Determining Optimal Dose of Chemical Fertilizer on Biofortified Bean in Sud-Kivu Highlands

Rational application of chemical fertilizer increases crop yield of biofortified bean. This study aimed at determining the optimal dose of chemical fertilizer to apply on two biofortified bean varieties used in the community in order to maximize their yield. Following a split-plot design, a field experiment was carried out on CODMLB001 and HM21-7 varieties, in Kashusha (Kabare territory) in Sud-Kivu Highlands, after a strategic application of increasing doses of chemical fertilizer NPK 17-17-17 (D0: Control; D1: 50 kg·ha; D2: 75 kg·ha; D3: 100 kg·ha; D4: 125 kg·ha and D5: 150 kg·ha). The said doses were applied on the sowing day in a parallel gutter at 5cm from the sowing line. The germination rate, the number of days at both the flowering stage and the stage of physiological maturity, as well as the number of harvested crops and beans per plant, number of beans per pod, weight per 1000 grains and yield were observed. Positive and negative interaction between different increasing doses of chemical fertilizers regarding the two varieties were observed. This strategic application allows increased performance according to considered varieties and doses. For instance, the HM21-7 variety gave the best performance with the D5 dose, while the CODMLB001 variety did better under D2.


Introduction
Africa has a complexity of climate conditions and a range of soils (Goudie, 1996;Griffiths, 2005;ODINAFRICA, 2007;UNEP, 2008;Casinga et al., 2015bCasinga et al., , 2016)).However, the latter have generally low fertility because of their ageing due to the lack of volcanic substrata for the establishment of new structures and their rejuvenation (Bationo et al., 2006).Nearly 16% of Africa's soils are of the best quality, 13% are of average quality, 16% of the least quality and 55% of poor quality although supporting a diversity of crops (Eswaran et al., 1996).On the one hand, approximately 900 million hectares of soils of high and average quality bear 400 million of persons or 45% of the African population.On the other hand, nearly 30% of the population (about 250 millions) live or depend on soils with low production potentiality and so leads to a situation of food insecurity (Eswaran et al., 1996;Bationo et al., 2006Bationo et al., , 2008)).
Phosphorus and Potassium are respectively about 22 kg•ha -1 , 2.5 kg•ha -1 and 15 kg•ha -1 (Steiner 1996).These bad farming practices have led some writers to characterize the agriculture of these countries as "mining agriculture" (Stoorvogel & Smaling, 1990) since outputs exceed inputs in these exploitation systems (Van Keulen & Breman, 1990;Van Reuler & Prins, 1993;Van Reuler, 1996).Due to rapid demographic explosion and dietary needs that follow, agricultural production must increase significantly in order to feed the population and eradicate malnutrition (Useni et al., 2012;Casinga et al., 2015aCasinga et al., , 2015bCasinga et al., , 2015cCasinga et al., , 2016)).Recourse to chemical fertilizers proves a key factor in the modernization of agriculture in developing countries due to the existence of a positive correlation between yield and the amount of chemical fertilizers used properly (Useni et al., 2012).The other advantage of fertilizers is that they not only improve efficiency but also crop residues serve as organic fertilizer from the previous crop (Batiano et al., 2006;Casinga et al., 2015b).In Sud-Kivu, the yields of the bean crop are low and do not exceed 500 kg•ha -1 in farming without fertilization (Casinga et al., 2015b).In large farms, with the use of improved varieties and the use of mineral fertilizers, yields varying from 800 to 1200 kg are achieved, HarvestPlus (2013).The present study aims to determine the optimal dose of chemical fertilizers NPK 17-17-17 able of inducing and enhancing increase in the yield of biofortified beans and the identification of the variety that best responds to mineral fertilization.

Location
The experiment was carried out during the growing season 2011A-2012A, in the experimental site of the "Université Evangélique en Afrique (UEA)" at Kashusha, Kabare territory, Sud-Kivu in the Democratic Republic of Congo.The geographical coordinates of the station are 028°47′ East longitude and 02°19′ South latitude while the altitude is 1712 m.Kashusha has an AW 3 climate type, according to Köppen's classification.The average annual rainfall reaches 1450 mm while the average annual temperature is 19.5 °C.The soil of the experimental site belongs to the class of Ferralsols, according to FAO-UNESCO classification (Baert, 1995;Beernaert, 1999;Baert et al., 2012;Botula et al., 2012;Casinga et al., 2015b); whereas its texture silty-clay (Casinga et al., 2015a).

Biological Material
Two biofortified beans varieties were used as biological material: the CODMLB001 and the HM21-7 varieties.

Method
The experiment was conducted on a split-plot design.Varieties of biofortified beans randomized and sown in line in the subplots at the depth of 4 cm with two seeds per hole, with a distance of 0.40 × 0.20 m were the main factor, while six doses of chemical fertilizer (D 0 : Control; D 1 : 50 kg•ha -1 ; D 2 : 75 kg•ha -1 ; D 3 : 100 kg•ha -1 ; D 4 : 125 kg•ha -1 and D 5 : 150 kg•ha -1 ) represented the secondary factor.Fertilizers were spread at sowing and applied in a gutter parallel to the sowing line, at a distance of more or less 0.5 cm, and a depth of more or less 5 cm.The latter were immediately covered with a little soil after application.At the beginning of vegetation germination rate was determined with the ratio of number of plants raised per the number of grains sown multiplied by one hundred.During vegetation, the numbers of days to flowering were determined by the difference in number of days between the date of the appearance of inflorescences (at least 50% on a plot) and sowing date.The yield components (number of pods, number of grains per pod and weight of 1000 grains) and the yield obtained by the formula were determined.
Yield (kg•ha -1 ) = Poids des Graines SU × 10000 Surface Utile (SU) The experimental results were assessed by the Analysis of Variance, correlation and multiple regression while the averages were separated by the test LSDα = 0.05.Excel 2013 software, R 3.3.0and Assistat 9.5.1 were used as calculation tool.

Results
Statistical analysis revealed significant differences in the parameters of the number of days at the flowering stage, the number of harvested plants and yield for the different increasing doses of mineral fertilizers.However, they revealed the lack of significant differences in the parameters of the germination rate, the number of days at the (1) stage of physiological maturity, number of pods per plant, number of grains per pod and weight of 1000 grains, depending on the different increasing doses of mineral fertilizers.The CODMLB001 variety presented a better rate of germination in comparison to the HM21-7 variety.Furthermore, it happened earlier than the latter since it reached flowering and physiological maturity earlier.At the harvest, the number of harvested plants for the CODMLB001 variety was greater than that of the HM21-7 variety.The number of pods per plant and the number of seeds per pod were similar for the two varieties, the increasing doses of chemical fertilizers and their interactions.The weight of 1000 seeds was positively correlated with the increasing doses of chemical fertilizers for the two varieties.The HM21-7 variety had the better yield in comparison to the CODMLB001 variety.

Table 1 .
Effect of increasing doses of chemical fertilizers on the growth of bean

Table 2 .
Effect of doses of increasing chemical fertilizers on the yield and its components *Note.