Soil samples (0-15 cm) were collected from a long-term experimental field located at the Bangladesh Rice Research Institute farm. The soil is Aerie Haplaquept. The treatments of long-term experiment were: absolute control (T1), NPKS = 40-8-12-5 kg/ha (T2), NPKS = 80-16-24-10 kg/ha (T3), NPKS = 120-24-36-15 kg/ha (T4), T2 + cow dung (5 t/ha) and rice husk ash (2.5 t/ha) (T5), T3 + cow dung and ash (T6). The experiment was continued for eight years. The initial soil characteristics were clay loam in texture, pH 6.4, organic carbon content 0.13%, total nitrogen content 0.08%, 9 ppm Olsen P, exchangeable K 0.2meq/100g, available S 14 ppm and available Zn 1.0 ppm. Soil samples for the present study were collected from the T1, T3 and T6 treated plots after eight years of cropping and the soil samples were labeled as T1, T2, and T3 respectively. The soil samples were then air dried, passed through 2 mm mesh sieve and stored in plastic bags prior to use. Phosphate adsorption was studied for the collected samples by shaking 2 g of soil in 20 ml of 0.01M CaCl2 solutions containing each of 0, 15, 30, 45 and 60 mg P/ml as KH2PO4 in 50 ml centrifuge tubes for 24 hours at room temperature (28°C). After shaking, the tubes were centrifuged at 10,000 rpm for 5 minutes and P in the supernatant was estimated following Murphy and Riley (1962) method. The adsorbed P was calculated from the difference between initial and final concentration of P in the solution. The equilibrium P concentration data were interpreted from Langmuir, Freundlich and Temkin equations as described below:
Langmuir equation:
C/(x/m)= 1/kb + C/b ……………. (1)
Where, C is the equilibrium P concentration (mg/L), x/m is the mg P sorbed per kg soil, b is the adsorption maxima (the maximum adsorption capacity) and k is a constant related to the bonding energy of adsorption. A plot of C (x/m) versus C gives a straight line. The constant b and k were obtained from the slope and intercept.
Freundlich equation:
x/m = aCb ………………………(2)
By rearranging,
Log(x/m) = log a +b log C ……(3)
Where, x/m is mg P adsorbed per kg soil, C is the equilibrium P concentration (mg/L) and a and b are constants. A linear plot of Log x/m versus Log C leads to a and b from the intercepts and slope respectively.
Temkin equation:
x/m = a + blnC…………………..(4)
Where, x/m is the amount of P adsorbed on unit mass of adsorbent (mg/kg), C is the equilibrium P concentration (mg/L) and a and b are constants. A plot of x/m against lnC gives a straight line. The constant a and b are obtained from intercept and slope. The b of the equation (4) is considered as the P buffering capacity.