Bhattacharyya, D.RAJKHOWA, MONTRISHNA2020-06-112020-06-112019-07http://krishikosh.egranth.ac.in/handle/1/5810147489The study investigated the Phosphate (P) adsorption in four (4) major soil orders of Assam. It was conducted in sixteen (16) surface (0-30 cm) and sub-surface (30-60 cm) soil samples, collected from two locations each of the order Entisol, Inceptisol, Alfisol and Ultisol. There were wide variations in physico-chemical characteristics of the soils with pH, organic carbon, texture, clay content, CEC, AEC, per cent base saturation (PBS) and free oxides of Fe and Al which ranged from 4.86 to 5.60, 0.66 to 1.17 per cent, sandy loam to clay , 18 to 39.5 per cent , 6.72 to 19.73 cmol (P+) kg–1, 1.64 to 2.80 cmol (-) kg-1, 21.16 to 44.39 per cent, 1.08 to 3.21 per cent and 0.60 to 1.08 per cent, respectively. In the soils forms of phosphorus present in variable amounts but had the same sequence of abundance viz., Fe-P>Al-P>Organic P>Residual-P>Ca-P in all soils. Total acidity of the soils ranged from 1.60 to 3.36 cmol (p+) kg-1. Forms of acidity revealed that total potential acidity was the most dominant forms acidity followed by pH dependent acidity, exchangeable acidity and total acidity in soils of Assam. The adsorption of P in the soils was studied by conducting batch experiment in the laboratory by equilibrating two (2) g soil in duplicate with 20 ml 0.01 M CaCl2 solution containing 5, 10, 20, 40, 60, 80, 100 and 150 mg P L-1 as KH2PO4. The results of adsorption studies on P were discussed in terms of per cent adsorption of added P (%AdsP), Langmuir and Freundlich adsorption parameters. The per cent of adsorbed P (%AdsP) decreased with the increased of added P. The P adsorption data was found to be the best fit in Langmuir adsorption isotherm when resolved into two parts.The Langmuir parameters viz., the adsorption maxima (b) were higher for part II (upper part) whereas the bonding energy constant (k) was higher for part I (lower part) of the isotherms. The highest b2 of part II was found in Ultisol (5882.35 mg kg-1 in S8d1) and the lowest was found in Entisol (1037mg/kg in S1d2).The highest bonding energy constant (k) value for part I was observed in Ultisol (0.795 L mg-1) and the lowest in Entisol (0.055 L mg-1). Phosphate adsorption maxima (b1) of part I had a significant positive correlation with OC (r = 0.814**), clay content (r = 0.632**), Ex. Ca (r=0.701**), CEC (r = 0.874**) and free Fe2O3(r = 0.840**), toal P (r = 0.541*), Fe-P (r = 0.607*) and Al-P (r = 0.512*) content of the soils. The bonding energy constant (k1) of the part I and part II of the isotherm was found to have significant positive correlations with OC (r = 0.731** and 716** ), clay (r = 0.655** and 0.642**), CEC (r=0.890** and 0.640**) and Fe2O3 (r = 0.732** and 815**) of the soils, respectively. Freundlich adsorption parameter K had a significant positive correlation with OC (r = 0.801**), CEC(r=0.908**), clay content (r = 0.660**), Ex. Ca(r=0.537*), Free Fe2O3(r = 0.810**) and Al2O3 (r = 0.571*) of the soils. The Bray 1 P significantly negatively correlated with P adsorption maxima (b1) (r = - 0.599*), bonding energy constant (k1) (r = - 0.594*), MPBC (r = - 0.599*) and Freundlich’s n (r = - 0.744*) and K (r = - 0.616*). Bray2 P also significantly negatively correlated with P adsorption maxima(r= - 0.668**), MPBC (r = - 0.533*) and Freundlich’s n (r = - 0.736**) and K (r= -.571**). P uptake was highest in soil S5d1 (0.042 g plant-1) and lowest in soil S4d1 (0.027 and 0.026 g plant-1) with as well as without addition of P. The P uptake by the Maize had no significant correlation with P adsorption parameters. It might be due to greater role of the intensity factor viz. P concentration and diffusion in supply of P to plant root than the P adsorption parameters. Therefore, amount of P required to obtain 0.2 mg P L-1 in equilibrium soil solution was different in different soil orders. It was highest in Ultisol S8d1 (90.89 mg PL-1) and the lowest in Entisol S2d1 (32.78 mg PL-1). The values of P supply parameter (SP) ranged from 0.38 in soil S7d1 to 49.38 in soil S3d2. Thus the findings of present investigation on P adsorption capacity of soils of Assam will be useful in P fertilizer management and recommendation of P fertilizer for growing crops in these soils.enThesis