Experimental site. The experiment was conducted during the winter seasons (November–March) of 2007–2008 and 2008–2009 at the experimental field of Bangladesh Agricultural Research Institute, Gazipur, Bangladesh (24.00°N, 90.25°E, 8.4 m a.s.l.). The soil of the experimental field was silt clay loam having a field capacity of 28.6%, wilting point 13.5% (dry weight basis) and bulk density of 1.46 g/cm3.
Experimental design and field management. Three levels of irrigation viz. 100, 75 and 50% of crop water requirement (ETc) with three mulches viz. no mulch (NM); black polyethylene mulch (PM) and paddy straw mulch (SM) were tested. There were nine treatment combinations as follows: T1 – drip irrigation at 100% ETc; T2 – drip irrigation at 75% ETc; T3 – drip irrigation at 50% ETc; T4 – T1 + PM; T5 – T2 + PM; T6 – T3 + PM; T7 – T1 + SM; T8 – T2 + SM; T9 – T3 + SM.
The experiment was laid out in randomized complete block design (RCBD) with three replications. Recommended fertilizer doses (N100, P100, K80 kg/ ha) for fertigation were used for all treatments. Total amount of P in the form of triple superphosphate (TSP) was applied at the time of final land preparation while N and K in the form of urea and muriate of potash (MOP), respectively, were applied with drip irrigation into four equal splits at 15 days intervals. The tomato (cv. BARI Tomato-3) seedlings 30 day old were transplanted in-unit plots of 4 m × 2.4 m with 60 cm × 40 cm spacing on 24 November 2007 and 28 November 2008. For mulching, 10 µm black polyethylene sheet having holes of 50 mm diameter at a distance of 60 cm × 40 cm was spread over the beds and tomato seedlings were transplanted in the holes. For straw mulch, paddy straw at 10 t/ha was used after 7 days of transplanting. For irrigation application, nine water tanks (each tank for each treatment combination) with drip system having a capacity of 250 L each were installed at a height of 1 m above the ground surface to irrigate 27 plots by gravitational flow. One plant was provided with a dripper of 3.5 L/h discharge capacity. The ripened tomato was harvested 9–10 times starting from the first week of February up to the second week of March.
Estimation of crop water requirement. The actual crop evapotranspiration was (ETa) computed by multiplying the reference evapotranspiration (ETo) with crop coefficient (Kc) for different growth stages of the crop. ETo was calculated on a daily basis from daily meteorological data (maximum and minimum temperature, relative humidity, wind speed and sunshine hours) using the CROPWAT 8.0 model (Rome, Italy). The model uses FAO Penman-Monteith equation, which was accepted as standard method to calculate reference evapotranspiration (Allen et al. 1998). 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 (0.4 × 0.6 m2)
Economic analysis. The cost of cultivation of tomato includes expenses incurred on land preparation, seeds, transplanting, cost of fertilizer, manure and their application, mulching, weeding, crop protection measures, irrigation water, and cost of harvesting. The cost for full irrigation (100% ETc) was considered as US$ 65/ha. The cost of drip irrigation system includes depreciation, current bank rate of interest, and repair and maintenance cost of the system. The useful life of the drip system and plastic mulch were considered to be 3 years and 1 year, respectively. The gross return from the produce was estimated from the prevailing average market price of US$ 115/t.
Statistical analyses. Treatment effects were analyzed using a one-way ANOVA followed by the Duncan’s multiple range test to calculate the least significant difference (LSD) between means. In all cases, differences were deemed to be significant if P < 0.05.