AGGREGATE STABILITY AS AFFECTED BY SHORT AND LONG-TERM TILLAGE SYSTEMS AND NUTRIENT SOURCES OF A HAPLUDOX IN SOUTHERN BRAZIL

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 m(3) ha(-1) y(-1) slurry cattle manure; and SM = 40 m(3) 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 loth 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 GMDAD 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 in layer in the NT system, in the layers 0.05-0.10 and 0.12-0.17 in 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 in. The effect of nutrient sources on GMD(AD) and GMD(WS) was small and did not affect SI.