The aim of the research is to investigate the impact of different doses of phosphogypsum as a chemical amendment on physical and physical-chemical properties of irrigated dark-chestnut soils in rice systems under constant flooding. The scheme of experiment: 1) moldboard tillage to the depth of 20–22 cm (control); 2) moldboard tillage to the depth of 20–22 cm + 10 t/ha of phosphogypsum (10 t/ha +PG); 3) moldboard tillage to the depth of 20–22 cm + 40 t/ha of phosphogypsum (40 t/ha PG); 4) imitation of soil milling to the depth of 20–40 cm + 10 t/ha of phosphogypsum (ISM + 10 t/ha PG). In the first year after applying phosphogypsum at a dose of 40 t/ha, soil density decreased in a layer 0–20 cm by 9.0 %, in 2012 – by 10.3 %. Results of applying 10 t/ha phosphorgypsum had intermediate position between the results of control variant and the variant with applying 40 t/ha phosphogypsum. Decreasing of soil density was also marked in a soil layer 20–40 cm but for less extent. In the variant of imitation of soil milling, positive results were obtained in the whole soil layer 0–40 cm preserving the obtained result for a long period. Soil layers 0–20 and 20–40 cm in this variant had uniform density which average values in 2012 equaled to 1.27 and 1.29 t/m³ respectively. Phosphogypsum application facilitated to silt content reduction and improving the structure of soil horizons. The imitation of soil milling showed the best results. Soil got homogeneous crumbly structure, which preserved during the whole period of the research. In a soil layer 0–20 cm the content of humus increased by 5.4–6.3 % in the variants with surface application of phosphogypsum against control. The variant ISM + 10 t/ha PG showed the best results: the content of humus increased by 7.2 % comparing to control variant.
Keywords: amelioration, phosphogypsum, rice, soil density, humus.
