effect of continuous cropping on phosphorus adsorption characteristics in some calcareous soils of north-west of iran

abstract phosphorus (p) sorption in soils is a key process governing the availability of p to crops. p sorption characteristics and their relationship to soil properties were determined in twenty-one paired surface soil samples (21 cultivated and 21 adjacent virgin soils) belonging to five soil types: typic calcixerepts, typic haploxerepts, typic endoaquepts, vertic endoaquepts, and vertic calcixerepts from sugar beet growing areas located in the north-west of iran. after determining p adsorption isotherms, adsorption data was fitted to langmuir and freundlich equations to derive adsorption parameters. in comparison with langmuir equation (r2 =0.82), freundlich model provided a better description of p adsorption isothems (r2 =0.97) in both cultivated and adjacent virgin soils. total p ranged from 1000 to 1398 and 1011 to 1394 mg kg-1, respectively. continuous cultivation resulted in a considerable increase in available p (olsen p) in cultivated soils. the amount sorbed p differed among soil types. the results showed that in all soils types except typic haploxerepts, equilibrium solution p concentration (epc) was almost lower than the requirement for most crops (<0.4 mg l-1). the amount of sorbed p at 0.4 mg l-1 epc ranged from 36 to 75 and 21 to 52 mg kg-1 for cultivated and adjacent virgin soils, repectively. the maximum p adsorption (b) ranged from 152 to and 171 to 220 mg /kg for cultivated and adjacent virgin soils, respectively. maximum p adsorption (b) ranged from 152 to 230 and 171 to 220 mg kg-1 for cultivated and adjacent virgin soils, respectively. the amount of sorbed p increased in most studied soil subgroups as a result of cultivation indicating improper management of p fertilizer application which resulted in the depletion of p from soils and an increase in p soption.