Effect of Various Levels of NPK Fertilizer on the Yield Attributes of Soybean ( Glycine max L . ) Varieties

Indonesian soybean production can only meet about 30% of national demand. Efforts to increase soybean production include the use of improved seed quality and fertilizer application. The aims of this research were to 1) examine the responses of six genotype soybeans to the application of different doses of NPK fertilizer; 2) determine the seed protein content of the six soybean genotypes following NPK application; 3) determine the interaction between soybean genotypes with NPK doses in affecting growth and yield. This research was conducted under a plastic house at the Faculty of Agriculture, University of Jenderal Soedirman, Central Java, Indonesia. The experiment was arranged in a randomized block design with three replications using six soybean varieties and four levels NPK fertilizer. Soybean varieties tested were “L/S: B6-G1” (V1), “L/S:B6-G3” (V2), “L/S:B6-G4” (V3), “Grobogan” (V4), “Burangrang” (V5), and “Argomulyo” (V6). Four levels NPK fertilizer tested were: without fertilizer (N0), 0.6 g per plant (N1) 1.2 g per plant (N2), and 18.g per plant (N3), or 0, 100, 200 and 300 kg.ha-1. “L/S: B6-G1” had the greatest number of seeds per plant (184) following treatment with 300 kg.ha-1 of NPK. The seed protein content increased with the increase doses of NPK. “Argomulyo” treated with NPK at 300 kg.ha-1 had the highest seed protein content (34.5%). Soybean responses to NPK treatment differed with varieties, particularly in the number of fl owers per plant, number of seeds per plant, grain weight per plant and seed protein levels.


Introduction
Soybean has been cultivated in Indonesia for a long time, but there have been very little expansion of the soybean growing area for the last ten years.Soybean is rarely grown as a single crop in the traditional farming systems.Soybean production in Indonesia in 2015 was 963,000 tons of dry beans with a productivity of 1.48 tons ha -1 whereas the national soybean demand was 2.4 million tons (CBS, 2016).National shortage of soybean demand has been supplied through imports, i.e.1.7 million tons in 2010, and increased to 1.9 million in 2012 (CBS, 2013).According to El-Shemy (2011) every 100 g of soybean seeds have 330 calories, 35 g of protein, 18 g of fat, 35 g of carbohydrates, 227 mg of calcium, 585 mg of phosphorus, 8 mg of iron, 110 SI vitamin A, and 1 mg of vitamin B. One of the efforts to improve Indonesian soybean productivity is through breeding to obtain high yielding varieties (Sumarno and Adie, 2010).Faculty of Agriculture, Jenderal Soedirman University has produced improved soybean varieties; these varieties were grown in different geographical and environmental conditions to identify high yields varieties with consistent results prior to releasing them as the new varieties.The promising varieties that have been developed at Faculty of Agriculture, Jenderal Soedirman University are "L/S: B6-G1", "L/S:B6-G3", and "L/S:B6-G4".These lines are progenies of crosses between "Lokon" (male) and "Sindoro" (female).The new lines are expected to have a higher yield, early maturing and higher seed protein content, which are important for the manufacture of tempeh (Sunarto et al., 2008).
Fertilization with inorganic compound fertilizer NPK will supply the nutrient requirements for healthy soybean growth (Adisarwanto and Wudianto, 1999).The research objectives were to 1) examine the response of six genotype soybeans to the dose of NPK fertilizers; 2) determine the seed protein content of the six genotypes of soybean; 3) examine the interaction between soybean genotypes with dose of NPK fertilizer in affecting soybean growth and yield.

Materials and Methods
The experiment was conducted in a plastic house at the Faculty of Agriculture, Jenderal Soedirman University, North Village Karangwangkal of Purwokerto District, Banyumas, Central Java, Indonesia located 110 m above the sea level with the average temperature of 29 o C, average rainfall of 284 mm per month, and an average humidity of 93%.The soil type was inceptisol.The study used three soybean lines, "L/S: B6-G1", "L/S:B6-G3", "L/S:B6-G4", and three released soybean varieties "Burangrang", "Grobogan" and ''Argomulyo".Fungicide and insecticide were applied as required.Soybean plants were grown in 15-L polyethylene bags and arranged in a factorial randomized block design.Factors tested in this research are soybean varieties "L/S: B6-G1" (V1), "L/S:B6-G3" (V2), "L/S:B6-G4" (V3), "Burangrang" (V4), "Grobogan" (V5), and ''Argomulyo" (V6), and four levels of NPK: 0, 0.6, 1.2, and 1.8 g per plant.All treatments were replicated three times, each treatment consist of three plants with a total of 216 plants.Variables scored were plant height, number of leaves per plant, number of fl owers per plant, days to fl owering, harvesting age, the number of productive branches per plant, number of branches per plant, number of pods per plant, number of seeds per plant, seed weight per plant, weight of one hundred seeds, and seed protein content.Data were analyzed using ANOVA to determine the effect of each treatment using SAS version 9.13.Signifi cant effects were further tested using DMRT at 5%.

Description of Soybean Varieties
The description of the soybean varities of this study is in Table 1."Argomulyo" and "Burangrang" have high resistance to leaf rust, whereas "Grobogan" has high adaptability to different climatic conditions (Arwin et al, 2012).

Growth and Yields of Six Soybean Genotypes
Soybean growth and yield as indicated by plant height, number of leaves per plant, number of fl owers per plant, days to fl owering, harvesting age, the number of branches per plant, number of productive branches per plant, number of pods per plant, number of seeds per plant, weight of seeds per plant, weight of one hundred seeds, and seed protein content, were signifi cantly different amongst the six soybean variety."L/S: B6-G4" was the tallest (101.7 cm at 10 weeks after planting) whereas "Grobogan" was the shortest (57.3 cm) (Table 2).
Genotype that had the greatest number of leaf is "L/S:B6-G4" had the highest number of leaves (24.8) and "L/S: B6-G3" had the fewest (15.6).Leaves are crucial for the interception and conversion of solar light energy into growth and yield through photosynthesis.
The number of productive branches in soybean is affected by nutrient uptake and light reception.The number of productive branches was the greatest in "L/S: B6-G4" (Table 2) whereas "L/S: B6-G4" has the greatest number of pods plant (63.4) (Table 3).The differences in the number of pods per plant amongst soybean genotypes were likely caused by different plant morphologies, which could obviously be seen in the plant height, the number of trifoliate leaves per plant, and the number of fl owers per plant.Number of seeds ranged from 72.3 to 122.0 seeds per plant."L/S: B6-G1" had the greatest number of seeds per plant (122.0), which were signifi cantly greater than "L/S: B6-G4" (103.4),"Burangrang" (92.4), and "Argomulyo" (94.6)."L/S: B6-G3" had the fewest number of seeds of plants of 72.3.The number of seeds per plant correlated with the large number of productive branches and number of pods per plant.In this study the soybean variety that has a large number of productive branches and number of pods per plant is "L/S: B6-G4".According to Somaatmadja (1985) the yield per plant is correlated to the number of pods, number of seeds and seed weight per plant.B6-G3" had the greatest 100-seed weight (18.6 g).These results were possibly related to a number of factors that determine the size of soybean seeds, including genetic factors.According to Adie (2005) the weight and size of the seeds is the quantitative nature of the genetic factors that in certain circumstances may change according to the growing environments.
Differences in levels of soybean protein are closely related to genotype tested and micro-climatic conditions at the time of the study.The seed protein content was the highest in "Argomulyo" (34.4%) whereas "L/S: B6-G4" had the lowest (33.3%) (Table 3).
Growing environment such as temperature might also indirectly affect the seed protein levels through the increase in plant's respiration and photo respiration (Lakitan, 2007).
The results showed that NPK doses signifi cantly increased all soybean growth attributes (Table 2 and  3) except for the time to fl ower and time to harvest (Table 2).Increased doses of NPK increased the average plant height from 59.7 to 73.4 cm, and number of leaves per plant from 16.5 in control to 21.6 at NPK 300 kg.ha -1 (Table 2).Increased plant height is related to the roles of NPK in promoting plant growth.
The results of this study demonstrated that the number of productive branches and the number of branches per plant were clearly affected by the doses of fertilizers (Table 2).The highest NPK doses of 1.8 g per plant, or 300 kg.ha -1 , resulted in the greatest number of fruits per plant, the greatest number of productive branches and total number of branches There were signifi cant differences in seed weight between the varieties tested.Seed weight per plant of "L/S: B6-G1" was 19.3 g and "L/S: B6-G4" had 16.3 g, which were signifi cantly greater than "L/S: B6-G3" (12.4 g), "Grobogan" (12.2 g), "Burangrang" (12.4 g), and "Argomulyo" (13.1 g; Table 3).According to Krisnawati and Adi (2007) seed weight per plant are closely related with the yield components including the number of pods per plant and seed size.The weight of 100 g seeds ranged between 13.7-18.6g with "L/S: per plant (Table 2).
Different doses of NPK fertilizer resulted in different soybean yields (Table 3).The seed weight per plant was the greatest (19.1 g per plant) when applied with the highest NPK, and the seed weight per plant was gradually decreased with the decrease of NPK doses (Table 3).As for other legumes, NPK played signifi cant roles in the development of soybean pods, seed formation, protein levels, and root growth (Rismunandar, 1993).Note: Values followed by the same letter are not signifi cantly different according to DMRT at 5%; dap: day after planting.
Table 3 shows that the number of pods and seeds per plant was the greatest (60.9 and 120, respectively) with the highest NPK dose, and the fewest without NPK (40.7 and 73.7, respectively).N, P and K have important roles in increasing the number of cells and chlorophyll, hence increased photosynthesis products, which resulted in the increase in the number of pods and seeds.The highest NPK dose also resulted in the greatest 100-seed weight (16.0 g) and the highest seed protein content (33.9%).The uptake of phosphorus during vegetative stage will be residing in the meristem tissues for protein synthesis, whereas during generative stage will accumulate in the fruit and seeds (Junkazayama, 2009).

Interaction between Soybean Varieties with NPK Doses
Different soybean varieties responded differently to NPK doses particularly in the number of fl owers per plant (Figure 1), number of seeds per plant (Graph 2), seed weight per plant (Figure 3), and seed protein content (Figure 4).
The results of this study demonstrated that the responses to NPK doses vary with soybean varieties (Figure 1).Increases in the dose of NPK fertilizer resulted in the higher seed yield per plant, but the increase was signifi cantly increased in V1 only (Graph 2).NPK has important roles in cell division, energy transformation, and fat metabolism (Hardjowigeno, 1995) which eventually affected seed production.
The seed protein content was the highest in "Argomulyo" (34.5%) fertilized with NPK at 1.8 g per plant, and it was signifi cantly higher than those in other genotypes at the same dose of fertilizer."L/S: B6-G1" had the lowest seed protein content of 33.2%.Phosphate plays important roles in plant growth as it is a constituent of ATP, ADP, NAD, NADPH.According to Gardner et al. (1991), nitrate reduction process that occurs before the production of amino acids requires electrons, and the main donor of electrons are nicotinamide adenine dinucleotide is (NADH) or nicotinamide adenine dinucleotide phosphate (NADPH), which are photosynthesis products.
Protein content was positively associated with the N 2 fi xation activity at all stages of the reproductive cycle, and different soybean genotypes could have different ability to fi x nitrogen and to assimilate nitrates (Fabre and Planchon, 2000).Effect of Various Levels of NPK Fertilizer on the Yield Attributes of Soybean ..........

Conclusion
Soybean genotypes had different responses to the increase of NPK doses."L/S: B6-G1" fertilized with 1.8 g per plant NPK produced 184 pods and 32.5 g of seeds per plant, which was 46% greater than the released variety "Argomulyo"."Argomulyo" treated with NPK of 1.8 g per plant, however, had the highest seed protein content of 34.5%.The increase in NPK doses increased the seed protein content in all soybean genotypes.There were positive interactions between the six soybean genotypes with doses of NPK in affecting the number of fl owers per plant, number of seeds per plant, seed weight per plant and seed protein content.Further fi eld research should be conducted using "L/S: B6-G1" and "L/S: B6-G4" in different locations to determine their growth, yield and adaptability.

Table 2 .
Matrix of analysis variance of the soybean varieties Note: V= varieties or lines; N = dose of NPK fertilizer; V x N= interaction of varieties with NPK doses; ** = highly signifi cant; *= signifi cant; ns = not signifi cant according to DMRT at 5%.

Table 3 .
Effects of NPK doses on growth of several soybean varieties

Table 4 .
Effects of NPK doses on yields of several soybean varieties