ISFM for banana systems

Piet van Asten, p.vanasten@cgiar.org, Severine Delstanche, Lydia Wairegi, Tony Muliele, Syldie Bizimana, Godfrey Taulia, Ken Giller, Peter Leffelaar, Laurence Jassogne, Philippe Baret, and Charles Bielders

Banana systems in Rwanda. Photo by Piet van Asten
Banana systems in Rwanda. Photo by Piet van Asten

Banana is the primary food crop in the Great Lakes Region, providing food and income for over 85% of the population. Unfortunately, current banana yields of 5–30 t/ha/year are low compared to potential yields of over 70 t/ha/year. Although between 25% and 70% of this yield gap can be explained by low soil fertility (Fig. 1), the use of external inputs such as fertilizers is virtually nonexistent and soil fertility is mostly managed by recycling local organic residues.

A study done by Severine Delstanche at the Catholic University of Louvain-la-Neuve (UCL) showed that very little nutrients were released from the soil through weathering of the soil minerals (Fig. 2). Hence, soil fertility depended almost entirely on the soil’s organic matter content. In banana-based farming systems, nutrient recycling is very important, as the harvest index is relatively small (<30%). This helps maintain relatively high organic matter content in the soil.

Figure 1. Primary crop constraints identified by use of boundary line approach.
Figure 1. Primary crop constraints identified by use of boundary line approach.

Furthermore, the large and perennial canopy and root system of banana help protect the soil from erosion. Banana therefore plays an important role in protecting the environment in this hilly landscape.

In addition to being an important component in sustaining soil fertility, banana plays an important socioeconomic buffer role for the smallholders. The crop provides food security as bunches are harvested throughout the year and any surplus can be sold to generate a continuous cash flow.

Banana systems particularly occur in areas with high population pressure and small (<2 ha) farm sizes. A study by Lydia Wairegi at Makerere University (Uganda), showed that fertilizer use was very profitable in the peri-urban area close to Kampala with marginal rates of return sometimes exceeding 500%. However, in areas far from the market (>150 km), the intensification process seemed less promising, but banana continues to play an important buffer role to maintain food security and protect the environment. In these remote locations, it seems wiser to invest in improved use of local (nutrient) resources, than to purchase mineral fertilizers.

Figure 2. The proportion of annual K fluxes in banana systems originating from soil mineral weathering (2%), crop<br /> residues (77%), and the crop harvest (21%).
Figure 2. The proportion of annual K fluxes in banana systems originating from soil mineral weathering.

To maintain its buffer role, banana can be integrated with other crops such as coffee and beans. The Ph.D. studies of Tony Muliele and Syldie Bizimana (both UCL) showed that intercropping beans with banana could be improved. Traditionally, farmers in Rwanda, Burundi, and DR Congo would till the banana field at the beginning of the wet season to suppress weeds and prepare the land for bean intercropping. Unfortunately, this practice damages a large proportion of the superficial root system of banana plants. Based on practices observed in Southwest Uganda, a technology of zero-tillage and mulching was tested. Beans are planted in the mulch. The planting holes for the beans were made using a stick. The use of external mulch greatly improved banana performance in eight trials across the region. However, beans did suffer some setback in some instances when the improved banana growth would lead to a more dense canopy, outshading the understory beans. In collaboration with Bioversity International and the Tropical Soil Biology and Fertility Institute of CIAT (TSBF-CIAT), a series of trials was conducted to reduce the banana canopy through leaf pruning. The results are almost ready, but they provide farmers with advice on how to best manage the trade-offs between banana and the understory legumes.

To improve fertilizer use efficiency and profitability, it will be important for farmers to apply the right nutrients at appropriate rates. To enable the identification of the most deficient nutrients (see photo) that need primary attention when applying fertilizers, compositional nutrient diagnosis (CND norms) were developed by Lydia Wairegi in Uganda and by Severine Delstanche in Rwanda. The CND norms are based on foliar analysis and allow a quick assessment of nutrient deficiencies observed within the plant. Contrary to critical norms for single nutrients, the CND allows for an integrated assessment of nutrient imbalances within the plant.

Farmers can use pictures to diagnose nutrient deficiencies for nitrogen, phosphorus, potassium, and magnesium.
Farmers can use pictures to diagnose nutrient deficiencies for nitrogen, phosphorus, potassium, and magnesium.

Besides developing fertilizer recommendations based on foliar analysis, IITA conducted a series of large nutrient omission trials in central and southwest Uganda. Based on the quantification of nutrient uptake, soil nutrient supply, and crop response, a QUEFTS model was developed to predict fertilizer requirements in collaboration with Wageningen University (WUR). This work was led by Ph.D. student Kenneth Nyombi and is currently being carried forward by Ph.D. student Godfrey Taulya. He observed that potassium nutrition was particularly important for banana to alleviate drought stress. The result from the ongoing research effort clearly shows that strong synergies can be achieved when integrating soil fertility, agronomic, and economic research approaches at the plot, farm, and regional levels.