Nitrate in soils and plants can be determined by the method of ultraviolet absorption of nitrowyl chloride. The extracts of plants need to be treated with hydrogen peroxide before the determination in order to exidize the organic matter. For the soil extracts the blank is obtained by eliminating nitrates with hydrazine sulfate wich server to correct the absorption of the soil solutions. The determination is carried on adding perchloric, sulfuric and hudrochloric acids to the solutions obtained from vegetable and soil materials to form nitrowyl chloride. The absorbances of solutions are read at 230 miliraicrons.