Kanako SuzukiIITA-Zambia, Christian Fatokun IITA-Ibadan, Ousmane Boukar IITA-Kano
Cowpea (Vigna unguiculata [L.] Walp.) is a major source of dietary protein for people living in the savanna and Sahel regions of sub-Saharan Africa. It is also a source of income and livestock feed, and can improve soil fertility by fixing nitrogen (N) from the atmosphere in its root nodules. Constraints to cowpea production in West Africa include low and erratic patterns of rainfall, and severe attacks by insects and diseases, but the major constraint is poor soils. Even with sufficient rainfall, soils with low fertility cannot supply enough nutrients for the crops.
The use of chemical fertilizers is a simple and quick method to enhance plant nutrients in the soil. However, in rural areas of Africa, the prices of chemical fertilizers are usually twice the international prices making them unaffordable for most farmers. In addition, there are many places where farmers do not have access to chemical fertilizers due to the inadequate infrastructure and difficult economic conditions.
Legumes need P to fix N
Phosphorus (P) deficiency is a serious problem which limits the productivity of cowpea in the dry savanna and Sahel regions. Low levels of P in the soils have negative effects on the formation of root nodules and limit the growth and survival of rhizobia in the roots of legumes. This reduces their effectiveness in symbiotic N2 fixation. On the other hand, sub-Saharan Africa has vast quantities of rock phosphate (RP) deposits of varying quality, some of which are of sedimentary origin and reactive. High- to medium-reactive (>15 g citrate-soluble P/kg) sedimentary rock P deposits are found in Burkina Faso, Mali, Niger, Senegal, and Togo in West Africa. Indigenous rock P is more readily available to farmers than chemical fertilizers.
Identifying effective rock P
The Improving the water and nutrient use efficiency of crops under dry savanna and Sahel regions in Africa project, supported by the Ministry of Foreign Affairs in Japan, conducted experiments in Nigeria to identify the effect of rock P application on cowpea. From the results, a minimum RP application of 57 mg P/kg is necessary to enhance cowpea yield. To verify the efficacy for indigenous rock P on cowpea cultivation and to identify the optimum method to apply RP in the field, the team conducted field tests from 2015 to 2016 in Fashola village, Oyo State, Nigeria. Fashola fields have extremely low soil P. The tested methods were broadcasting (BC) and micro-dosing (MD). To identify the effect of RP application with BC and MD on cowpea growth and yield, treatments using single
super phosphate (SSP) were also established. Zero application treatment was used as the control.
From the study, the differences among the treatments compared to zero application were not significant except in two clones— IT97K-556-4 and IT90K-284-2. Although no significant differences were identified for the positive effects of the RP application on cowpea cultivation, the shoot dry weight (SDW) at 8 WAS in seven good responder genotypes except IT97K-499-35 tended to be higher under the RP application with BC and MD methods when compared to the control. Focusing on the application methods, MD and BC, the SDWs under MD application of 114 kg P/ha in 2015 and 60 kg P/ha in 2016 as RP tended to be higher than under BC application. The pictures show the positive effects of the MD method on cowpea growth. The same trends were identified under MD or BC application of 30 kg P/ha as SSP.
As the next step, our study focused on co-application of indigenous RP and arbuscular mycorrhizal fungi (AMF) to promote P nutrient uptake from rock P. In pot experiments, we inoculated cowpea with Glomus intraradices and established the treatments of no-inoculant and inoculant with AMF under no-P application and rock P application at a level of 60 mg P/kg. Using 15 cowpea genotypes, the pot experiment was conducted for verifying the effects of co-application of RP at 60 mg P/kg and AMF inoculation. One of 15 genotypes (Sanzi) showed significantly (P <0.05) higher shoot dry weight (SDW) at 8 WAS with AMF co-application than in the treatment of only rock P application. The SDW of 14 other cowpea genotypes were slightly higher than when only rock P was applied. AMF inoculation appears to be ineffective under high soil P conditions. Therefore, these results indicated that rock P application at 60 mg P/kg is too high for AMF inoculation to work. The optimum application amount of rock P should be less than 60 mg P/kg.
In another pot test, four levels of rock P application: 0, 20, 40, and 60 mg P/kg were established with AMF inoculation using six cowpea genotypes selected from the nine genotypes. The SDW at 8 WAP showed that 20 mg P/kg was the optimum level for cowpea with AMF inoculation.
Additionally, AMF can contribute to increase water as well as P uptake. We observed that
AMF inoculation has a positive effect on increasing drought tolerance of cowpea genotypes that are susceptible to drought. More detailed studies.