Collection and processing of soil samples:
The sampling site selected for collection of the soils is an adjacent area of Faridpur Sadar which belongs to Ishurdi series. The geographic location of the sampling site is 23°40.842’ N and 89°45.749’ E. The bulk of soil samples representing 0 - 15 cm depth from the surface were collected by composite soil sampling method as suggested by the Soil Survey Staff of the USDA (1951). The samples were collected with spade and dried in air for 3 days (40°C) by spreading in a thin layer on a clean piece of paper. Visible roots and debris were removed from the soil sample. After air drying, a portion of the larger and massive aggregates were broken by gently crushing them with a wooden hammer. Ground samples were screened to pass through a 2 mm stainless steel sieve. The sieved samples were then mixed thoroughly for making a composite sample. These soil samples were used for various physical analyses.
A portion of soil samples (2 mm sieved) was further ground and screened to pass through a 0.5 mm sieve. The sieved samples were mixed thoroughly for making composite samples. These soils were used for chemical and physico-chemical analyses. The rest of the soil samples collected for pot experiment was air dried, cleared off the debris and crushed to smaller clods and passed through a 5 mm sieve.
Analyses of soil samples:
Various physico-chemical properties of soils were analyzed in the laboratory. Soil pH was measured electrochemically using a glass electrode pH meter at a soil : water ratio of 1 : 2.5. Particle size analysis was done by Hydrometer method. The available and total N in soil were determined by alkali distillation either of the Kjeldahl digest or of the extract for available-N. The available P of soil was determined spectrophotometrically at a wavelength of 580 nm by developing blue color using ascorbic acid after extracting the soil with 0.5M NaHCO3. The available K of the soils was determined by flame photometer afler extracting the soils with lN ammonium acetate at pH 7. For the determination of total P, K and S in soil, the samples were digested with aqua-regia (HC1: HNO3 = 3:1). The total P content in the digest was determined colorimetrically in a spectrophotometer at 490 nm by developing yellow color with vanadomolybdate. Total S content was determined by turbidimetric method. The total K in the digest was determined by flame photometer. The procudures for the determination of pH, per cent moisture, particle size, available and total N, P, K and S were followed as outlined in Imamul Huq and Alam (2005). The determination of soil As was done by Hydride generation-Atomic absorption spectrometer (HG-AAS) after digesting the sample using aqua-regia (HC1 : HNO3 = 3 : 1) (Portman and Riley 1964). Total lead, cadmium, iron, manganese and zinc content of the background soil samples were determined by Atomic absorption spectrometer (AAS) following digestion with aqua-regia (HCl : HNO3 = 3 : 1).
Experimental Set-up:
For the pot culture experiment, seedlings of hybrid variety (BARI-3) tomato (Sola rium lycopersicum) were planted in the net-house of the Department of Soil, Water and Environment, University of Dhaka.
The treatments in the experiment were designed in two sets - soil with no arsenic treatment and soil with arsenic treatment. Recommended doses of fertilizers (NPK) were applied at three different rates via., 1/2 of recommended dose, 3/4 recommended dose and full recommended dose. Nitrobenzene was sprayed on to the growing plants having these three different rates of fertilizer doses. Therefore, there were 14 treatments including the control treatment (7 treatments in each set). Each treatment had three replications. Accordingly, there were 42 pots: 3 pots for each treatment; 21 pots for each set. The treatment combinations were as follows:
- Control (C): No fertilizer and no nitrobenzene was applied.
- Nitrobenzene (N): Only nitrobenzene was applied.
- 1/2 dose of recommended fertilizer (1/2 RD).
- 1/2 dose of recommended fertilizer plus nitrobenzene (1/2 RD+N).
- 3/4 dose of recommended fertilizer (3/4 RD).
- 3/4 dose of recommended fertilizer plus nitrobenzene (3/4 RD+N).
- Full dose of recommended fertilizer (FRD).
Spikîng of the soif was done with arsenic (As) in solution at a rate of 20 kg/ha. Na- arsenate was used as a source of arsenic. A solution was made by calculated amount of As and 100 ml of solution was added to each pot of the sets having As treatment.
Nitrobenzene solution was prepared by dissolving 10 ml of liquid in 1000 ml of water. Fertilizer was used according to BARC (2005) guideline. Nitrobenzene. solution was sprayed at 10, 15, 25, 30, 40 and 50th day of plant growth.
Processing of the plant samples:
Tomato plants were harvested at 100 days after planting. The plants were harvested by uprooting. The harvested roots were washed with deionized distilled water several times to remove ions from the ion free space as well as to dislodge any adhering particles on the root surface. The upper parts of plants were also washed. The plant samples were separated into two parts : aerial and underground. The fresh weights of the collected plant samples were taken, air dried and then oven dried at 70+5°C for 48 hours and the dry weight of the plant sample was noted. Afier grounding and sieving at 0.2 mm sieve the dried samples were preserved in small plastic bottles separately.
Analyses of plant samples:
Plant sample (0.5 g of both aerial and underground parts) was taken separately into 100 ml Pyrex glass tubes. Five m1 of HNO3 was added and the tubes were left for half an hour. Then the sample tubes were placed on a digestion block. Samples were normally predigested overnight over 50 - 70°C before increasing the temperature to 140°C for the final dissolution of organic material. When dissolution was complete, samples were diluted to 25 ml, shaken and filtered. This extract was used for the determination of P, K, S (Imamul Huq and Alam 2005) and As (Portman and Riley 1964) contents of the plant samples. The total N in plant samples was determined by Kjeldahl’s method following H2SO4 digestlor. (Imamul Huq and Alam 2005). All elements were calculated on oven dry basis.
Statistical analyses of data:
Data were statistically analyzed by using Microsoft Excel and/or SPSS (version 16) Packages.