, iron (Fe) and manganese (Mn) increased. In addition, low indigenous supply by soils restricted the uptake of K and Mg in the Upper zone, while Ca-Mg imbalances limited the uptake of Mg by plants in the Lower zone. Conversely, the uptake of boron (B), cupper (Cu) and zinc (Zn) was limited by low availability in soils, and strong alkaline soil conditions in the lowlands further restricted the uptake of these nutrient elements. As the main source of soil derived N, smaller quantities of decomposing organic material in the surveyed banana home gardens were responsible for the observed smaller N concentrations in Mchare banana plants grown in the low altitude zone. Apparently, soil fertility problems in banana home gardens cannot be rectified by a general fertilizer recommendation, rather they require appropriate soil fertility management strategies adapted to specific areas. Second, field experiments were conducted at Tarakea, Lyamungo and Tengeru sites with varying soil types along the altitudinal gradients on the slopes of Mount Kilimanjaro and Meru to assess the benefits of N fertilization on Mchare banana production at 0, 77, 153, and 230 kg N ha−1 year−1. The experiment consisted of eight fertilization treatments, which included (N rates expressed in kg ha−1 year−1): T1 = 0 N (control); T2 = 77 kg N (derived from urea, 50 % below the N dose applied by resource-endowed farmers); T3 = 153 kg N (derived from urea, corresponding the traditional N rate derived from cattle manure); T4 = 230 kg N (derived from urea, 50 % above the traditional N rate); T5 = 50 % urea (containing 77 kg mineral N) + 50 % cattle manure (containing 77 kg organic N); T6 = 100 % cattle manure (containing 153 kg organic N, which mimicked resourceendowed farmers’ practice); T7 = 50 % urea (containing 77 kg mineral N) + bean haulms (containing 52 kg organic N); and T8 = 100 % bean haulms (containing 52 kg organic N). The first four treatments intended to estimate the optimum N fertilization dose for Mchare banana as an attempt to improve the traditional soil fertility management practices in banana home gardens. In general, yield increased significantly and positively with increase in N dose and was higher in the 2nd cycle than in the 1st cycle. The results further revealed that combined use of urea and cattle manure at 153 kg N ha-1 year-1 each contributing 50 %, produced the highest yield of up to 54 t ha-1 year-1 in the cycle two, almost similar to that attained with cattle manure only. On the contrary, profitability analysis indicated that the use of cattle manure alone was most costly. Subsequently, household income attained in the integrated soil fertility management (ISFM) strategy, which combined cattle manure and urea was ten-fold larger than the current income generated by the crops produced in the home gardens and foothill fields all together. Remarkably, the ISFM approach reduced production costs by 45 %, while increasing Mchare banana yield, and hence households’ income. Third, an agronomic survey was carried out in smallholder maize fields in the lowlands of the Hai and Siha districts to provide a preliminary evaluation on quantities of stover transported to the uplands as additional fodder for zero-grazed dairy cattle, and the resulting nutrient flows in banana home gardens. During the survey, data on fertilizer use were collected, size of the surveyed maize fields, plant spacing at harvest, and stover yield were measured. Plant spacing and stover yield were measured in a 2 m by 4 m sampling unit (SU), and was repeated four times in each surveyed field. Stover samples from the SUs were bulked per field, and analyzed for nutrient concentrations, and the resulting mineral nutrient removal from the field was quantified. The findings demonstrated that maize production in the foothill was constrained by (i) land scarcity same as banana production in the uplands, and (ii) low nutrient inputs, which relied on mineral N fertilization only, and mostly below the recommended dose of 50 kg N ha-1. The average plant density was 21,944 plants ha-1, only 50 % of the optimal recommended density of 44,444 plants ha- 1, and seemed to restrict farmers from attaining potential crop yield of up to 5 t ha-1 (Kihara et al., 2014). As such, the study highlighted sub-optimal plant population density as an important agronomic factor responsible for the low maize grain yield under smallholder farming settings in the central-northern highlands Tanzania. The findings further revealed that stover transfers from maize fields in the lowlands to the uplands as animal fodder resulted in larger nutrient flows in banana home gardens following manure applications. However, nutrient exports e.g., K in both agroecosystems exceeded the inputs, which indicated nutrient mining. We can conclude from this study that: i. Mchare banana production in Arusha and Kilimanjaro regions was constrained by low soil fertility, ii. quantities of manure produced in the homesteads by zero-grazed dairy cattle were insufficient to replenish nutrient elements and maintain high soil fertility in banana home gardens when used alone, iii. agronomic performance of mineral N fertilization only was inferior to that of cattle manure alone, which represented the traditional farmers’ practice, iv. the ISFM technique, which combined cattle manure and urea each contributing 50 % of annual N budget improved the internal and utilization efficiency of the applied N while it did not supply adequate amounts of Cu and Zn to the crop same as cattle manure only, v. the conventional mode of production, which relied on cattle manure only was less profitable compared with the ISFM technique, vi. soil fertility management practices in smallholder maize fields, which relied on urea applications only caused mining of other nutrient elements and is expected to degrade the land. Overall, this study validates the urgent need to optimize the current soil fertility management practices in both agroecosystems to enhance crop production in a more sustainable way.