Zeolites have a negative charge on their surface; preferentially attract cations and scarcely anions such as and PO which are components of fertilizers. To modify the surface characteristics of the natural clinoptilolite type zeolite, sieved on 14 mesh, it was washed several times with distilled water, dried and placed in colloidal suspension of ferric hydroxide Fe(OH)3 for 24h. The zeolite was washed with distilled water and then it dried at 60°C for 2h. To evaluate the adsorption capacity of natural and modified zeolite, the adsorption isotherm was determined at different phosphate concentrations.The evaluation of the phosphate concentration before and after the adsorption process was determined by spectrophotometry, a method that is based on the formation of the blue ammonium phosphomolybdate complex (λmax=650nm), by reaction of a reducing solution of ammonium molybdate and ascorbic...