Soil physical properties and soybean (Glycine max, Merrill) root abundance in conventionally- and zero-tilled soils in the humid Pampas of Argentina.

摘要

In the humid Pampas of Argentina soybean is cultivated in different soil types, which were changed from conventional- to zero tillage systems in the last decade. Little is known about the response of soybean roots to these different soil physical environments. Pasture, and conventionally- and zero-tilled field lots cropped to soybean (R1 and R2 ontogenic stages) were sampled in February-March 2001 in a sandy clay loam and two silty clay loam Mollisols, and in a clayey Vertisol. In the 0-0.05 m layer of conventionally- and zero-tilled lots soil organic carbon represented 53-72% of that in pasture lots, and showed an incipient recovery after 4-11 years of continuous zero tillage. Soil aggregate stability was 10.1-46.8% lower in conventionally-tilled than in pasture lots, and recovered completely in zero-tilled lots. Soil relative compaction ranged 60.8-83.6%, which was below the threshold limit for crop yields (90%). In change, soil porosity 50 micro m ranged 0.91-5.09% soil volume, well below the minimum critical limit for root aeration and elongation (10%, v/v). The threshold of soil resistance (about 2-3 MPa) was only over passed in an induced plough pan in the conventionally-tilled Bragado soil (5.9 MPa), and in the conventionally- and zero-tilled Ramallo soils (3.7-4.2 MPa, respectively). However, neither the low macroporosity nor the high soil resistances impeded soybean roots growth in any site. According to a fitted polynomial function, root abundance was negatively related to clay content in the subsoil (R2=0.84, P0.001). Soybean roots were only abundant in the subsoil of the sandy clay loam Mollisol, which had 350 g kg-1 clay. Results show that subsoil properties, and not tillage systems, were the primary effect of root growth of soybean..