MAINTENANCE OF YIELDS AND SOIL FERTILITY IN NONMECHANIZED CROPPING SYSTEMS, BOLIVIA

Slash and burn farmers in the tropical rain forests of eastern Bolivia are abandoning land after one rice (Oryza sativa L.) crop because of declining yields. A trial was conducted on a Typic Paleudult for 41 mo to investigate whether alternative low-input nonmechanized cropping systems could prolong soil fertility and yield maintenance, and whether soil fertility or weeds were responsible for declining yields. Twelve cropping systems were investigated in a factorial design, with three summer-winter crop sequences: rice-peanut (Arachis hypogaea L.), com (Zea mays L.)-bean (Phaseolus vulgaris L.) later substituted by cowpea [Vigna unguiculata (L.) Walp.], and rice-fallow (control); two weed control treatments: minimal and optimal; and two fertilizer treatments: with and without 60 kg N ha-1 and 17.5 or 35 kg P ha-1. Crop sequences significantly increased exchangeable acidity; the rice sequences significantly reduced exchangeable Ca, and cornbean/cowpea and rice-peanut significantly reduced exchangeable Mg. Fertilization significantly increased soil P but decreased Ca. Foliar analysis revealed N, Mg, and Zn deficiencies in all cropping systems. Rice yields, unlike corn, were significantly increased by optimal weeding. Corn yields were dominated by fertilization, whereas rice yields were mainly influenced by fertilization in the first and fourth years, and by weeds in the intervening years. Without fertilizers, rice-fallow was not sustainable, and only corn-bean/cowpea was sustainable for 3 yr. With fertilization, rice-fallow plus optimal weed control and corn-bean/cowpea with minimal or optimal weeding were sustainable for 3 yr. Additional fertilization and future liming would be necessary for more prolonged sustainability.