Covering the soil with plastic film is an important method to modify soil temperature and improve the conditions for plant growth. For the optimal use of films in horticulture, it is necessary to quantify this effect, which depends particularly on the optical properties of the film. Five films, used mainly for soil covering in asparagus cultivation, were tested by measuring the soil temperature as well as the transmittance and reflectance in the short-and long-wavelength bands: 'Taschenfolie schwarz/weiß', a film with black and white surfaces and pockets for fastening; 'Antitaufolie', a film with anti-condensation coating, 'Solartherm plus Folie', a film with black top and transparent sides; 'MaterBi Folie', a biodegradable film; and 'Thermoplus Folie', a black polyethylene with transparent sides. All films were produced in Germany. The temperature measurements were carried out under controlled irradiation (short-wavelength band) and ambient temperature conditions in the laboratory: first, 182 W m −2 and 11 • C at a depth of 0.4 m, and second, 340 W m −2 at 20 • C. A significant relationship between the temperature increase or decrease by covering the soil with the film and the calculated apparent quantities of the reflectance in the short and long-wavelength band was derived (r 2 = 0.85, n = 12). Furthermore, first tests with a physical model were carried out to obtain basic information, which should be used in the development of strategies for film management in the field.