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Soil & Water Management & Conservation

Change in Physical Properties of "Terra Rossa" Soils in Paraguay under No-tillage

^a Former experts of CETAPAR-JICA, Paraguay
^b Current mailing address: DAPO 7777, Metro Manila, Philippines
^c CETAPAR-JICA, Ruta 7, km 45, Distrito Yguazú, Depto. Alto Paraná, Paraguay

^* Corresponding author (akikubota{at}yahoo.com)

The introduction of no-tillage farming stabilized soybean [Glycine max. (L.) Merr.] yields on "Terra Rossa" soils of eastern Paraguay at a yield level that is among the highest in the world. The objective of this study was to clarify the effect of long-term no-tillage in a soybean-based farming system on physical properties of "Terra Rossa" soils with different texture. Four farmers' fields of different soil texture were studied in 1990 and 2000. Two fields had fine-textured soil and other two had coarse-textured soil. A 1-m deep pit was prepared at each site at the exact same location in 1990 and 2000, and clay contents, bulk density, cone index, saturated hydraulic conductivity (K_sat), aggregate-size distribution, and Ap horizon thickness were investigated. To evaluate changes in Ap horizon thickness over time, data from several additional pits were included. Effects of long-term no-tillage on physical soil properties differed with soil texture. High erosion rates were associated with coarser soil texture, low K_sat, poor aggregate stability, and high compaction. Coarse-textured soils lost 34% (about 7 cm) of their Ap horizon in 10 yr compared with 11% in fine-textured soils. Cone index readings in the subsoil of a coarse-textured soil became extremely high (4.72 MPa). Soil compaction was furthermore associated with rapid soil drying and on the sixth day after rain reached a level that could restrict vertical root development of germinating seedlings. These results suggest that the no-tillage system as currently practiced in eastern Paraguay may not be suitable for coarse-textured "Terra Rossa" soils.

Abbreviations: CETAPAR, Centro Tecnologico Agropecuario en Paraguay • JICA, Japan International Cooperation Agency • SF, soybean field • VF, virgin forest