Study area: The south western coastal zone is covered by the Sundarbans mangrove forest, covering greater Khulna and part of Patuakhali district. Greater Khulna district consists of nine Upazilas. Out of them four land sites of Jalma (Batiaghata), Krisnanagar (Dumuria), Kismat (Fultola), Bajua (Dakop) and four rivers, viz: Rupsha River ( Rupsha), Shailmari River (Koyra), Vadra River (Dakop) and Kazibachha River (Terokhada) were selected as the study sites. These sites were selected to investigate the top soil and river water salinity and their effects on crop production. The areas lie at 0.9 to 2.1 m above mean sea level. Soil characteristics of the western coastal zone are silty loam or alluvium.
Data collection: The study area was the south western part of Bangladesh more specifically, the Khulna district. In this study, two types of data were collected. The soil and river water salinity data of year 2004 to 2009 for study site was collected from “Soil Resources Development Institute (SRDI, 2009), Khulna” and crop yield data of 2004 to 2009 for Khulna from “Agricultural Extension Institute (AEI, 2009), Khulna”.
Data analysis: From the soil and water salinity data, monthly salinity of 2004 to 2009 was calculated and illustrated in graphs which faciliated better comprehension of top soil salinity pattern of the study area of Jalma (Batiaghata), Krisnanagar (Dumuria), Kismat (Fultola) and Bajua (Dakop) as well as river water salinity pattern of four rivers like Rupsha River (Rupsha), Shailmari River (Koyra), Vadra River (Dakop) and Kazibachha River (Terokhada), under greater Khulna district.
Analysis of topsoil and river water salinity (ECe:ds/m) conditions: Monthly variations of topsoil and river water salinity of the study area in different in year wise
IMPACT OF SALINITY OF SOIL AND WATER ON CROP PRODUCTION: Rising of salinity level will decrease agricultural production by unavailability of fresh water and soil degradation. Salinity also decreases the terminative energy and germination rate of some plants (Rashid et al., 2004; Ashraf et al., 2002). Ali (2005) investigated the loss of rice production in a village of Satkhira district and found that rice production in 2003 was 1,151 metric tons less than the year 1985, corresponding to a loss of 69%. The production rate of different types of crops in different Upazila in Khulna districts is shown from Figures 11 to 14.
Salinity adaptation strategy: Worsening sea water storm surges and over use of irrigation have left fields, wells and ponds in parts of southern Bangladesh too salty to grow crops, leading to a growing exodus of farmers from the region. During Cyclones Sidr and Aila, in 2007 and 2009 respectively, sea water was driven into ponds and rivers in Khulna, Bagerhat and Satkhira districts in southern Bangladesh, and some fields remained flooded by sea water long enough to raise levels of salinity in the soil and in underground aquifers used for irrigation. If we can control natural disasters then we can control salinity level. Now farmers on hundreds of thousands of acres in the region are watching their rice crops wither and die before reaching maturity. In some cases, farmers have sown rice plants several times in a season but seen none survive. Binoy Singh, a farmer in Surigati village in Bagerhat district, recently lost nearly his entire 10 - acre rice crop to salt contamination. Severe storm surges and sea level rise linked to climate change, as well as over use of irrigation, threaten to make soil salinity a worsening problem across broad areas of southern Bangladesh, a vast and heavily populated river delta region that sits barely above sea level. In the Tala, Debhata and Kaliganj sub-districts under Satkhira district, salinity in wells 70 to 80 feet deep is now 10 times higher than the tolerable limit for rice cultivation , researchers say. This poses a grave threat to food security in southern Bangladesh, and is causing farmers to relocate in sea rch of other work to feed their families. Institute researchers are installing wells in some of the worst hit areas in an attempt to find out whether there is water suitable for irrigation still available deeper underground. In many areas, farmers now have to dig wells at least 500 ft deep to get water that is safe for irrigation. Earlier, such water was available at 200 to 250 ft. Over the last 25 years, sea water from the Bay of Bengal has pushed 40 km inland throughout underground aquifers, replacing fresh water. Topsoil salinity in cropped lands came down to 4.0 ds/m or less at least one month earlier than fallow lands. Such a low salinity level opened up an opportunity to grow modern rice varieties (whether salt - tolerant or not) in the wet season . Taking advantage of rainfall, much earlier than the present cropping schedule offered a yield advantage of about 1.5 t/ha over the present productive level. Slightly saline groundwater was used to irrigate dry season crops. Of all the crops grown only se same responded positively to irrigation. Yields were not significantly different for any crop between irrigated and rain - fed conditions mostly because about 60 mm rainfall occurred at the pod formation stage of crops, a week after irrigation. Although groundwater was slightly saline, it reduced topsoil salinity when used to irrigate dry season crops. Therefore groundwater can be used to irrigate profitable non-rice crops in the dry season. The marginal rate of return indicated that sesame and mungbean were highly profitable in saline soil environments, but mungbean grown under rainfed conditions maximized farmer's income from dry season cropping. The deepest water table was within 1.0 to 1.5 m below ground surface at both locations. The aquifer was fully recharged and the water table remained either above or close to the soil surface from the last week of June to the first week of December.