Aggregation Formation in Chestnut and Meadow–Chestnut Soils of the Terek–Sulak Plain and Its Impact on Their Agrophysical Properties

Abstract

The physical properties of plowed and fallow chestnut and meadow–chestnut soils were studied in the Terek–Sulak Plain. It is shown that soil density depends both on the aggregate size and specific features of the soil formation process. In each particular case, this dependence is closely linked to the soil genesis and agricultural use. Analysis of the aggregate porosity estimated based on the percentage content of fractions shows that the major share of interaggregate porosity is attributed to large aggregates. The results can be extrapolated to other parts of the soil cover in the study area. On each studied site, an experimental soil profile cut was established, the morphological soil properties were described, and soil samples were taken layer by layer (cm) from horizons A, B₁, B₂, and C₁ (A_plow, arable layer) for chestnut soils and from horizons A, B₁, and C₁ for meadow–chestnut soils. The structure index was computed based on the the sifting data as the ratio between the sum of macroaggregates (0.25–10 mm in size) and the sum of aggregates with diameters of more than 10 mm and less than 0.25 mm. The porosity of aggregate fractions 7–5, 5–3, 3–2, 2–1, 1–0.5, and 0.5–0.25 mm in size was determined from randomly selected samples of the respective fractions. Mechanical sampling was used to ensure the random selection of samples for analysis. In all of studied soils, the porosity of aggregates of the same size decreases down the profile. The sharpest decrease is observed in large (7–5 mm) aggregates: their porosity drops from 37% in horizon A to 31% in horizon C₁. In small aggregates, this trend is manifested not so clearly. In chestnut soils, aggregate porosity gradually decreases with depth, while the minimum porosity in meadow–chestnut soils is observed in the B₁ and C₁ horizons. This pattern is manifested most clearly in aggregates 2–1 mm in size. However, the minimum aggregate porosity in horizon B₁ is pronounced quite clearly not only in the 2–1 mm fraction but also in all other fractions up to 7–5 mm in size. In the C₁ horizon, the aggregate porosity drops to almost identical values that are close to the computed close-packed porosity (a method involving mutual coupling of mineral particles in a clastic rock). Earlier studies have shown that aggregate porosity is closely linked to the soil composition, structure, and origin, as well as the general soil genesis that determines the fertility.