Abstract
The ability of a soil to keep its structure under the erosive action of water is usually high in natural conditions and decreases under frequent and intensive cultivation. The effect of five tillage systems (NT = no-till; CP = chisel plowing and one secondary disking; CT = primary and two secondary distings; CTb = CT with crop residue burning; and CTr = CT with removal of crop residues from the field), combined with five nutrient sources (C = control, no nutrient application; MF = mineral fertilizers according to technical recommendations for each crop; PL = 5 Mg ha-1 y-1 fresh matter of poultry litter; CM = 60 m³ ha-1 y-1 slurry cattle manure; and SM = 40 m³ ha-1 y-1 slurry swine manure) on wet-aggregate stability was determined after nine years (four sampled soil layers) and on five sampling dates in the 10th year (two sampled soil layers) of the experiment. The size distribution of the air-dried aggregates was strongly affected by soil bulk density, and greater values of geometric mean diameter (GMD AD) found in some soil tillage or layer may be partly due to the higher compaction degree. After nine years, the GMD AD on the surface was greater in NT and CP compared to conventional tillage systems (CT, CTb and CTr), due to the higher organic matter content, as well as less soil mobilization. Aggregate stability in water, on the other hand, was affected by the low variation in previous gravimetric moisture of aggregates, which contributed to a high coefficient of variation of this attribute. The geometric mean diameter of water-stable aggregates (GMD WS) was highest in the 0.00-0.05 m layer in the NT system, in the layers 0.05-0.10 and 0.12-0.17 m in the CT, and values were intermediate in CP. The stability index (SI) in the surface layers was greater in treatments where crop residues were kept in the field (NT, CP and CT), which is associated with soil organic matter content. No differences were found in the layer 0.27-0.32 m. The effect of nutrient sources on GMD AD and GMD WS was small and did not affect SI.
Highlights
Soil structure has been defined as the size, shape and arrangement of the solid particles and voids, and is highly variable and associated with a complex set of interactions among mineralogical, chemical and biological factors (Letey, 1991)
Differences among tillage systems and sampling layers and stability index (SI) were observed in the dry (GMDAD) and water-stable size distributions, with interaction between these two sources of variation at the end of the ninth year of treatment application
Air-dried aggregate size distribution is strongly affected by bulk density, and greater values of geometric mean diameter are found in no-till and chisel plow systems, or in compacted layers under conventional tillage system
Summary
Soil structure has been defined as the size, shape and arrangement of the solid particles and voids, and is highly variable and associated with a complex set of interactions among mineralogical, chemical and biological factors (Letey, 1991). Soil structure is not considered a factor directly related to crop production, it plays an important role in water and air supply to roots, root elongation, nutrient availability and macrofauna activity. A soil should have a good structure, and a structure which will persist for a long time, e.g., a structure of high quality and stability (Dexter, 1988). This author classifies structure stability in two principal types: (a) the ability of a soil to keep its structure under water action; and (b) the ability of moist soil to keep its structure under the action of external mechanical stresses. The structure stability under external stresses can be determined in tests of compressibility (Gupta et al, 2002) and shear strength (Fredlung & Vanapalli, 2002)
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