Geology and Mineral Resources of Paraguay A Reconnaissance

SOILS OF PARAGUAY 101investigations, for, in the absence of surface exposures,useful information as to existence or distribution of freshwater can be gained only from close observation andcontrol of the drilling operations and from study of therecords so obtained. Once some guides are establishedby the suggested exploratory wells, geophysical methodsmight be used to trace out fresh-water-bearinghorizons. Careful attention to and experimentationwith methods of drilling, casing, and screening are atleast as important as is geologic control, for there isalways grave danger of missing fresh-water aquifers entirely,or drilling through them and contaminating thefresh water with underlying salt waters.This suggested program of exploration has the virtueof flexibility as to scope; it could easily be restricted tosmall areas where the need for water is the greatest.Moreover, whatever its scope, it bears promise ofpossible reward far beyond its cost.SOILS OF PARAGUAYBY PEDEO TIRADO SULSONAAbout 70 percent of eastern Paraguay is covered byrelatively deep residual soils that range from loamy sandto sandy clay in texture. They are classified in sevenmajor soil series, the differences between them moreclosely related to the parent materials from which theywere derived than to any other single factor. The remainderof eastern Paraguay is covered by transportedsoils, divided into four series; like the residual soils, allare sandy. As would be expected from its geologichistory, virtually all of the Gran Chaco is covered bytransported soils ranging from sandy loam to clay thatare classed in six major soil series, with distinctionsbased more on drainage and topographic position thanon parent materials or other features. The soils ofParaguay are described briefly in the accompanyingtable; their general distribution is shown in plate 3.The soils of eastern Paraguay provide favorableto adequate conditions for root penetration, aeration,and water absorption and retention. The areas oftransported soils, however, are generally characterizedby a water table near or at the surface.The residual soils are low in exchangeable calciumand phosphorus, and low to adequate in organic ma­terial and in nitrogen. They are commonly acceptableto high in exchangeable magnesium and potassium.Most are acid, the pH ranging from 5.8 to 6.8. Thealluvial soils are higher in all nutrients than are theresidual ones and can generally be considered as adequatefor agricultural purposes if proper drainage canbe attained.All the eastern soils, transported and residual, tend tobe very low in the minor elements, such as copper, zinc,manganese, and cobalt, which are known to be essentialto healthy plant or animal growth. This shortcomingis doubtless a dominant factor in the slow growth andmaturity of cattle, and perhaps it even contributes tothe comparatively small stature of most of the people.The absence of minor elements, as well as the lowcontent of such elements as calcium and phosphorus, isa direct result of the geologic and soil-forming historyof the region. As most of the soils are derived fromsandstone, whose constituents represent long periods ofweathering and of rather thorough sorting, it is only tobe expected that they contain much smaller amounts ofmany essential elements than would soils formed fromthe weathering of rocks or mineralized zones that werericher in such materials. Moreover, all the easternsoils have evolved under tropical conditions, with heavyprecipitation, so that much of the nutrient materialthey may have had originally has long since been removedby leaching.The Gran Chaco soils are, as would be expected,richer in all the essential chemical elements than arethose of eastern Paraguay. They represent alluvialmaterial that, geologically speaking, has been freshlywashed down from a great variety of rocks in the Andesand that has had little opportunity to lose these ele­ments by leaching. Were it not for the high watertable and the abundance of magnesium sulfate andother salts, there is little doubt that the Gran Chacosoils would be far more amenable to agriculture and tostockraising than are those east of the Rio Paraguay.REFERENCES AND ANNOTATED BIBLIOGRAPHYAhlfeld, Federico, 1936, Ein neues Nutzglimmer Vorkommen inBolivia: Zeitschr. filr prakt. Geologic, v. 44, p. 143-145.Mica deposits in northeastern Bolivia.1946, Geologfa de Bolivia, Re vista de museo de la Plata:Sec. Geol., v. 3, p. 5-370, La Plata, Argentina, 1946. Alsopublished separately by Univ. de Econ. Nac., DireccionGeneral de Minas y Petroleo, La Paz, Bolivia.Excellent summary of Bolivian geology.Almeida, F. M. de, 1945, Geologia do Sudoe"ste Matogrossense;Dept. Nac. da Produgao Min. Div. de Geol. e Min., Bull.116, 118 p., Rio de Janeiro.Detailed description of stratigraphy in vicinity of Corumba,with maps.Azara, F. de, 1790 (republished 1904), Geografia fisica y esfericade las provincias del Paraguay y Misiones Guara.nies; An alesdel Mus. nac. Montevideo, Sec. his.-filos., v. 1.Quoted extensively by Carnier (191 Ic) with regard to geologyof central Paraguay. (Not seen.)Baker, C. L., 1923, The lava field of the Parana basin, SouthAmerica: Jour. Geology, v. 31, p. 66-79.Clear description of the Serra Geral lavas, their environmentand history.