The laboratory experiment conducted at the Department of Soil Science, Patuakhali Science and Technology University, Dumki, Patuakhali during January to June 2009 involves thirteen surface (0-15 cm) soil samples collected from cultivated rice fields in January to March 2007 after T-Aman harvest. The selected soil sampling sites includes 7 Upazila–Dumki, Kalapara of Patuakhali, Amtali of Borguna, Wuzirpur and Gouranadi of Barisal and Borhanuddin and Lalmohan of Bhola district. The collected soil samples were air-dried and ground to pass a 2-mm sieve and then mixed to form a composite sample.
Phosphorus sorption
Phosphorus sorption isotherm studies followed the procedure of Nair et al. (1984). Briefly, one gram of the soil sample was taken in 25 ml test-tube with various addition of P (0, 1, 3, 6, 12, 18, 24, 30, 37, 43, 55, 68 and 80 ppm) in 20 ml of 0.01M CaCl2 solutions separately. The resultant P content was 0, 20, 60, 120, 240, 360, 480, 600, 740, 860, 1100, 1360 and 1600 mg kg-1. Then the test tubes were shaken for 24 hrs on horizontal shaker with low oscillation. After equilibration, the supernatant was passed through a Whatman # 42 filter paper and the filtrate was analyzed for P following Murphy and Riley (1962) method. The equilibrium P concentration data were interpreted by Langmuir, Freundlich and Temkin Equations.
Calculation of Smax, Adsorption energy and P-buffering capacity
The phosphorus adsorption maximum (Smax) was calculated using the Langmuir adsorption equation (Eq. 1). Smax was calculated by regressing C/(x/m) versus C, where C is the equilibrium solution P concentration and (x/m) is adsorbed P. The reciprocal of the slope of the linear regression is Smax (Olsen and Watanabe, 1957; Syers et al., 1973). The energy of adsorption for phosphorus (binding energy) was also calculated from Langmuir adsorption equation, where k is a constant related to the bonding energy of adsorption. The P-buffering capacity of soils has been calculated from the Temkin equation (Eq. 3), where b is considered the P-buffering capacity.
Phosphorus depletion experiment was performed in pot culture with a test crop of local rice variety Mala. Four rice crops of 6 weeks duration were grown with P0 and P50.
Plant dry weight (g pot-1) was recorded and P uptake (mg pot-1) by rice plant was calculated. Relative dry matter yield (RDMY) of rice was calculated as
--------------------------------------------------(1)
Where
DM0 = dry matter yield without P application
DM50 = dry matter yield with 50 mg kg-1 P application
Relationship of RDMY of rice (1st crop) with soil available P extracted by different extraction methods and different fractions of phosphorus was tested. Critical level of different soil test P (Olsen P, Bray I P, Mehlich II and Mehlich III) for dry matter yield of rice was done by Cate-Nelson (1965) graphical method. Phosphorus concentration (%) in plant and P uptake (mg pot-1) by rice plant was determined at first, second, third and fourth harvest. Relationship between dry matter weight and P content, dry matter weight and P uptake of rice plant at different harvests were established. Different P pools were estimated.