In 2013-2014, field experiments were conducted at the Agricultural Research Station in Pawłowice (Department of Crop Production of the Wroclaw University of Environmental and Life Sciences), using sorghum as a test plant. The purpose was to determine the influence of the type of fertiliser on the content of mineral nitrogen (N–NO3 and N–NH4) in the top soil layer (0-60 cm). Soil samples were collected three times during the growing season: in the spring before sowing, during the phase of shoot formation (July), and after the harvest (October). The fertilisers used in the experiment were ammonium nitrate, urea and slow-release coated urea (Meister). Regardless of the weather conditions in 2013 and 2014, similar tendencies were found with respect to the differentiation of the nitrogen content in soil. The total nitrogen content was the highest in the middle of the growing season, and the amounts of nitrate form differed greatly among the treatments. The content of N-NO3, measured in that period was 20-44% lower in the soil fertilised with coated urea than with  standard urea. After the harvest, the stock of nitrogen in soil fertilised with ammonium nitrate was 58-61 kg per hectare, which was classified as a medium-high level (posing a threat to water quality), while being on a low level of 31-46 kg ha-1 in soils fertilised with standard urea and coated urea, which is considered to be harmless for the ground- and surface waters under climate conditions of Poland. The stocks of N-NO3 left in soil after sorghum harvest fell into the medium-high level according to the limits recommended for sandy soils in Poland after fertilisation with ammonium nitrate, and into the low level if fertilised with standard urea and coated urea, in both years.

22.4.2016