CULTURAL AND ECOLOGICAL FACTORS AFFECTING NUTRIENT DYNAMICS AND CROP YIELDS IN SHIFTING CULTIVATION (PANAMA)

Four crops of maize were grown under conditions of slash-and-burn agriculture on heavy clay soil in Panama. The field was established in an area of ten-year-old secondary growth forest. The soil was low in pH and in most essential nutrients. Crop yields and nutrient dynamics were monitored over the two years of the study.;Dynamics of the exchangeable nutrients potassium, calcium, and magnesium were dominated by fluctuations in the soil pools of these elements, as only small quantities of exchangeable nutrients were bound up in vegetation. Potassium stocks began to decline rapidly and steadily after the first crop, but transient recharge of the exchangeable calcium and magnesium pools from weathering was observed.;The vegetational component of the ecosystem was more important in the dynamics of nitrogen and phosphorus. Decay of burned tree trunks and vegetation cut down after each harvest contributed to the soil nitrate pool until, at one point, nitrate was abundant in the soil. This nitrate was lost to the ecosystem by leaching during the fourth crop, when vegetational growth was quite poor due to inadequate supplies of other soil nutrients.;Phosphorus dynamics were even more concentrated in the vegetational component of the ecosystem than nitrogen: phosphorus cycled rapidly and did not remain in the soil long. Retention of phosphorus in vegetation may have been important. Since, however, phosphorus stocks dropped most severely in periods of heavy rain regardless of plant growth rates, erosion and loss of organic matter particles in runoff were suspected as being more important in phosphorus than in nitrogen dynamics.;Soil pH and nutrient availability were found to be more limiting to maize yield than was weed competition. Both soil nutrient deficits and weed competition were more intense later in the cropping cycle and could thus have caused yield declines. Herbivory (by mammals) was episodic and occurred early in the cropping cycle.;It is suggested that attempts to construct sustained yield tropical agroecosystems aim to promote nutrient retention in vegetation and prevent erosion losses in the contexts acceptable to marginal-land farmers.