4.1 physical and chemical property of the soil
The soil analysis revealed that the soil of the testing site was loamy sand with available phosphorus of 7.72 ppm, which is very low for most crops including groundnut (Sahlemedhin, 2001). At the experimental site, it was phosphorus that was the major restraining factor for the groundnut production and thus, its application was important to enhance optimum yield. Phosphorus deficiency constitutes a severe limitation to crop production in weathered tropical soils containing high Fe and Al oxides that quickly fix added P. The pH of the soil at the trial site was 7.25, considered favorable for groundnuts (Murata, 2003). According Nkot et al. (2011) stated reduced groundnut nodulation and fixation of nitrogen in acid soils pH of 3.6 -6.9.
Very low content (0.07%) of total nitrogen in the soil was observed at the experimental site, This was may be due the past frequent farming with little and/or no application of inorganic as well as organic fertilizers. The level of organic matter (0.09%) is also very low to meet the crop growth requirement, which impels to low water holding capacity. Nitrogen is not mandatory in large amounts to groundnut since it fixes atmospheric nitrogen by nature. Tubbs et al. (2012) reported that nitrogen availability in the soil is essential for vegetative growth as such, however, addition of nitrogen fertilizers at low rate as a starter dose to groundnut crop is important mainly where content of the nitrogen in the soil is low.
According to Moazed et al. (2010) limited soluble Phosphorus in soils is reduce crop production and productivity. Nevertheless, zinc is also a very important nutrient for building metabolites due to activating effect of several enzymes involved in biosynthesis of growth substances, which produce more plant cells and dray mater accumulation (Darwish e t al., 2002). However, the amount of zinc found in the experimental area as the soil laboratory result displayed (5.52ppm) was below enough to for groundnut production.
Furthermore, the soil analysis results showed that potassium 238 kg/ha were acceptable for the crop overall growth (Landon, 1991). Results also revealed that cation exchange capacity and electrical conductivity (2.7cmol (+)/kg/ha) and (0.045ms/cm) respectively are very low to groundnut cultivation (Landon, 1991) and it implied that the soil have low holding exchanging cations but free of salt problem.
4.2 Phenological Parameters
Flowering is a very important physiological process in plant developmental phase and has a profound effect on the economical final yield of the crop. The statistical analysis revealed a clear effect of the genotype as well as the application of a combination of phosphorus with foliar application of zinc fertilizer on days to 50% flowering and days to 50% maturity on groundnut. In the main effect groundnut genotypes and the combined fertilizers showed significant effect, where the standard check variety Sedi flowered earlier at the highest level of combined fertilizer (30 kg P/ha + 1.5 g Zn/L).
In the study genotypes flowered earlier produce greater yields. Similar study by Sastry et al. (1985) stated that groundnut genotypes, which flower early during first and the second week of the flowering period produce better yield. The differences observed among the groundnut genotypes in relation to days to flowering can be attributed to the difference in growth characteristics among the genotypes. Verma et al. (2009) reported variable growth patterns in some groundnut genotypes, which could be due to differences in their genetic makeup.
Furthermore, the variation occurred due the main effect of combined fertilizer also may be resulted from the zinc applied as foliar application that enhances photosynthesis at early growth of the plants that improves nitrogen fixation and phosphorus utilization which promotes rapid growth stage. In addition to that, combination of Phosphorus fertilizer and foliar spray with zinc may improve utilization of nutrients and water, which was revealed and reflected in a good growth and biological yield. This result is in full agreement with Mirvat et al. (2006) who showed phosphorus application from 0 - 60 kg/fad could possibly shortens maturity of groundnut.
4.3 Growth Parameters of Groundnut
Analysis of the result indicated that, growth parameters, leaf width and leaf length of groundnut did not show a significant variation for the interaction of combined fertilizers and genotype. However, the main effect of the combine phosphorus with foliar application of zinc on leaf length was significant. The difference in leaf length may be the large amount of phosphorus with zinc application, as phosphorus plays crucial roles in enhancing development of extensive root system which resulting into increase nutrient uptake. Besides the foliar spraying with zinc may encourage the vegetative growth and by thereby increased dry mater accumulation via the plant capacity for building metabolites. Increasing the joint application of phosphorus with foliar zinc up to 30 kg P/ha + 1.5 g Zn/L are increasing the leaf length on Sedi variety. In line with current study, Mirvat (2006) reported that increasing P fertilization up to 60 kg P2O5/fad with zinc concentration up to 1 g/L improves leaf length. These effects revealed that foliar nutrition of groundnut with zinc might increase the efficiency phosphorus utilization and enhancing vegetative growth.
Furthermore, the significantly enhanced leaf length of the genotype might be due to increasing of phosphorus level with foliar application of zinc plays an important role to in conservation and transfer of energy in metabolic reaction of living cells and root development. The current finding is in agreement with the report of Jeetarwal (2013) that application of phosphorus enhance nutrient availability and uptake of N, P and Zn that results vigorous growth parameters.
4. 4 Yield and Yield Components
The application of different fertilizer levels had significantly affected pod number/plant, pod yield/ha, number of seeds/pod, seed yield/ha and shelling percentage. This result was concurrent to the study carried out by Mirvat (2006) who reported a significant influence of combined application of phosphorus with foliar zinc on the yield component traits that the combined nutrient application may enhance the nutrients and water utilization that enhances yield component traits.
The interaction and main effect of combined fertilizers and genotypes had on influence on the yield and yield traits. Mainly the interaction of combined phosphorus with foliar zinc and genotype had influence on pod number/plant, pod yield/ha, number of seeds/pod, seed yield /ha and shelling percentage.
The increasing trend of the combined fertilizer level from 0-30 kg P/ha + 1.5 g Zn/L with different genotypes showed an increment on yield and yield parameters might be due to the effect of combination of phosphorus with foliar zinc fertilizer level on number and weight of nodules and nitrogen activity which in turn affects positively the yield component traits. Furthermore, the increment in yield may also be resulted due to the phosphorus fertilizer attributed to the activation of metabolic process as stated by Marschner (1986). Similarly, the foliar spraying with zinc might lead to increase in number of pods and seeds/plant, weight of pods and seeds/plant. Since zinc fertilizer often attributed to the vital role of activating enzymes in biological process that may, help to increase yield characters.
In general, the higher seed yield depicted by the variety Sedi under the combined application of phosphorus and foliar zinc might be characteristics of higher yielding potential of the variety as well as efficient utilization of growth resources. Similar to this result, Ofori (1996) also stated that variety of Bambara groundnut with the spreading growth habit produce flower throughout growing season and gives high yields.
Highly significant increase seed yield was obtained from the application of 30 kg P/ha+1.5 g Zn/L on Sedi variety (2,529 kg/ha). Under this treatment significant increase in seed yield may be due to associated improvement in leaf length, leaf width, pod number/plant, pod yield/ha and number of seeds /pod as reported by Jeetarwal, (2013). The seed yield has a highly significant and positive correlation with those yield attributes that further support their direct and indirect effect on seed yield in the crop.
Application of combined fertilizers on the standard check variety sedi could increase productivity of groundnut per unite area from the current farmers yield of Tanqua-Abergelle (1200 kg/ha) which is very low compare to the yield obtained from the current research finding 2,529 kg/ha that proved more than double yield advantage than the farmers practice in the woreda. Thus, result also exhibited promising increment than the average yield (700 kg/ha) of the region Tigray and 1,330 kg/ha of the national productivity (CSA, 2016).
In the result also observed that the phonological traits (days to 50% flowering and maturity) had a strong negative correlation with most of the yield attributes (except hundred seed weight) such as pod number/plant, pod yield/ha number of seeds/pod and seed yield that was consistency with the report of Bethlehem (2011).
This indicated that when the crop matures earlier particularly under stress condition and when the growing season is too short and when terminal drought is frequent, early maturing genotypes may escape the stress and yield better than the long maturing genotypes. Moreover, the late maturing genotypes may be affected by weather fluctuation such as shortage of moisture (rainfall) and high temperature that was apparent in the latter growth stage of the crop. This was in agreement with the findings of Samalulu et al (2014) who reported the effect and the influence of micro climatic conditions and soil has on the growth of difference groundnut genotypes. As the variety Sedi matures earlier than others and provided relatively higher yield as compared with the other testing new genotypes, under a well-managed condition (better nutrient application), the local genotype would still be suitable to the area where terminal drought is the norm.
Hundred seed weight is one of the yield-contributing traits in groundnut. In this study, the combined application of phosphorus with foliar application of zinc on groundnut has not revealed any significant effect on hundred seed weight. However, hundred seed weight of groundnut was significantly influenced by genotypes main effect, where the higher seed weight was recorded by ICGV91114 (47.23 g) than variety Sedi (45.12 g). This result is may be due to the genetic differences between the genotypes that is consistent with the idea of Mulatu (2014) who stated that the seed weight characters is more influenced by genetic factors than environment.
In other hand, hundred seed weight (45.12g) of the genotype had found in the rage of 35-70 g is under the rage, which fulfills the international market quality of seed grade (Acland, 1971).
Shelling (%) is an indication of pod filling efficiency so the highest percentage value had a direct effective on pod filling. In this finding the interaction of combined fertilizer and genotype significantly affected shelling percentage (%) of groundnut in a linear fashion with increasing phosphorus and foliar zinc application. The variety Sedi had the highest record for this trait. This might be due to fertilizer rates hasten the pod formation then efficient assimilates rather move to the seed when the moisture is available especially at dryland area.
Furthermore, it has a positive correlation with seed yield (r=0.62) and this was may be due to the application of more fertilizer to the soil and foliar, which enhances these parameters. This was in agreement with mulatu (2014) who reported variation in variety and efficient partitioning of assimilates in to seed than pods.
4.5 Crude Protein and Fat (%)
The application of combined fertilizer had an interaction effect with the genotypes for crude protein and fat. The crude protein and fat (%) ranged 36.69 - 37.79 and 41.44 - 43.95% by combined application of 30 kg of P/ha + 1.5 g Zn/L fertilizer on Sedi and ICGV00308, respectively. Okello et al. (2010) who found 20-50% and 40-50% protein and fat content on groundnut due to fertilizer application, respectively also reported similar finding.
The increment in protein and fat % may attributed to the contribution of the genotype difference and combined application of phosphorus with foliar zinc as it was revealed from the significant interaction effect. The current result was in line with Majumdar et al. (2001) also reported that, the percentage of protein increase as a result of an application of phosphorus up to 70 kg P2O5/ha. Gobarah et al. (2006) also noted that P application significantly increased protein (25.82%) contents on groundnut over control.
The interaction of the combined fertilizer level and genotypes was strongly and positively affected the fat percentage, which recorded highest percentage from the treatment, which had received the highest combined fertilizer levels.
As the result displayed the highest fat percentage is extracted from the ICVG00308 genotype that showed an increasing trend (linearly) from control (zero application) to the maximum level of combined fertilizer. This could possibly due the combined effect of phosphorus with zinc that have on the metabolic processes and growth that in turn reflected positively on the biochemical content of the groundnut seed. Similar to the current study, Mirvat et al., (2006) and Kausale et al. (2009) also reported maximum oil content and fat in seeds of groundnut with the application of those fertilizers. The crude fat content 43.95% had found from the highest fertilizer combination and the result is in line with finding of Evans et al. (2015) reported that fat content of groundnut in the range 33.6-54.95%.
4.6 Partial Budget Analysis
The dominance as well as economic analysis on marginal rate of return (MRR) showed that the combination of P10Zn0.5 fertilizer on ICGV00308 genotypes had better gain over the other treatments (380.58%) (Table 4. 12). Generally, the combined application of P10Zn0.5 fertilizers on the genotype coded as ICGV00308 had the highest percentage of MRR that clearly confirmed that, the additional capital invested may economically feasible than the unfertilized treatment while the other genotypes with the fertilizer application was non-significant.
In addition, this rate of fertilizer application was economically acceptable than the other dominant treatments although the marginal rate of return obtained from all dominant treatment was above the minimum acceptable marginal rate of return. This implies that for one birr invested in groundnut production, the producer can get 3.8 ETB. The most important key finding of this result would be low input cost and relatively good seed yield of the genotype. While the lowest marginal rate of return (26%) from the highest combined fertilizer level on the genotype ICGV00308 due to the cost of fertilizer (Table 12).