Introduction

Most of the research system in Africa emphasizes on managing the three macronutrients nitrogen (N), phosphorus (P), and potassium (K) (Kang and Balasubramanian 1990; Smaling et al. 1993; Smaling and Braun 1996). In this region, crop production is critically dependent on sub-optimum nutrient application, especially N and P which are very low in amount with less than 8 kg ha/year (Crawford and Jayne 2010; Morris et al. 2007; Smaling 2006). Such agricultural practice accelerates the depletion of other macronutrients and micronutrients (Cobo et al. 2010; Sanchez 2002; Smaling et al. 1997) and causes negative soil nutrient balances. In Ethiopia, the nutrient depletion has been 41, 6, and 26 kg/ha/year for N, P, and K, respectively (Stoorvogel and Smaling 1990). This problem is aggravated by the inherent poor fertility in most tropical soils (Okalebo et al. 2003). These practices, consequently, lead to the less responsive or non-responsive soil (Foli 2012; Vanlauwe et al. 2014). When farmers applied fertilizers on this soil, they did not get benefit by increasing crop productivity. Due to this reason, farmers became reluctant to apply inputs, besides the cost of fertilizer is increasing.

A study conducted across Ethiopia showed that most sites are low in soil organic carbon (SOC), available N, and available P, K and S contents and some micronutrients (Hailu et al. 2015; Laekemariam et al. 2016). These nutrient deficiencies can affect not only plants but also soil microbes including rhizobia populations (O’Hara 2001) and its activities such as N2 fixation (Giller 2001). To lessen these problems, the use of chemical fertilizer has been limited mainly because of little accessibility and lack of buying ability of the poor farmers in the region (Morris et al. 2007; Giller et al.1998; Mugwira and Murwira 1997; Rufino et al. 2010). For instance, mineral fertilizers in Africa cost at the farm gate, two to six times as much as in Europe, North America, or Asia (Sanchez 2002). Therefore, organic inputs are a viable alternative source of plant nutrients for resource-poor farmers. Application of organic input usually leads to increased crop yields (Ogundare et al. 2012; Kimetu et al. 2004; Mugwira 1984; Vanlauwe et al. 2001a, b). However, yield reduction by organic fertilizer has also been reported (Mugwira and Murwira 1997; Nhamo 2002). The negative effect of low-quality organic fertilizer on the productivity of peanut has been resolved when it has been applied it’s with chemical fertilizer and/or fly ash (Burgos et al. 2006).

As residue quality influences the rate of decomposition, the amount of nutrients immobilization–mineralization and timing of nutrient release (Heal et al. 1997). Vanlauwe et al. (2005) found that Class I residues caused net N mineralization, Class II residues had no effect on mineral N, and Class III and IV residues resulted in net N immobilization. The addition of fertilizers with intermediate quality organic inputs may increase organic matter decomposition (Sakala et al. 2000; Zingore et al. 2003) and, thus, enhances the crop production. Animal manures and composts have shown in several trials to increase nutrient availability and to partly substitute mineral fertilizers (Goyal et al. 1999). However, availability of manure in smallholder farms was generally lower in Ethiopia (Lupwayi et al. 2000). In this region, some crop by-products such as Khat leftover are available for use as soil amendments because of fewer alternate uses.

Keeping the above in view, the present investigation was undertaken with a major objective to make the influence towards growing peanut production by promoting the use of integrated nutrient management that is capable of increasing soil fertility status in the small plot based farming systems in the Eastern Ethiopia using locally available resources. The specific objectives of the study were to compare the effect of combined application of different organic input and inorganic fertilizer with Bradyrhizobium inoculation on nodulation and yield of peanut and to assess how these inputs influence selected soil properties.

Materials and methods

Description of the study sites

Field experiments were initiated during the rainy season of 2014 cropping season at Fedis (09°06.941′N and 042°04.835′E at an altitude of 5476 ft above sea level [asl]) and Babillae (09°13.234′N and 042°19.407′E at 5478 ft asl) experimental sites, Eastern Hararghe, Ethiopia under Haramaya University. The study sites comprise lowland and midland agroecological zones that obtain 400–800 mm of annual rainfall in a bimodal pattern. In normal years, 60–70% of the rainfall occurs during the main rains season, between May and October, while the rest falls during the short rains between March and May. At Fedis site, the mean maximum and minimum temperatures are 27.8 and 8.8 °C, respectively, with annual mean rainfall of 714.5 mm. The climate of the Babillae is semi-arid tropical, with the mean annual air maximum and minimum temperature for the duration from 2000 to 2014 of 20 and 9 °C, respectively, and the mean annual precipitation of about 566 mm.

The topography of Babillae is rolling with low SOC concentration (0.56%) and total N (0.06%), available P (2.22 mg/kg), cation exchangeable capacity of 6.26 cmol(+)/kg, exchangeable Ca+2, Mg+2, Na+1 and K+1 of 4.18, 3.5, 0.15 and 0.34 cmol(+)/kg and sand, silt, clay contents of 76, 6 and 18%, respectively (Tekalign 1991). The soil pH and electric conductivity are 6.66 and 0.04 mS/cm, respectively.

The surface soil in the plot area in Fedis site before commencing the experiment is a silty clay loam, containing 36% sand, 45% silt, and 19% clay. The soil (0–20 cm depth) had a pH of 7.76, electric conductivity of 0.06 mS/cm, ammonium acetate-extractable K of 1.09 cmol(+)/kg, Mg of 12.87 cmol(+)/kg, Na of 0.12 cmol(+)/kg, Ca of 23.12 cmol(+)/kg with cation exchangeable capacity of 32.22 cmol(+)/kg. The soil had low organic carbon (1.32) and Olsen extractable P (1.78 mg/kg) and medium total N (0.12%) (Tekalign 1991).

Source of the test variety

Groundnut variety “Baha Jidu” which has been recently approved as high yielder in this region was obtained from groundnut improvement project, Haramaya University, Ethiopia. BaHa-jidu variety is the runner type and medium seeded (Kebede and Bushra 2012).

Organic fertilizer sampling and analyses

The farmyard manure was prepared mainly from cow dung and hay, which is normally used as a bedding material in the cow shed. Compost was prepared following the conventional method using locally available khat leftover organic waste. The following properties were analyzed according to the recommended testing methods: pH by potentiometric; Ammonium acetate extraction, flame photometry for available K determination; organic C by loss of weight on ignition; Olsen (available P); Kjeldahl (total N); DTPA extraction(Zn); Azomethine-H method (available B); KCl extract (NO3 −) and KCl extract-Magnesium oxide distillation (NH4 +). The average nutrient composition of FYM and compost applied in the experiment during this period are given in Table 1.

Table 1 The selected chemical properties of khat leftover and manure compost
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Treatments and crop management

The experiment consisted of eleven treatments: (1) 2 ton manure/ha + 4 ton compost/ha + no inoculation; (2) 2 ton manure/ha + 4 ton compost/ha + Bradyrhizobium inoculation; (3) 2 ton manure/ha + no inoculation; (4) 2 ton manure/ha + Bradyrhizobium inoculation; (5) 4 ton compost/ha + no inoculation; (6) 4 ton compost/ha + Bradyrhizobium inoculation; (7) DAP—(46 P2O5 kg/ha + 19 kg N/ha); (8) (46 P2O5 kg/ha + 19 kg N/ha) + Bradyrhizobium inoculation; (9) 20 kg N/ha; (10) 20 kg N/ha + Bradyrhizobium inoculation and (11) the control check. The rates of organic inputs were developed based on its inorganic N (NO3 − and NH4 +) concentration. The treatments were replicated three times, and laid out according to a Randomized Complete Block (RBD) design, with plot dimension of 3 m × 3 m.

The experimental plots preparation involved one plowing immediately after getting the first rainfall in June followed by blade harrowing. The organic inputs are surface applied and incorporated minimally with a hoe to a depth of approximately 10 cm. Inorganic N and P fertilizers were applied in the form of urea and triphosphate, respectively. The entire dose of inorganic and organic fertilizers was applied as basal at the beginning of growing season as per the treatment.

Soil sampling and analysis

After the peanut harvest in November 2014, soil samples were taken from 0 to 20 cm soil layers from each plot of two experimental sites. In each plot, the soil samples were collected from four points and were mixed to get a composite sample. Soil samples were air dried, gently ground and passed through a 2-mm sieve. The major chemical composition of the manure and compost was then analyzed using standard laboratory methods of soil and plant analysis.

Agronomic data collection

At the R 2 stage of peanut, five plants from the central three rows were uprooted. The nodulation status (nodule number dry eight) and shoot dry weight were recorded. At harvest, the pod weight, the total biomass yield, and the kernel weight were measured. The yield was calculated by harvesting central three rows of peanut (3.6 m2).

Plant samples and analysis

At late flowering stage (R 2), three plant samples were uprooted for plant N tissue analysis. The oven dried plant sub-samples were then ground and analyzed for N by the micro-Kjeldahl procedure.

Statistical analysis

Statistical analyses of the data were done using the SAS version 9.2 to analyze variance and to determine the statistical significance of the treatment effects. Analysis of variance (ANOVA) was performed on a fully randomized plot design to test for significance of treatments and means were compared by least significance difference (LSD) at the 5% level (SAS, 1996).

Results

The nodulation and yield of peanut showed a significant response to organic (compost and manure) and inorganic fertilizer (Urea and DAP) integrated with Bradyrhizobium inoculation (Tables 2, 3, 4). At Fedis, the nodule number was found to be increased by 141, 142.8 and 143.6% due to the applied compost (C) + Bradyrhizobium, Manure (M) + Bradyrhizobium, and DAP + Bradyrhizobium, respectively, compared to the control check (Table 2). Only DAP + Bradyrhizobium application resulted in a significant increase in the nodule number of peanut at Babillae. However, the treatments did not affect the pooled NN peanut. Likewise, the nodule dry weight of peanut at Babillae was enhanced significantly by M + C + Bradyrhizobium and C + Bradyrhizobium application (Table 2). At Fedis, an increase in nodule dry weight of peanut by starter N (20 kg N/ha) application was found. Application of M + C + Bradyrhizobium and C + Bradyrhizobium resulted in a significant increase in the pooled nodule dry weight.

Table 2 Nodule number, nodule dry weight and shoot dry weight of peanut as affected by combined application of organic and inorganic fertilizer application at Babillae and Fedis sites, eastern Ethiopia
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Table 3 Number of pegs per plant, hundred seed weight and total biomass yield of peanut as affected by combined application of organic and inorganic fertilizer application at Babillae and Fedis sites, eastern Ethiopia
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Table 4 Total weight of pods per hectare, shelling percentage, grain yield and plant tissue N accumulation of peanut as affected by combined application of organic and inorganic fertilizer application at Babillae and Fedis sites, eastern Ethiopia
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Excluding C + M, the other organic and inorganic combination of fertilizer improved the effectiveness of Bradyrhizobium inoculation on nodulation at Babillae site (Figs. 1a, 2a). At Fedis site, manure and compost application enhanced the effectiveness inoculation on nodule number and dry weight at Fedis site (Figs. 1b, 2b). However, starter N and DAP application did not improve the effectiveness of Bradyrhizobium inoculation at Fedis site.

Fig. 1
figure 1

Effect of Bradyrhizobium inoculation on nodule number of peanut at a Babillae and b Fedis experimental sites

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Fig. 2
figure 2

Effect of Bradyrhizobium inoculation on nodule dry weight of peanut at a Babillae and b Fedis experimental sites

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There was no significant effect of organic and inorganic fertilizers’ combination with Bradyrhizobium on shoot dry weight measured at late flowering and a number of pods per plant at Babillae site (Table 2). In contrary, M, C, and DAP applied with Bradyrhizobium caused a significant positive influence on the shoot dry weight at Fedis site. The pooled shoot dry weight significantly increased by DAP applied with Bradyrhizobium inoculation. The plants receiving DAP + Bradyrhizobium and M + Bradyrhizobium were found to record significantly higher number of seed per plant than the control at Babillae and Fedis sites, respectively. The pooled number of seeds per pod was not affected by the treatments.

The highest total biomass yield (kg/ha) of peanut at Babillae and Fedis was recorded by combined application of manure, compost and Bradyrhizobium (Table 3). This treatment increased the total biomass yield by 46.3 and 35.1% over the control check at Babillae and Fedis site, respectively. The total pod’s weight (kg/ha) under application of M + C + Bradyrhizobium and Urea + Bradyrhizobium was significantly (P < 0.05) superior to that of control check at Babillae site (Table 4). Similarly, the total pod’s weight at Fedis and pooled total pods weight were found to be significantly increased due to combined application of manure, compost and Bradyrhizobium.

The shelling % was not influenced by the treatment at Babillae site (Table 4). However, most of the organic and inorganic fertilizers applied with Bradyrhizobium inoculation encouraged the pooled shelling % at Fedis site. A significant increase in plant N accumulation at Babillae site was found in combined application of manure, compost and Bradyrhizobium. Most of the organic and inorganic fertilizer combination increased the accumulation of N in plant tissue at Fedis site and the pooled plant N accumulation (Table 4).

At Babillae site, manure and compost applied with Bradyrhizobium inoculation significantly increased the plant N accumulation compared to sole application of manure and compost (Fig. 3a). Bradyrhizobium inoculation did not increase the plant N accumulation when integrated with manure and compost, and manure at Fedis site (Fig. 3b), but inoculation improved the plant N accumulation when integrated with DAP and Urea.

Fig. 3
figure 3

Effect of Bradyrhizobium inoculation on grain yield of peanut at a Babillae and b Fedis experimental sites

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The kernel yield of peanut was significantly influenced by Bradyrhizobium inoculation integrated with organic and inorganic fertilizer at Babillae site (Table 4). At this site, the highest kernel yield was recorded at Bradyrhizobium integrated with manure, followed by manure applied with compost. However, Kernel yield at Fedis site and pooled kernel yield were not significantly affected by the treatments. The highest kernel yield (1562.1 and 1818.8 kg/ha) of peanut at Babillae and Fedis site was 66.6 and 44.1% increase over unamended control of respective sites, respectively.

Inoculating Bradyrhizobium integrated with compost and manure recorded the higher kernel yield than those organic fertilizers without inoculation at Babillae (Fig. 4a). However, compost and manure application together did not enhance the effect of Bradyrhizobium on kernel yield. At Fedis site, Bradyrhizobium in combination with organic and DAP application produced higher kernel yield than those obtained from fertilizer without inoculation (Fig. 4b).

Fig. 4
figure 4

Effect of Bradyrhizobium inoculation on plant N accumulation of peanut at a Babillae and b Fedis experimental sites

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The effect of organic and inorganic fertilizers on soil N and organic carbon was not significant at Fedis site (Table 5). The treatments did not also significantly influence the pooled soil N and organic C content. Slight increase in soil N and organic C by the organic and inorganic application was found at Babilale site. However, organic and inorganic application significantly improved the soil available P in both experimental sites. The pooled available P significantly increased when applied DAP alone and in combination with Bradyrhizobium over the control check.

Table 5 Total N, organic matter and available P of Babillae and fedis soils as affected by combined application of organic and inorganic fertilizer application
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Discussion

The result from this experiment showed that Bradyrhizobium inoculation in conjunction with organic (Compost and manure) and inorganic fertilizer (NP) significantly increased the nodulation and yield of peanut at both experimental sites. Inoculating Bradyrhizobium integrated with organic fertilizer and DAP at both sites were found to increase significantly the nodulation when compared to the corresponding sites control check. This finding is similar to those reported by Panda et al. (2012) who found that poultry manure boosts the effectiveness of Rhizobium in cowpea. With the present study, all fertilizer combination improved the effectiveness of Bradyrhizobium inoculation on the NN and NDW at Babillae site. However, an increase in NN and NDW due to inoculation was recorded only with the sole and combined application of manure and compost at Fedis. These difference results could have been attributed to the correction of the deficiencies of essential macronutrients and micronutrients on top of NP in Babillae soil. Previous work on organic fertilization effect on the soil has found to buildup of essential plant nutrients (i.e., N, P, K, S, Ca, Mg, Zn, Fe, Mn, and B) in the soil (Dotaniya et al. 2016; Ogundare et al. 2012; Rezig et al. 2012). Organic manure application enhanced the native rhizobia population nodulating cowpea by 23% above control (Kimiti and Odee 2010). This positive influence leads to enhance the root growth and the uptake of nutrients (Ibrahim et al. 2011) and thus improve the nodulation (Basu et al. 2007; Mohammadi et al. 2011; Tsai et al. 1993). This observation is in agreement with other studies where organic matter has been shown to increase the viable number of rhizobia and nodulation of peanut (Basu et al. 2008). These authors found a significant increase in native rhizobia population and nodule formation.

The application of organic fertilizer rich in nitrate did not suppress nodulation in both experimental sites though this N-rich material with C: N ratio less than 17:1 enhanced mineralization by microorganisms. However, the negative effect of N on nodulation did not observe. Lack of inhibition effect of N might be because of the attenuated effect of other essential nutrients found in the organic input (Burgos et al. 2006). Wu and Arima (1992) reported that N applied with other nutrients had increased nodulation whereas N applied alone reduced the nodule formation. This result suggests that application of medium quality organic fertilizer is essential to enhance nodulation in degraded and low fertile sand soils of Ethiopia.

Organic and inorganic fertilizer application did not increase significantly the shoot dry weight measured at the late flowering stage at Babillae site but it was increased significantly at Fedis site. The effect of organic fertilizer with Bradyrhizobium inoculation at Babillae site did not improve the number of pod per plant and hundred seeds weight. However, the treatment where Bradyrhizobium was combined with DAP significantly increased the hundred-seed weight at Babillae site. Insufficient nutrient supply at flowering stage of the plants due to immobilization (Petersen et al. 2005) might have been the reason, which limited the remarkable effect of organic inputs. Limitation in plant growth due to nutrients immobilization particularly N and P had also been stated by Herencia et al. (2011).

In contrary, there was an increase in a number of pods per plant, hundred seeds weight, and shoot dry weight of peanut by Bradyrhizobium when integrated with organic fertilizer (manure and/or compost) at Fedis site. This result indicates the need of organic fertilizer application at Fedis site beside Bradyrhizobium inoculation. This increase in pod number might be through reducing bulk density besides supplying adequate nutrients (Mittra et al. 2005), thereby increasing pegging and pods formation.

The highest total biomass yield, kernel yield and plant N accumulation of peanut at either of the experimental sites were found where Bradyrhizobium in conjunction with organic fertilizer was applied. This shows the importance of organic fertilizer application in increasing yield of peanut. This pronounced effect of organic inputs on final yield of peanut could be associated with the mineralization and releasing of nutrients from the organic inputs at a late stage of the plants. It has been known that organic fertilizer is the major source of mineral nutrients (Eghball et al. 2002). These responses of grain yields to organic fertilizer are consistent with other studies on peanut such that there was a significant increase in peanut production by organic input enriched by fly ash (Burgos et al. 2006). The benefits of organic fertilizer and inorganic fertilizer application in combination with rhizobia on the agronomic productivity of food legumes have been previously reported in chickpea (Namvar et al. 2013; Shahzad et al. 2013). The present results are also in accordance with the findings of Shahzad et al. (2013) who demonstrated a significant increase in soil nutrients including available P by organic matter application.

The effect of organic and inorganic fertilizer on soil N and organic C was not significant at both sites indicating that one year application organic fertilizer did not affect the soil N and soil organic matter. However, the results showed that DAP and organic inputs increased the available P in both soils. The increase in P due to organic fertilizer application could be organic acids production as a result of microbial decomposition of organic matter, which can solubilize the native unavailable inorganic P beside serve as source of inorganic P (Ramesh et al. 2009).

Conclusion

In general, organic fertilizer applications are relevant to boost the effectiveness of Bradyrhizobium on nodulation and yield of peanut in degraded and sand soil of eastern Ethiopia. Compost and farmyard manure prepared from locally available materials and inoculation of elite Bradyrhizobium isolate are needed to increase the productivity of peanut sustainably in sandy and degraded soil of eastern Ethiopia. Although this study finds a handful of beneficial effects of organic amendments with Bradyrhizobium on the yield of peanut, its effect on selected soil properties has been negligible. Therefore, further study on the long-term application of organic inputs effect on soil fertility and peanut production would be suggested.