Tomato Yield and Economic Performance Under Vermicompost and Mineral Fertilizer Applications

2.1. Experimental Design and Treatments

The present study was carried out in Northern Ethiopia, at Aksum University Selekleka Demonstration Site during 2016/2017. The experimental area is situated at 14° 08’ 57”N latitude and 38° 17’ 02” E longitudes at an altitude of 1864m. According to the climatic zone classifications of Ethiopia, Alemneh [20] which was based on altitude, rainfall, average annual temperature and length of growing season; the study areas belong to cool sub-humid agro-climatic zone. The study area receives mono-modal type of rainfall with the main rainy season extended from June to September with a long-term average annual rainfall of 1031.3mm. The average temperature of these study area for the past 15 years revealed 20.92°C with a mean maximum temperature record (30.97°C) in April and the mean minimum (11.4°C) in January.

Shanti-PM variety of tomato was used as a test crop to evaluate the agronomic and economic performance of tomato under vermicompost and mineral fertilizer (Urea and NPS) combination levels using drip irrigation. Eight fertilizer combination treatments were laid out in RCB design with three replications. The treatments were:

1. Control (zero vermicompost and mineral fertilizer use)
2. Full recommendation of mineral fertilizer (150kg ha^-1 NPS and 150kg ha^-1 urea)
3. 4 ton ha^-1vermicompost + 25% recommended mineral fertilizer
4. 4 ton ha^-1vermicompost + 50% recommended mineral fertilizer
5. 6 ton ha^-1vermicompost + 25% recommended mineral fertilizer
6. 6 ton ha^-1vermicompost + 50% recommended mineral fertilizer
7. 8 ton ha^-1vermicompost + 25% recommended mineral fertilizer
8. 8 ton ha^-1vermicompost + 50% recommended mineral fertilizer

The recommended amount of mineral fertilizer used was 150kg ha^-1 urea combined with150 kg ha^-1NPS fertilizer for tomato production [21]. The nitrogen content in 100kg urea is 46kg. Whereas, 100kg NPS fertilizer constitutes 46kg phosphorus, 18kg nitrogen and 7kg sulfur. Vermicompost was applied 14 days before sowing and thoroughly mixed in the upper 15-20cm soil depth. The blended fertilizer was applied all during sowing. To minimize loss and increase efficiency, urea was applied as a split at planting and during the flowering period. Tomato seeds were sown in a flat nursery bed of 1m by 5m area. Seedlings were transplanted from nursery after 35 days and planted in the main field at a spacing of 45cm between plants and 120cm between rows. The plot size was 4.5m by 6m. All agronomic practices such as weeding, cultivation, irrigation, diseases and insect-pest control were kept the same for all the experimental plots throughout the growing season.

2.2. Data Collection and Analysis

A composite soil sample was taken from two depths of 0-20cm and 21-40cm using auger prior to transplanting to represent the entire experimental field. From this study area, 30 subsamples were collected, dried, composited, and sieved through 2mm sieve to have a 1kg composite sample. The collected soil samples were then analyzed for selected soil properties mainly soil texture, pH, EC, organic matter content, at Shire Soil Laboratory and total N, available P, and available S at National Soil Testing Center, Addis Ababa. Routine soil analysis procedures described in the soil and plant laboratory manual by Sahlemedhin and Taye [22] were followed to determine the selected pre-planting characteristics of the experimental soil. Total Nitrogen was determined by macro Kjeldahl method [23]. Available phosphorous content of the soil was determined by Olsen method [24] and available S was determined using the turbidimetric method [25].

The fresh marketable, unmarketable and total tomato fruit yield was recorded from the three middle rows of 12 tomato plants to avoid border effect. Harvesting of fresh tomato was done four times and the values were added to determine total yield. All the data collected from each plot were first checked for their normality and analyzed statistically through Analysis of Variance (ANOVA) using Statistical Analysis System version 16 software package [26]. Least Significance Difference (LSD) tests were used for mean comparison among treatments.

Besides, an economic analysis was carried out for every treatment using the Partial budget analysis (also known as marginal analysis) based on the CIMMYT (International Maize and Wheat Improvement Center, known by its Spanish acronym CIMMYT for Centro Internacional de Mejoramiento de Maíz y Trigo) approach [27]. The partial budget analysis was carried out to indicate the economic superiority of alternative treatments over the control treatment by estimating the varying costs and benefits based on the current market prices for 2017.

The varying fertilizer and labour costs were estimated based on the existing rate of fertilizer purchase and daily labourer payment. Since vermicompost was prepared from the locally available decomposable raw materials, only labour cost for the vermicompost preparation was included and the cost of locally available raw materials was excluded. For the partial budget analysis, only costs that vary among all the treatments (fertilizer and labour costs) were considered to obtain Total Varying Cost (TVC). Basically, partial budget analysis is the computation of Marginal Rate of Return (MRR) compared with Acceptable Minimum Rate of Return (AMRR).

Identification and elimination of inferior treatments (dominance analysis) was carried out first by listing the treatments in order of increasing cost variation. The marginal rate of return was compared with the farmers’ minimum acceptable rate of return to identify the economically preferable treatment. Farmer’s acceptable minimum rate of return to adopt an alternative treatment/technology ranges between 40% and 100% [27, 28]. However, many researchers [29-31] suggested a minimum rate of return to be 100%, especially for poor farmers in developing countries. Accordingly, in this study, 100% minimum acceptable marginal rate of return was considered.

3.1. Pre-Planting Soil Properties of the Study Area

According to Soil Survey Staff [32], textural class of the study soil was silt clay loam. Besides, soil of the study site has low organic matter content with around neutral pH and low EC level indicating nonsaline soil (Table 1). Value of the soil parameters decreases across depths except, pH level, which showed a relative increase with depth. Besides, nutrient status of the experimental soil prior to transplanting of tomato seedlings was low in both soil depths, except the concentration of phosphorus which was rated as high.

The soil property of the study area was in conformity with that of ATA [35], except the rate of P which was reported by ATA as very low in the study district.

3.2. Yield and Yield Parameters

It is evident that tomato fruit yield was markedly influenced by vermicompost and mineral fertilizer combinations (Table 2). Significantly surpassing marketable, unmarketable and total fruit yield performances were recorded when 8ton ha^-1vermicompost combined with 50% recommended dose of mineral fertilizer was applied. This result agrees with the findings of Singh et al. [12] who found that application of 7.5 ton ha^-1 of vermicompost along with 50% of recommended NPK fertilizer (60:30:30 kg ha^-1) was optimum for obtaining better quality and productivity of field grown tomatoes. Similarly, Mengistu et al. [16]; Islam et al. [36]; Chatterjee et al. [37]; Ogundare et al. [38]; Olatunji and Oboh [39]; and Togun et al. [40] also reported an increased tomato yield when vermicompost and mineral fertilizer were applied in combination.

All the fertilizer combination treatments had a marketable yield advantage over the control and over the sole mineral fertilizer, except the treatment which received 4 ton ha^-1vermicompost together with 25% dose of recommended mineral fertilizer scored lower marketable fruit yield than the full dose of mineral fertilizer recommendation. As suggested by Srivastava et al. [41], the integrated application of vermicompost and mineral fertilizer in appropriate ratio resulted in a more balanced nutrient supply to crops. This might have led to increased uptake of essential nutrients which in turn resulted in the increased vegetative growth of the plant to help for better carbohydrate build up that subsequently contributed to higher fruit yield and quality components [42].

Moreover, as Namazi et al. [43] elaborated, adding vermicompost to the soil not only increased the nutritious elements needed for the plant but also improved the soil environment, encouraging the proliferation of roots to draw more water and nutrients from larger areas, finally resulting in an improved biological function of the plant. However, almost all of the unmarketable yield records in the present study were due to the occurrence of the blossom end rot. The incidence of blossom end rot increased with increasing application rates of vermicompost and mineral N. The highest unmarketable yield, 5.37 ± 0.28 and 4.64 ± 0.45 ton ha^-1 were recorded when the tomato was cultivated under 8 ton ha^-1 vermicompost combined with 25% and 50% mineral fertilizer levels, respectively. This implies, though plant nutrients sourced from vermicompost and mineral fertilizer increase tomato fruit yield, this yield may not be marketable due to the occurrence of blossom end rot which might be caused due to imbalanced water and nutrient levels.

3.3. Partial Budget Analysis

The result displayed in Table 3 reveals that, higher net return, 10760.88 USD was recorded when the tomato was cultivated using 8 ton ha^-1of vermicompost combined with 25% mineral fertilizer recommendations. Lower net return, 5630.86 USD was observed when the tomato was grown with no any fertilizer applications. The maximum MRR, 1362.09% was analyzed when the tomato was cultivated using full recommendation of mineral fertilizer followed by 1147.20% MRR recorded from tomato cultivated using 8 ton ha^-1vermicompost with 50% recommended mineral fertilizer. Nevertheless, result of the stepwise treatment comparison (also known as dominance analysis) indicated that the higher marginal rate of return, 5681.07% was recorded from tomato produced under the application of 4 ton ha^-1vermicompost combined with 50% mineral fertilizer recommendation.

However, treatments which received 4 ton ha^-1 and 6 ton ha^-1vermicompost combined with 25% recommended mineral fertilizer were dominant and resulted in a rate of return that was below the farmer’s minimum acceptable rate of return (100%). This implies that, when changing from no fertilizer application to 4 ton ha^-1vermicompost with 50% mineral fertilizer application, farmers can recover one USD plus an extra of 56.8 USD in net return for each one USD on average invested. On the other hand, application of 25% mineral fertilizer with 4 ton ha^-1 or 6 ton ha^-1vermicompost could lead to negative return of 11.88 USD/ha and 2.16 USD/ha for 1 USD invested, respectively. This was associated with the yield obtained from the application of 25% of the mineral fertilizer which was not significantly different from the yield obtained from the application of 50% mineral fertilizer in combination to 4-6 ton ha^-1vermicompost. However, the cost of their application has a difference. Therefore, the application of more than 4 ton ha^-1vermicompost in combination with mineral fertilizer was economically inferior in this study. However, Mengistu et al. (2017) reported that tomato cultivation using 50% recommended a dose of mineral fertilizer together with 7.5 ton ha^-1vermicompost was found to provide the highest rate of return.