Potato yield data from field experiments performed in Bangladesh (Noakhali), Canada (Ontario), Peru (Huancayo), and U.S.A. (Alaska, Hawaii and Idaho) were used to evaluate the P requirements of potatoes. Estimates of the concentration of P in soil solution were obtained from phosphate sorption curves plotted for soils from the various sites. The quantity of P required to establish solution P at standard concentration (0·2 p.p.m. P) ranged from 15 ?g/g (Ontario) to 1000 ?g/g (Alaska - Cryorthod). The concentration of P in a solution equilibrated with a soil to which no P had been added ranged from about 0·002 p.p.m. for a Gibbsihumox of Hawaii to 0·05 p.p.m. for soils from Bangladesh and Canada. Although rates of P applied were high, the Alaska site - a soil influenced by volcanic ash - gave no evidence that maximum yields were attained. Inadequacy of P fertilization was confirmed by foliar analysis. The P sorption curve for this location predicted that the P fertilizer requirement was much greater than that actually applied. This information is consistent with other information that P sorption by soils which contain weathered amorphous materials is in the range of 1000 to 3000 ?g P/g of soil. At one location -Wahiawa (Hawaii) - maximum yields were attained at levels of P in solution which were greatly in excess of that required for potato plant nutrition per se. Except for the highest P levels, these plants were severely affected by Rhizoctonia solani. Yield data from the remaining five locations were plotted against estimated P concentrations in solution to give a composite yield response curve. Phosphorous concentration of about 0.2 p.p.m. in soil solution was associated with approximately 95 per cent of maximum yield. This approach should have wide application as a means of utilizing existing field trial data for determining standard external P requirements for crops in areas where few data are now available. Phosphate sorption curves can then be used to transfer result of field experimentation from one location to another.