TROPICAL SOILS — PENDLETON 443 



Too often, however, particularly in the vast interiors of equatorial 

 Africa and equatorial South America, there has been too little erosion. 

 The residual products of weathering have accumulated, after the 

 principal nutrient materials have long since been leached from the 

 soils : iron has accumulated as laterite, less often aluminum as bauxite, 

 or more often as kaolinitic clays, and silica as quartz sand. It may 

 seem rank heresy to emphasize the fact that any region can suffer 

 from too little erosion, but this certainly is the case in considerable 

 portions of the one tropical region which I know best, Thailand 

 (Siam). 



THE SOIL-FOREST COMPLEX 



It is likely that these magnificent forests in humid equatorial low- 

 land regions started when the soil was not so poor, when rocks had 

 not yet weathered so deeply, so that there was not such a scarcity of 

 plant nutrients in the surface soil within the reach of the roots of 

 the forest trees and other plants. The forest developed great luxuri- 

 ance, while the tree roots went deeper and deeper. As long as there 

 was no general destruction of the forest vegetation, there was very 

 little loss of plant nutrients within the reach of the tree roots, for as 

 soon as one tree died and fell to the ground, it was quickly attacked 

 by termites, mold, fungi, etc., and within a year or two, practically all 

 the plant nutrients were liberated in the mineralization of the plant 

 materials. Roots of the surrounding trees and plants immediately 

 took up these nutrients. They were taken back into the forest vege- 

 tation without any considerable proportion being lost to the deeper 

 percolating waters. In other words, the plant nutrients were being 

 cycled — used in the forest vegetation over and over again. But the 

 soil itself was changing. Weathering had been progressing deeper 

 and deeper. Wliile most of the plant nutrient substances were being 

 held by the roots of the vegetation, small portions were lost by too 

 rapid percolation and perhaps some by erosion. The mineral soil 

 itself was becoming poorer and poorer. 



Felisberto de Camargo has described one of the most striking ex- 

 amples of a magnificent dense and tall equatorial forest (selva) which 

 had developed on a soil that has proved a serious disappointment for 

 annual agricultural crop production. This is the country through 

 which runs the railway from Belem to Braganga, state of Para, 

 Brazil.* South of the mouth of the Amazon River is an enormous 

 region of low, rolling country on which once stood a magnificent 

 high tropical rain forest. Some 70 years ago the government under- 



* Felisberto C. de Camargo, Terra en colonizagao no anticro e novo Quaternario 

 da zona da estrnda de Ferro de Braganca, Estado de Pard, Brasil, Bol. Mus. 

 Pnraense E. Goeldi, vol. 10, pp. 123-147, 1948. (In English in Dept. State Publ. 

 3382, International Organization and Conference Series 2, American Republics 4, 

 sect. 3, pp. 213-220.) 



