The experiment was conducted at the research farm of Irrigation and Water Management Division, Bangladesh Agricultural Research Institute, Gazipur, (latitude, longitude, and elevation from MSL were 24000'N, 90025'E and 8.40 m) during 2008 – 2009 using micro lysimeter. The weather data during the crop period was collected from the Agro-meteorological station which was very close to the experimental field. The soil was silty clay loam with field capacity and bulk density of 28% and 1.5 g/cc, respectively.
Potato seeds (variety: BARI Alu -7, Diamant) were planted on 25 November, 2008 in four lysimeter tanks maintaining row to row and plant to plant spacing of 60 cm × 25 cm. Recommended fertilizer was applied at the rate of N121, P27, K125, S12, Mg7, Zn2, and B1 kg/ha and cowdung 5 t/ha. During final land preparation, half of N and total amount of other fertilizers were applied and rest of N was top dressed after 30 DAP. Intercultural operation (ear thing up after 30 DAP, and weeding) and pesticides were applied according to necessity. A pre-irrigation of 25 mm was applied in the field for proper seed germination and plant establishment. Also, to maintain a similar atmosphere, the same crop was grown in the entire field around the lysimeter tanks. The crop was harvested at 01 March, 2009. The selected treatments were as follows:
T1= Irrigation at 25 and 40 days after planting (DAP) allowing drainage (AD) T2= Irrigation at 25, 40 and 55 DAP AD
T3= Irrigation at 25, 40 and 60 DAP AD
T4= Irrigation at 25, 40, 55 and 60 DAP AD
The micro lysimeter contains 4 tanks, each having an area of 1.0 m2 which was designed and installed by Khan et al. (1993) at IWMD, BARI, Gazipur. As this was a controlled structure from surrounding soil so no further replication of the treatments was done (Islam and Hossain, 2010) for calculating ETc. CROPWAT software was used to calculate potential evapotranspiration (ETo) by inserting climatic data of maximum and minimum temperatures, air humidity, sunshine hours and wind speed together with the location information. The crop was irrigated in according to the design of the treatments. Measured quantity of water was applied to the tanks to ensure drainage. Soil moisture was measured in gravimetric method during plantation, prior to irrigation and at maturity to determine the irrigation quantity and depleted soil moisture. During the crop season, negligible amount of rainfall was occurred (Fig.4). Crop evapotranspiration (ETc) was calculated by using water balance equation (Eq. 2) (Itier et al., 1997). Therefore, the stored soil moisture during the period under consideration and drainage amount were subtracted from the applied water and rainfall to obtain ETc i.e.,
ETc = Wa – (Dw ± ΔSs) -------------- (1)
Where, ETc = Crop evapotranspiration in mm for time t Wa = Applied water + rainfall (mm) for time t Dw = Drainage water (mm) for time t
ΔSs = Stored soil moisture (mm) for time t
Then from the potential evapotranspiration (ETo) estimated for the specified period, the value of Kc for the period was determined using the following equation.
ETc = Kc * ETo --------------------------- (2)
At 65 days after sowing, the data on plant height were recorded from 8 plants in each lysimeter. The yield and yield contributing data (no of tuber/plant, tuber length, tuber diameter, and tuber weight/plant) were collected during and after harvest.