Effect of Mulching and Amount of Water on the Yield of Tomato under Furrow Irrigation

Description of the Study Area

The experiment was conducted during the dry season From 2020 to 2021 at experimental field of Pawe Agricultural Research Station (11°18´40´´ to 11º19´29´´N 36º 24´ 26´´ to 36º 25´ 27´´E, 1120 m a.s.l.) to evaluate the effect of mulch and amount of water on the yield and yield components of tomato under furrow irrigation system and to assess the potential of deficit irrigation to improve the economic efficiency of tomato production. Pawe is located in the western part of Ethiopia and regarded as a warm temperate climatic zone where there is distinct dry months in winter. The soil of the experimental field was dark color (vertisol) with clay in its textural class. The area has an annual mean maximum temperature of 32.64 ºC and monthly values range between 27.72 ºC and 37.62ºC while, the mean annual minimum temperature is 16.49ºC and monthly values range between 12.03 and 19.54ºC during the experiment. The area receives a total annual rainfall of 405 mm (Figure 1).

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Experimental Design and Field Management

Four levels of irrigation viz. 125, 100, 75 and 50% of crop water requirement (ETc) with three mulches types viz. no mulch (NM); Dried banana leaf mulch (DBLM) and Dried elephant grass mulch (DEGM) were tested. The 4x3 factorial combinations with a total of twelve treatments were arranged in randomized complete block design (RCBD) with three replications. Recommended fertilizer doses (200 kg DAP ha-1 and 150 ka Urea ha-1) were used for all treatments. Total amount of DAP was applied at the time of final land preparation while urea was applied half at the time of transplanting and half at 45 days after transplanting [13]. The variety of Melka-Shola (processing type; released by Melkasa Agricultural Research Center) was used as the testing crop. Seedlings were raised before a month and undamaged, reasonably uniform and clean seedlings were selected and transplanted to properly prepared plots in unit plots of 2.8m×2.4m with 70cm×30cm spacing on December 20, 2020. Each plot consisted of four rows. The spacing between adjacent blocks and plots was 1.5 m and 1 m, respectively. For mulching, the dried and chopped banana leaf mulch (BLM) was applied at the rate of 2 kg m-2 while dried and chopped elephant grass mulch was applied at the rate of 2.5 kg m-2.

For irrigation application, polythene sheet was buried with a width of 60 cm to prevent water flow through seepage. Plot designing, furrow slope aligning and all specification was accomplished using water level. Parshall flume was installed at 7m away from the experimental plots to control the flow rate at different water head. Once it installed, water was released through it for calibration purpose by uniform velocity and constant head. Then, the water pass through the partial flume was directly given to plots and the time required was also recorded. The end of each furrow was blocked to prevent the out flow of tail water and also to maintain the required depth of water within the furrow.

Estimation of Crop Water Requirement

The actual crop evapotranspiration (ETa) was computed by multiplying the reference evapotranspiration (ETo) with crop coefficient (Kc) for different growth stages of the crop (Table 1). The crop water requirement (ETc) in the AquaCrop model was determined using the Penman–Monteith method following the procedures in Allen [14]. Irrigation was set until the soil reaches field capacity (back to field capacity). The Kc for different growth stages of tomato determined locally by lysimeter study were used in the calculation of actual crop evapotranspiration. Thus, volumetric water required for a tomato plant was computed as:

ETa (m3) = Kc × ETo (m) × projected area (0.3 × 0.7 m2).

IWA=AxETcxIixKr/Eax1000

where IWA is irrigation water applied (m3); A is area of plot (m2); Ii is irrigation intervals (day); Kr is coverage coefficient (Kr = (0.10 + Gc) ≤ 1) [15], Gc is ground cover; Ea is application efficiency (85%). Therefore, based on the model output, amount of irrigation water (mm) for 125%, 100%, 75%, and 50% ETc was 679.87, 543.9, 407.92, and 271.95 mm, respectively. Water use efficiency was determined as marketable fruit yield per unit of water applied [16].

Data Collection and Analysis

Soil Sampling and Analysis: A representative soil samples were collected from the experimental field before transplanting at three depths of 0–20, 20–40, and 40–60 cm using an Auger. A composite of disturbed soil sample was collected, dried, composited, and sieved using 2mm sieve to prepare a 1 kg composite sample for the determination of selected soil physical and chemical properties which include soil texture, organic matter, pH, EC, FC and PWP. Texture (particle size distribution) was determined using the Bouyoucos hydrometer method [17]; soil pH in a suspension of 1:2.5 soil-water ratio was determined by using a pH meter [18]; Organic carbon (%) was determined following the wet digestion method or titration method using chromic acid (potassium dichromate + H2SO4) digestion as described by Walkley and Black [19]. Organic matter content of the soil was estimated from the organic carbon content determined using the Walkley and Black [19] method. The FC and PWP were determined using pressure plate and membrane apparatus by applying a pressure of 1/3 (0.33) and 15 bars, respectively on saturated soil paste until no change in moisture is detected [20]. The ECe was determined by measuring the conductivity of saturated soil extract using electrical conductivity meter [21]. Soil analyses were conducted in the soil testing laboratory of Pawe Agricultural Research Center’s Soil Laboratory at the beginning of experiments. Tables 2 and 3 show some physical and chemical properties of the soil at the station.

Table 3: Some chemical properties of the experimental soil EC: electrical conductivity; OC: organic carbon; OM: organic matter content Agronomic Data Collection: Ripened tomato was harvested 9–10 times starting from the first week of February up to the second week of March. Fruit length, fruit diameter, number of fruits per plant, and marketable fruit yield were measured at each harvest time. Parameters like number of fruits per plant and marketable fruit yield (Ym) were summed up at the end of the experiment, while the values of fruit diameter and fruit length in each harvest were summed up and the average value was considered. The tomato plants in the middle rows were considered for data collection. To collect data on fruit diameter and fruit length, five randomly collected fruits were considered while the length and diameter of each fruit were measured using a digital caliper and the average value was computed for each plot. Economic Analysis: The cost of cultivation of tomato includes expenses incurred on land preparation, seeds, transplanting, mulching, weeding, irrigation water, and cost of harvesting. The economic analysis for every treatment was carried out using the partial budget analysis based on the CIMMYT [22] approach which utilizes partial budgeting combined with marginal analysis to determine the most economically acceptable treatment by estimating the varying costs and benefits based on the current market prices. Farm gate price of tomato yield harvested was calculated based on the existing market price. Statistical analyses: All data were subjected to analysis of variance (ANOVA) according to the procedure outlined by Gomez and Gomez [23] using SAS statistical software package 9.4 [24]. Treatment effects were analyzed by Fisher’s least significant difference (LSD). In all cases, differences were considered to be significant if P ≤ 0.05.

Tomato Yield and its Components

Tomato number of fruit per plant and average fruit weight was influenced significantly by different levels of irrigation and mulching treatments (Table 4). The number of fruits per plant varied significantly with different levels of irrigation and was maximum with 100% ETc and minimum with 50% ETc without mulch. Similarly, the heaviest fruit (45.95 g) was achieved with 100% ETc however; such result did not significantly differ from the treatment of 75% ETc. This may be due to the sufficient available water in the root zone for the 100% ETc treatment, thereby leading to an increase in both water and nutrient absorption. Table (4) indicates that both number of fruits per plant and average fruit weight were significantly affected by mulch treatments. The maximum number fruits (65.55) and heaviest fruit weight (47.16 g) were attained when EGM was applied; which, however, number of fruits per plant recorded from plants grown under EGM was not significantly different from BLM. On the contrary, the lowest number of fruits (63.88) and average fruit weight (35.38 g) were recorded from tomato plants grown with NM. This could be due to mulch improves the soil microenvironment, cools the soil, conserve soil moisture and return nutrients to the soil through decomposition. When optimum moisture is available for plant, fruits per cluster and total number of fruits per plant increase as flower dropping will be reduced. The result is in line with the findings of Abouziena and Radwan [25] who reported that mulching could mitigate the effects of water stress on plant growth and produce the maximum number of fruits and unit fruit weight. Candido [26] reported that pepper plants subjected to periods of water shortage exhibited a significant reduction in the number of fruit than plants in peppers with those regularly watered plants.

Means followed by the same letters within columns are statistically not significant at 5% level of probability, NM: no mulch, BLM: banana leaf mulch, EGM: elephant grass mulch, * significant at P = 0.05, significant at P = 0.01, * significant at P = 0.001 Table 4: Main effect of amount of water and mulch on the mean fruit yield components of tomato in 2021/2022 Tomato plants are sensitive to water stress and they show high correlation between evapo-transpiration and crop yield. The statistical analysis in Table 5 indicates that fruit diameter, fruit length, marketable and total fruit yield were positively influenced by the interaction of irrigation and mulching treatments. The fruit diameter, fruit length, marketable and total fruit yield increased by 127% 194% 138%, and 48% due to EGM integrated with 100% ETc irrigation, respectively as compared to NM with 50% ETc irrigation. In general, all aforementioned parameters of tomato increased with the increase in water supply without mulch up to 100% ETc. however, irrigation of the same level without mulch produced the lowest yield. The increased yield under EGM with full water regime might have resulted from better water utilization, excellent soil-water-plant relationship and higher uptake of nutrient. The yield increased due to the applied mulching, which can be attributed to a lower rate of water loss from the soil by evaporation, leading to significant conservation of soil moisture [27].

Table 5: Effect of amount of water, mulch and interaction of mulch and amount of water on the mean fruit yield and yield components of tomato in 202/2022. Means followed by the same letters within columns are statistically not significant at 5% level of probability, NM: no mulch, BLM: banana leaf mulch, EGM: elephant grass mulch, significant at P = 0.01, * significant at P = 0.001 Furthermore, the applied mulch increased transpiration, thereby leading to more photosynthetic efficiency that resulted in increased yield as reported by Liu [28]. The lowest values of marketable and total fruit yield of tomato at 125% ETc irrigation with all mulch treatments could be due to excess water in the soil decreased the oxygen diffusion rate in the root zone, which negatively affected crop yield. Similar finding was reported by Wan and Kang [29].

Different irrigation regimes combined with mulch has a significant influence on the water use efficiency of tomato. Agronomically, water use efficiency (WUE) is, simply the efficiency in which water is used to produce an economic yield. Table 6 shows that WUE varies both with irrigation regimes and mulches. Under all levels of irrigation, mulches with irrigation gave higher WUE over irrigation alone. As indicated in Table 6, a larger effect of mulches on WUE was observed when it was combined with lower irrigation regime. The application of 100% ETc (544 mm) combined with elephant grass mulch gave the maximum marketable (38.44 t ha-1) fruit yield. However, the highest water use efficiency value (8.76 kg m-3) was recorded at the lowest water level (272 mm) with elephant grass mulch, while the lowest WUE (2.45 kg m-3) was obtained at 680 mm without mulch treatment, which indicated that the dry elephant grass mulch clearly improve the water use efficiency of tomato (Table 6). To maximize WUE it is necessary to conserve water and to promote maximal crop growth. Maximizing WUE requires minimizing losses through runoff, seepage, evaporation, and transpiration by weeds.

The result is in accordance with Ayars [30] who reported that the lower the amount of water use, the higher was the WUE, so, low irrigation regime reduced deep percolation and increased water use from root zone soil. Similarly, Seyfi [31]

who showed that drip irrigation with black plastic mulch markedly decreased the amount of water applied, increased water use efficiency (WUE) and increased crop yield of cantaloupe.

Economic Analysis

The highest net return (ETB 480,777) was recorded in elephant grass mulch with 100% irrigation regime. In case of NM and BLM with the same irrigation regime, the returns were ETB 262, 677 and 315,327, respectively (Table 7). In order to recommend this result for farmers, it is necessary to estimate the minimum rate of return acceptable to farmers in the recommendation domain. According to CIMMYT [22], the minimum acceptable marginal rate of return (MRR) must be between 50 and 100%. There is no estimated cost for irrigation water in Ethiopia so far. Hence, considering the cost of irrigation water as zero with 10% prices increment of other variable costs, the highest net benefit was obtained via 544 mm with elephant grass mulch amid a net benefit (480,777 birr/ha) and a marginal rate of return (MRR) 552.40%. The marketable yield advantage of 544 mm/with EGM over 272 mm/without mulch was 134% (Table 8 and 9).