AIP Conference Proceedings 2199, 040009 (2019); https://doi.org/10.1063/1.5141296 2199, 040009
© 2019 Author(s).
Application bio-fertilizers to increase yields of zero-tillage soybean of two varieties under different planting distances in dry season on vertisol land of Central Lombok, Indonesia
Cite as: AIP Conference Proceedings 2199, 040009 (2019); https://doi.org/10.1063/1.5141296 Published Online: 23 December 2019
W. Wangiyana, and N. Farida
Application Bio-Fertilizers to Increase Yields of Zero-Tillage Soybean of Two Varieties Under Different Planting
Distances in Dry Season on Vertisol Land of Central Lombok, Indonesia
W. Wangiyana
1,a)and N. Farida
1,b)1Faculty of Agriculture, University of Mataram (Unram), Mataram, Lombok, Indonesia.
a)Corresponding author: w.wangiyana@unram.ac.id
b)nihla_farida@yahoo.com
Abstract. Soybean crop is capable of establishing symbiosis with both Rhizobium bacteria and arbuscular mycorrhizal fungi (AMF) to form tripartite symbiosis. Symbiosis with Rhizobium bacteria enables a host plant to perform biological nitrogen fixation, while symbiosis with AMF enables host plants to increase nutrient uptake and water absorption so that they can be more tolerant to drought in dry seasons. This study aimed to examine the effect of application of Rhizobium and AMF bio-fertilizers on growth and yield of two soybean varieties under two treatments of plant spacing in vertisol ricefield during the dry season 2009 following rice crop without tillage. The experiment was arranged in a Randomized Block Design, with three blocks (replications) and three treatment factors, namely soybean varieties (V1= Anjasmoro;
V2= Wilis), plant spacing (30x20 and 25x25 cm), and types of fertilizers (F1= without fertilizer; F2= Rhizobium application; F3= NPK only; F5= Rhizobium + AMF). Results indicated that application of both bio-fertilizers (Rhizobium and AMF) significantly increased soybean yield components, including grain yield, weight of 100 grains, grain number and total biomass per clump, compared with fertilization only with NPK or no fertilizers. There was no significant effect of plant spacing, but both varieties showed differences in plant height, grain yield, harvest index, and weight of 100 grains. However, there were interaction effects especially between variety and fertilization on plant dry weight, grain number and grain yield per clump, in which the highest grain yield was on soybean bio-fertilized with both Rhizobium and AMF, both in V1 and V2, but the average was higher in V1 (25.58 g/clump) than in V2 (15.03 g/clump). V1 was more responsive to dual application of the bio-fertilizers than V2.
INTRODUCTION
In Indonesia, 60% of soybean production areas are in the riceland, in which soybean crops are grown in the dry seasons following rice crops. In terms of domestic production and consumption, there have been deficits in domestic soybean production so that Indonesia still imports soybean. In 2009 for example, domestic production was 0.928 million tons while import was 1.05 million tons [1]. The results of prediction on production and consumption of soybean in Indonesia, from 2013 to 2020, it has been projected that consumption will still be much higher than the domestic production, and based the analyses applied, it is projected that the deficits in domestic soybean production will be more than 1.6 million tons per year, so that import will still have to done [2].
Based on the national Bureau of Statistics of the Republic of Indonesia, the national average of soybean productivity in 2015 was 1.568 ton/ha (https://bps.go.id/subject/53/tanaman-pangan.html#subjekViewTab3). Based on the description of the Indonesia national soybean varieties, there are soybean varieties having potential productivity of higher than 3.5 ton/ha, such as Mutiara-1 (4.1 ton/ha), Dega-1 (3.82 ton/ha) and Detap-1 (3.58 ton/ha) (https://balitkabi.litbang.pertanian.go.id/). Therefore, the average productivity of 1.568 ton/ha is still very low. This means that there are big gaps in the application of soybean production technologies between the farmers and researchers. According to So and Ringrose-Voase [3], the low soybean productivity achieved by farmers is not surprising because they mostly grow soybean without fertilization and without tillage. However, the results of complete tillage and fertilization studies carried out in the vertisol Lombok area by Adisarwanto et al. [4], also showed that soybean productivity was still relatively low (<1.5 tons/ha). The study reported no significant effect of fertilization with N, P and K soybean yields averaging only 1.29 and 1.21 tons/ha in Sengkol (Central Lombok) and 1.48 and 1.47 tons/ha in Keruak (East Lombok), between fully fertilized and those without fertilization [4].
Other possible factors causing low soybean productivity achieved by farmers are the low moisture contents of the soil in the rhizosphere of soybean grown on paddy fields during the dry season and low nutrient availability. In general, farmers grow rice in the rainy season (first rice crop), followed by second rice crop at the end of the rainy season or the beginning of the dry season (MK-1) as MK1 paddy rice crop. Soybeans or mungbeans are generally planted in the dry season (MK 2) after harvest of the second rice crop without tillage, so that the moisture content of the soil is usually low. In dry conditions, vertisol land is generally hard and cracked so that the growth of plant roots, as well as absorption of water and nutrient uptake become constrained. Application of soil amendments on to vertisol of Southern Lombok was found to significantly reduce cracks because of reduced COLE (coefficient of linear extensibility) value of the vertisol resulted from application of soil amendments [5]. The application of soil amendments to vertisol of Southern Lombok, either sand only of mixed with cattle manure, in addition to reduce soil strength [5], also increased establishment rate of soybean in the dry season and the subsequent mungbean crop, as well as increased their grain yields [6].
Other ways of improving growth and yield of crops in the dry season on vertisol riceland include establishment of effective symbiosis with arbuscular mycorrhizal fungi (AMF). The fungi will produce extraradical (external) hyphae, which function as extension of the plant root system to help the host plants increase water absorption and nutrient uptake. According to Sieverdings [7], the volume of land that can be explored by plants in symbiosis with AMF can reach 100 times greater than non-mycorrhizal plants. That is why plants that establish effective symbiosis with AMF become more resistant to drought conditions due to higher rates of water absorption and nutrient uptake by mycorrhizal than non-mycorrhizal plants [8], [9]. Wangiyana et al. [10] also reported that AMF inoculation on soybean direct-seeded following harvest of rice in vertisol growing media in pot experiments, significantly increased growth and grain yield of the soybean plants, compared with the uninoculated soybean.
In addition to drought conditions as the constraints for growing upland crops, changing conditions of land from inundated conditions, because it was previously used to grow flooded rice crop, to dry conditions for use to grow upland crops such as mungbeans and soybeans, can significantly reduce the availability of P nutrients for non-paddy crops [11], [12], [13], [14]. In addition, after the use of land for growing rice under flooded irrigation (conventional rice), especially after the second cycle of rice crop, the population of AMF becomes very low due to flooding even though rice is actually also a host plant for AMF [15]. [16], [17]. Soybean plants are also hosts for AMF, and soybean is classified as having a high degree of dependence on symbiosis with AMF [18]. In addition to symbiosis with AMF, soybean plants are also capable of establishing, and require, symbiosis with Rhizobium bacteria, which together produce a tripartite symbiosis, i.e. between soybeans as the host plants, and Rhizobium and AMF as the micro-symbionts [19], [20]. This study aimed to examine the response to application of Rhizobium and AMF bio- fertilizers of two varieties of soybean grown in dry season on vertisol land previously used to grow flooded rice crop in Central Lombok, Indonesia.
MATERIALS AND METHOD Treatments and Experimental Design
In this study, the field experiment was carried out on farmers' riceland in Mujur village (Central Lombok) with vertisol soil types, from August to October 2009. The experiment was arranged according to the Randomized Block Design by testing three treatment factors namely soybean varieties (V1= Anjasmoro and V2= Wilis), plant spacing (S1= 25x25 cm, S2= 30x20 cm), and fertilization treatments consisting of 4 levels of treatments (P1= No fertilizer application as what the farmers normally practiced; P2= application of Rhizoplus bio-fertilizer (containing Rhizobium bacteria); P3= NPK (Phonska 15-15-15) fertilizer only; P4 = Combination of Rhizoplus (Rhizobium sp) and Technofert (AMF) bio-fertilizers. Each treatment combination was made in three blocks (replications).
Implementation of the Experiment
After harvest of previous rice, no soil tillage was done. The land only divided into treatment plot of 3 x 2 m separated with a drainage furrow of 40 cm width and 20 cm depth surrounding each treatment plot. Soybean seeds of two varieties were dibbled at two planting distances, i.e. 30 x 20 cm, and 25 x 25 cm, depending on the treatments. For the P2 and P4 treatments, the seeds were coated with Rhizobium inoculants (Rhizoplus bio-fertilizer,
in the form of powder) prior to burying in the planting holes. For the P4 treatment, the base of the seed planting holes was first filled with “Technofert” bio-fertilizer (a bio-fertilizer containing mixed species of AMF mixed in zeolit growing media) of 5 gram per hole then it was thin-covered with soil, then Rhizobium-coated soybean seeds were placed on it and thin-covered with soil. For the P3 treatment, NPK fertilizer was applied by dibbling it at 7 cm depth and 5 cm next to young soybean plants at 7 days after seeding (DAS) the soybean seeds. For the P1 and P3 treatments, soybean seeds were direct-dibbled without prior coating with Rhizoplus. The maintenance of soybean plants afterwards includes weeding every two weeks and insecticide spraying as necessary. Harvest was done at 85 DAS for Anjasmoro variety (V1) and 90 DAS for Wilis variety (V2).
Observation Variables and Data Analysis
Observation variables included growth variables and yield components, which were measured on two types of sample plants, i.e. destructive samples harvested at the start of R3 growth stage, and final harvest samples. The destructive sample plants were used to measure R3 shoot dry weight, plant height, leaf number, nodule number and weight, while the final harvest samples were used to measure grain yield and other yield components, including biomass weight (above-ground dry weight, including stover, pods and grains), grain yield, number of grains, and weight of 100 grains. Data were analyzed with analysis of variance (ANOVA) and the Tukey’s HSD test at 5% level of significance, using the statistical software CoStat for Windows ver. 6.303. In addition, regression and correlation analyses between several variables were also carried out as necessary. For graphic presentation, the mean values were used together with the error bar using the standard error values based on Riley [21].
RESULTS AND DISCUSSION
Results of ANOVA summarized in Table 1 show that among the three treatment factors examined, type of fertilizers applied resulted in significant effects on more observation variables, especially those related to yield components, than types of the soybean varieties used, while plant spacing does not show any significant effects on the observation variables.
TABLE 1. Summary of ANOVA results for all observation variables.
Observation variables Main effects Interaction effects
Varieties Spacing Fertilizer VxS VxF SxF VxSxF
R3 Plant height *** ns ** ns ns ns ns
R3 Leaf number ns ns ns ns ns ns ns
R3 Shoot dry-weight ns ns ns ns * *** **
R3 Nodule number ns ns ns ns ns ns ns
R3 Nodule weight ns ns ns ns ns * ns
Dried harvested biomass ns ns *** * ns * ns
Grain number per clump ns ns *** ns * ns ns
Grain yield per clump *** ns *** ns *** ns ns
Grain yield per plot *** ns *** ns *** ns ns
Weight of 100 grains *** ns ** ns ns ns ns
Harvest index *** ns *** ns *** ns *
Remarks: ns = non-significant; *, **, *** = significant at p-value <0,05, <0.01, and <0.001, respectively
However, there are some interaction effects especially between varieties of soybean and types of fertilizers on grain yield and yield components, such as harvested biomass, grain number, grain yield per plot, weight of 100 dry grains, and harvest index, but there are also two three-way interaction effects, i.e. on R3 shoot dry weight and harvest index (Table 1).
Based on the main effect of varieties, both varieties show significant growth differences only in plant height (at R3 stage or 49 DAS), i.e. V2 was taller than V1 (Table 2), but in terms of yield components, V1 was higher in grain
yield per plot or per clump as well as in weight of 100 grains (Table 3). Crop (harvested) dry weight (biomass), which in this study is the dry weight of the above-ground plant parts (stems, branches, leaves, pods and seeds) and the total number of grains per clump are not significantly different between the varieties although they tended to be higher in V2. However, in terms of weight of 100 dry grains, V1 is significantly higher, and V1 (Anjasmoro), according to the variety description, is categorized as a big seed variety, while V2 (Wilis) is categorized as a small seed variety. In addition, V1 showed significantly higher harvest index (31.09%) compared with V2 (21.61%), indicating that the Anjasmoro variety (V1) is more efficient in partitioning dry weight to seeds compared with Wilis (V2) variety, considering that the dry (harvested biomass) weight of the plants was not significantly different between the two varieties. Therefore, the average grain yield, either per clump or per plot, was higher in V1 than in V2 (Table 3).
TABLE 2. Average plant height, leaf number, shoot dry weight, nodule weight, and nodule number at the R3 growth stage of two varieties of soybean as affected by plant spacing and types of fertilizers.
Levels of Treatment
factor R3 plant height
(cm) R3 Leaf number
per clump R3 shoot dry
weight (g/clump) R3 nodule weight (g/clump)
R3 nodule number per
clump
V1: Anjasmoro 42.42 b 38.29 a 12.68 a 1.01 a 36.92 a1)
V2: Wilis 50.35 a 33.67 a 12.19 a 0.88 a 30.50 a
HSD0.05 4.02 5.30 1.10 0.25 10.06
S1: 30x20 cm 45.90 a 35.63 a 12.33 a 0.88 a 31.42 a
S2: 25x25 cm 46.88 a 36.33 a 12.54 a 1.01 a 36.00 a
HSD0.05 4.02 5.30 1.10 0.25 10.06
P1: No fertilizer 39.50 b 34.33 a 11.60 a 0.80 a 29.25 a
P2: Rhizobium 49.50 a 38.83 a 13.14 a 1.14 a 38.17 a
P3: NPK only 47.08 a 32.00 a 11.50 a 0.91 a 34.42 a
P4: Rhizo+AMF 49.46 a 38.75 a 13.50 a 0.93 a 33.00 a
HSD0.05 7.57 9.98 2.08 0.48 18.93
1) Mean values in each column followed by the same letters are not significantly different beween levels of a treatment factor based on the Tukey’s HSD at 5% level of significance
In relation to the effects of different types of fertilizers between inorganic and bio-fertilizers, in terms of the effects on growth, the treatment factor resulted in significant effect only on plant height in which the unfertilized plants showed shorter height (Table 2), but the treatment factor showed significant effects on all yield components (Table 3). Among the levels of the treatment factor of fertilizer types, all mean values of yield components are lowest on unfertilized crops and highest on those fertilized with both bio-fertilizers. On some yield components, application of the Rhizobium bio-fertilizer only could also increase grain number and grain yield per clump or per plot compared with application of NPK fertilizer, but in some yield components, application of NPK fertilizer resulted in non-significant different with unfertilized treatment. This means that application of bio-fertilizer was more effective than application of NPK fertilizer in increasing soybean yield on vertisol Riceland following conventional rice crop.
Soybean crop is capable of establishing symbiosis with both Rhizobium species to form root nodules and AMF to form arbuscular mycorrhiza, and when these symbiosis established in one soybean plant, it is called tripartite symbiosis, and tripartite symbiosis has been reported to be more effective in improving growth and yield of soybean compared with dipartite symbiosis [19], [20]. Among the 24 species of seed plants examine by Sinclair and de Wit [22], it was concluded that soybean is a seed plant requiring the highest amount of N for production of seeds, which during the seed-filling stages, the amount required is much higher than the ability of it root systems in taking up N from the soil, so that these authors categorized soybean plants as self destructive, because they will have to remobilize N from leaves to growing seeds, which eventually will result in faster leaf senescence. Due to the high N requirement of soybean for seed production [22], and high P requirements of legume crops for nodulation and biological N2 fixation [23], then in addition to symbiosis with Rhizobium species, soybean and legume crops also require a good symbiosis with AMF for better P nutrition [24], [8], [9]. Therefore, this dual bio-fertilizer application would be better in supporting soybean yield than application of only one type of bio-fertilizer, i.e. Rhizoplus (containing Rhizonium species).
TABLE 3. Average harvested biomass, grain number per clump, grain yield per clump and per plot, weight of 100 dry grains, and harvest index of two varieties of soybean as affected by plant spacing and types of fertilizers.
Levels of Treatment factor
Dried harvested biomass (g/clump)
Grain number per clump
Grain yield (g/clump)
Grain yield (g/plot)
Weight of 100 grains
Harvest index (%) V1: Anjasmoro 51.81 a 112.67 a 16.30 a 1596.59 a 14.40 a 31.09 a1)
V2: Wilis 55.19 a 123.61 a 11.64 b 1140.72 b 9.37 b 21.61 b
HSD0.05 4.35 12.45 1.44 140.26 0.29 2.99
S1: 30x20 cm 54.81 a 115.92 a 13.75 a 1374.50 a 12.01 a 25.34 a
S2: 25x25 cm 52.19 a 120.36 a 14.20 a 1362.82 a 11.76 a 27.37 a
HSD0.05 4.35 12.45 1.44 140.26 0.29 2.99
P1: No fertilizer 44.35 c 92.19 c 10.60 c 1038.60 c 11.60 b 23.97 b P2: Rhizobium 57.33 ab 122.21 b 14.31 b 1402.14 b 11.95 ab 25.00 b
P3: NPK only 51.70 bc 95.77 c 10.67 c 1044.67 c 11.65 b 21.82 b
P4: Rhizo+AMF 60.63 a 162.40 a 20.31 a 1989.22 a 12.35 a 34.61 a
HSD0.05 8.18 23.43 2.71 264.10 0.55 5.63
1) Mean values in each column followed by the same letters are not significantly different beween levels of a treatment factor based on the Tukey’s HSD at 5% level of significance.
However, although types of fertilizers did not show significant effects on plant dry weight at R3 growth stage, there were significant interaction effects among the treatment factors, including a three-way interaction effect. In addition, harvest index also show a three-way interaction effect of the treatment factors, but unlike the R3 shoot dry weight, harvest index shows significant effects of both varieties and types of fertilizers (Table 1). Graphically, the three-way interaction effect is as in Fig. 1 for the R3 shoot dry weight and Fig. 2 for the harvest index.
0 2 4 6 8 10 12 14 16 18 20
P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4
S1: 30x20 cm S2: 25x25 cm S1: 30x20 cm S2: 25x25 cm
V1: Anjasmoro V2: Wilis
R3 shoot D.W. (g/clump)
FIGURE 1. Averages (Mean r SE) of R3 shoot dry weight (g/clump) of the two varieties of soybean to show the three-way interaction effects of the treatment factors tested.
0 10 20 30 40 50 60
P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4
S1: 30x20 cm S2: 25x25 cm S1: 30x20 cm S2: 25x25 cm
V1: Anjasmoro V2: Wilis
Harvest index (%)
FIGURE 2. Averages (Mean r SE) of Harvest index (%) of the two varieties of soybean to show the three-way interaction effects of the treatment factors tested.
Although there is no three-way interaction effect on grain yield per clump, the patterns of mean differences are likely more similar to those for harvest index than for the R3 shoot dry weight, as can be seen from Fig. 3.
0 5 10 15 20 25 30
P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4
S1: 30x20 cm S2: 25x25 cm S1: 30x20 cm S2: 25x25 cm
V1: Anjasmoro V2: Wilis
Grain yield (g/clump)
FIGURE 3. Averages (Mean r SE) of grain yield (g/clump) of the two varieties of soybean to show the three-way interaction effects of the treatment factors tested.
However, there are also significant positive correlation coefficients between grain yield per clump and harvest index, R3 leaf number, and R3 shoot dry weight per clump. Among the observation variables, harvest index shows the highest R2 values with grain yield, either per clump or per plot (Table 4), which means that grain yield per clump was most closely related to the rates of assimilate partitioning from the shoot dry weight to the growing seeds during the seed-filling stages. Since soybean crop has a strong capability to perform remobilization of assimilate and nutrients, especially nitrogen, from shoot including leaves [22], then increased leaf number and shoot dry weight prior to the peak of seed-filling stages would significantly support high grain yield per clump. In this study, it is also shown by the results of the correlation analysis, in which grain yield per clump was significantly correlated with harvest index, R3 leaf number and R3 shoot dry weight (Table 4). Based on the average grain yield per clump in Fig. 3, it can be seen that the Anjasmoro variety (V1) is more responsive to dual application of the bio-fertilizers than the Wilis variety (V2) of soybean.
TABLE 4. Results of correlation analysis (correlation coefficient and p-value) between variables Observation variable Statistic R3 shoot
DW (g/clump)
R3 leaf number per
clump
Harvest biomass (g/clump/
Harvest index (%)
Grain yield (g/clump) R3 leaf number per clump r_xy 0.519
p-value 0.039
Harvest biomass (g/clump) r_xy 0.020 0.033
p-value 0.942 0.903
Harvest index (%) r_xy 0.540 0.570 -0.056
p-value 0.031 0.021 0.838
Grain yield (g/clump) r_xy 0.536 0.579 0.336 0.918
p-value 0.032 0.019 0.203 0.000
Grain yield (g/plot) r_xy 0.533 0.584 0.346 0.910 0.998
p-value 0.033 0.017 0.189 0.000 0.000
CONCLUSION
It can be concluded that dual application of the bio-fertilizers, i.e. Rhizobium species and AMF, could significantly increase grain yield per clump and harvest index in soybean, especially in the Anjasmoro variety, direct-seeded following rice crop in vertisol riceland in the dry season.
ACKNOWLEDGMENTS
Through this article the authors would like to thank the Directorate of Research and Development of the Ministry of Research, Technology, and Higher Edication of the Republic of Indonesia, the Chairman of the Institute for Research and Community Service and the Rector of the University of Mataram for the “Stranas” research grant funded through the “2009-UNRAM-DIPA” with contract No. 0234.0/023-04.2/XXI/2009, from which data reported in this article were partly taken.
REFERENCES
1. Zakaria, A.K., Sejati, W.K., and Kustiari, R. Jurnal Agro Ekonomi, 28(1): 21 – 37 (2010) (Text in Indonesian).
2. Aldillah, R. Jurnal Ekonomi Kuantitatif Terapan, 8(1): 9-23 (2015). (Text in Indonesian).
3. So, H.B., and Ringrose-Voase, A.J. In: G. Kirchhof and H.B. So (Eds), Management of Clay Soils for Rainfed Lowland Rice-based Cropping Systems. Canberra, Australia: ACIAR (1996).
4. Adisarwanto, T., Suhendi, R., Sinaga, M.A., and Ma’shum, M. In: Suyamto H., Achmad Winarto, Sugiono and Sunardi (Eds), Proceedings of a seminar on farming systems, held in Mataram, 22-26 October, 1991. Malang, Indonesia: Balai Penelitian Tanaman Pangan Malang, Indonesia (1992).
5. Kusnarta, I.G.M., Wangiyana, W., Dulur, N.W.D., and Mahrup. IOSR Journal of Agriculture and Veterinary Science, 10(9, Ver.1): 58-62 (2017).
6. Wangiyana, W., Kusnarta, I.G.M., Farida, N., and Zairin, M. Crop Agro, 5(1): 26-32 (2012).
7. Sieverding, E. Vesicular-Arbuscular Mycorrhiza Management in Tropical Agrosystems. Eschborn, Federal Republic of Germany: Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH (1991).
8. Harrier, L.A., and Watson, C.A. Advances in Agronomy, 79: 185-225 (2003).
9. Smith, S.E., and Read, D.J. Mycorrhizal Symbiosis. Third Edition. Elsevier, Amsterdam (2008).
10. Wangiyana, W., N.W.D. Dulur, and N. Farida. International Journal of Environment, Agriculture and Biotechnology, 4(3): 884-891 (2019).
11. Sah, R.N., and Mikkelsen, D.S. Plant and Soil, 92: 265-278 (1986).
12. Sah, R.N., and Mikkelsen, D.S. Soil Sci. Soc. Am. J., 50: 62-67 (1986).
13. Sah, R.N., and Mikkelsen, D.S. Soil Sci. Soc. Am. J., 53: 1718-1722 (1989).
14. Muirhead, W.A., and Humphreys, E. 1996. In: G. Kirchhof and H.B. So (Eds), Management of Clay Soils for Rainfed Lowland Rice-based Cropping Systems. Canberra, Australia: ACIAR (1996).
15. Ilag, L.L., Rosales, A.M., Elazegui, F.A., and Mew, T.W. Plant and Soil, 103:67-73 (1987).
16. Wangiyana, W., Cornish, P.S., and Morris, E.C. Experimental Agriculture (Cambridge), 42: 427–439 (2006).
17. Wangiyana, W., Cornish, P.S., and Ryan, M.H. IOSR - Journal of Environmental Science, Toxicology and Food Technology, 10(12, Ver.III): 51-57 (2016).
18. Anderson, J.M., and Ingram, J.S.I. Tropical Soil Biology and Fertility: A Handbook of Methods. 2nd edition.
Wallingford, UK: CAB International (1993).
19. Meghvansi, M.K., and Mahna, S.K. American-Eurasian Journal of Agronomy, 2: 21-25 (2009).
20. Subramanian, K.S., Jegan, R.A., Gomathy, M., and Vijayakumar, S. Madras Agric. J., 98: 224-228 (2011).
21. Riley, J. Experimental Agriculture (Cambridge), 37:115–123 (2001).
22. Sinclair, T.R., and de Wit, C.T. Science, 189: 565-567 (1975).
23. Leidi, E.O., and Rodriguez-Navarro, D.N. Frontiers in Microbiology, Vol. 6, Article 1559, pp. 1-13 (2016).
DOI: 10.3389/fmicb.2015.01559.
24. Miyasaka, S.C. and M. Habte. Commun. Soil Sci. Plant Anal., 32: 1101-1147 (2001).
