Quantity-intensity (Q/I) relationships for phosphate (P) in long-term cropping systems were studied for soils obtained from two research sites; Clarion-Webster Research Center (CWRC) at Kanawha and Galva-Primghar Research Center (GPRC) at Sutherland, in Iowa. Soil samples were collected from three cropping systems: continuous corn (CCCC), corn-soybean-corn-soybean (CSCS), and corn-oats-meadow-meadow (COMM), with or without N fertilizer treatment. In these studies, a 1-g sample of <60-mesh (250 μm) soil was equilibrated with 25 mL of standard KH2PO4solution containing 0 to 10 mg P L-1at 25°C for 24 h. Inorganic P in the filtrate was determined. Quantity-Intensity diagrams were constructed by plotting the amount of P adsorbed by or desorbed (ΔP) from the soil against the solution P concentration. Statistical analysis (ANOVA) revealed significant differences in P adsorption/desorption among replicates of the experimental field plots as the result of spatial variability of phosphate in these fields. Results showed that Q/I parameters (intercept labile P,a; equilibrium buffering capacity, EBC; and equilibrium P concentration, EPC) varied significantly between and within sites for the cropping systems studied. Results also showed that N application, in general, increased the equilibrium buffering capacity of the soils, leaving less P in the labile and solution phases. At the CWRC site, application of N fertilizer significantly increased the EBC for soils under various cropping systems in the order: COMM > CSCS > CCCC. At the GPRC site, however, the order was CCCC > COMM > CSCS. Theavalues were correlated significantly with the results of seven P tests evaluated, and thervalues ranged from 0.637**to 0.846***. Theavalues were significantly, but negatively, correlated (r= -0.743***) with the ECB values, and positively correlated (r= 0.893***) with the EPC values. Useful relationships were developed between EBC and organic C, CEC, clay content, and exchangeable Ca.