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Soil Fertility Dynamics after Clearing a Tropical Rainforest in Peru¹

ABSTRACT

This paper describes changes in soil properties during the first 8 y after clearing a fine loamy, siliceous, isohyperthermic Typic Paleudult at Yurimaguas, Peru. Three adjacent fields under a 17-year old secondary forest were slashed, burned, and planted to three crops per year with or without fertilization for 8 years. Ash from the burn increased soil pH, available N, P, K, Ca, Mg, and decreased exchangeable Al. Six months after burning, however, the levels of available N and K were reduced, along with sporadic S, Cu, and B deficiencies. Topsoil organic C and total N decreased at an annual rate of 25% during the first year but approached an equilibrium afterwards. The rapid organic matter decomposition probably released organic matter-bound Al which reversed the liming effect of the ash. Phosphorus and Mg became deficient during the second year, Ca within the first 30 months and Zn and Mn during the fourth year. Molybdenum deficiencies occurred sporadically, particularly when locally produced legume seed was used. Soil chemical properties have improved with continuous cultivation because of liming and fertilizer additions. After 8 years and 21 crops, topsoil pH increased to 5.6, exchangeable Ca increased by 10 times, effective CEC doubled, available P increased from 5 to 39 mg kg^–1, and Al saturation decreased from 82 to 1%. The 15- to 50-cm layer of the subsoil has undergone significant increases in exchangeable Ca and Mg and a decrease in Al saturation. This should promote deeper root development. The time at which nutrient deficiencies appeared and the amounts of fertilizer or lime needed to correct them varied substantially between the three fields, despite their close proximity, same preclearing vegetation, geomorphic position, and same soil classification at the family level.

¹ Paper no. 8549 of the North Carolina Agric. Res. Serv., Raleigh, NC 27650. Contribution of the Tropical Soils Research Program conducted jointly with the Instituto Nacional de Investigación y Promoción Agraria of Peru (INIPA) and supported by Contract AID/ta-C-1236 with the U.S. Agency for International Development.

² Professor of Soil Science, Former Graduate Research Assistant, and Assistant Professor of Soil Science, respectively, North Carolina State Univ. Dr. Villachica is now Professor of Tropical Soils, Universidad Nacional Agraria, La Molina, Lima, Peru.

Received for publication October 4, 1982. Accepted for publication July 28, 1983.

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