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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

View larger version (14K): [in a new window]   Fig. 1. Location of surveyed sites in Yguazú district and distribution of "Terra Rossa" soil in eastern Paraguay. Locations of soybean field (SF) 1, 2, 3, and 4 are 1 through 4 in the map, respectively.

View larger version (32K): [in a new window]   Fig. 2. Monthly average precipitation and air temperature of CETAPAR experimental station, Yguazú District, Paraguay (1990–2000). Mean annual precipitation was 1803 mm.

View larger version (26K): [in a new window]   Fig. 3. Clay content, bulk density, cone index, and saturated hydraulic conductivity (Ksat) of the soils measured in 1990 and 2000. No data of clay content is available for SF1 and SF2 in 1990. The Ksat value was measured by the constant-head method in 1990, and by falling-head method in 2000 (VF: virgin forest, SF: soybean field).

View larger version (22K): [in a new window]   Fig. 4. Distribution of aggregates bigger than 0.5 mm in the Ap horizon of fine-textured and coarse-textured "Terra Rossa" soils. General contrast were used to determine differences between soil groups within years and groups over years. Changes over years were not significant. ** indicates significance between soil groups at P = 0.01. Error bars: SD (n = 2).

View larger version (41K): [in a new window]   Fig. 5. Change in Ap horizon thickness of soybean fields in 5 yr, from 1990 to 2000, under no-tillage system. The Ap horizon thickness of 1995 was measured by Miura et al. (1998).

View larger version (18K): [in a new window]   Fig. 6. Change in Ap horizon thickness of two texture groups of "Terra Rossa" soil in 10 yr. The vertical bars indicate LSD at 5% probability. Number of samples were 4, 21, 4, and 10 from the left to the right bars.

View larger version (19K): [in a new window]   Fig. 7. Relationship between matric water potential and cone index measured at 15 and 25 cm (CETAPAR experimental field).

View larger version (12K): [in a new window]   Fig. 8. Change in cone index at different depths after rain (108 mm) on 14 Dec. 1999 (CETAPAR experimental field). Error bars: SD (n = 5). The dotted line indicates a cone index of 1.27 MPa, which was found to restrict soybean taproot growth (Sato et al., 2001). *greater than 1.27 MPa at P = 0.05. **greater than 1.27 MPa at P = 0.01.