Influence of the Duration of the Use of Perennial Grasses on the Fertility of Irrigated Meadow-Chernozemic Soils in the Omsk Irtysh Region

Abstract

Studies were carried out to establish the influence of legume–grass mixtures on the fertility indicators of irrigated meadow-chernozemic soil depending on the duration of their cultivation in grain-grass crop rotation. Two factors were studied in the experiment: (A) grass mixture: (1) awnless brome + alfalfa in the sixth year of life, (2) awnless brome + eastern goat’s rue + hill mustard in the 12th year of life, (3) awnless brome + eastern goat’s rue in the 21st year of life; (B) mineral fertilizers: (1) without fertilizers (N₆₀P₆₀). Nitrogen fertilizers were applied for each of the two mowings and phosphorous fertilizers were applied as reserves. The dependence of the amount of agronomically valuable aggregates and their resistance to the erosive water action on the age of legume–bluegrass mixtures has been revealed. With increase in the age of herbs, the amount of these aggregates in the 0–20-cm soil layer increased from 48% under the grass mixture in the sixth year of life to 69% under the grass mixture in the 12th year of life, reaching 79% under the grass mixture in the 21st year of life. In variants with the addition of nitrogen–phosphorous fertilizers, fractions of agronomically valuable aggregates under the grass stand in the 12th and 11th years of life decreased by 7.9% compared to variants without fertilizers. With increase in the age of grass stand, the content of humus and humic acids in its composition, exchange cations, and the reaction of the medium did not change significantly; the recorded changes are more determined by the influence of mineral fertilizers. The amount of humus under the herbs in the 0–20-cm soil layer was high and reached 6.7 to 7.1% in unfertilized variants and 7.5 to 7.8% in variants without fertilizers. The proportion of humic acid carbon in the humus composition was 1.9–2.4 and 2.6–2.7%, respectively. The reaction of the medium in the test variants varied from 5.8 to 6.8 units of pH of aqueous suspension. The proportion of calcium in the composition of exchangeable cations was 70 to 87%. In the test variants without fertilizers, the content of mobile phosphorus (according to Chirikov) in the soil was 55.90 mg/kg; in variants with fertilizers, its amount increased by 68.95 mg/kg in the 0–20-cm layer and by 40 to 70 mg/kg in the 20–40-cm layer. Indicators of the chemical and physicochemical properties of the soil reached optimal values after 6 years of cultivation of the legume–bluegrass mixture. Therefore, the further use (up to 12 and 21 years) of the soil under grass is unpractical.