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The effects of water and nutrient supplementation on the annual aboveground biomass production of Chenopodium petiolare H.B.K. were measured in an old field of north-central Chile by simulating rainfall events of 5 mm per month and by adding fertilizers to experimental quadrats in two years with different precipitation levels. In irrigated quadrats, the biomass of C. petiolare was higher than in non-irrigated quadrats. In the nutrient supplementation experiment, the only fertilizer that elicited a plant response was nitrogen. Biomass yield in the nitrogen-supplemented quadrats was over twice that in the control quadrats. As expected from experimental results, biomass in the wet year (1991) was twice as high as the biomass yield during the dry year (1990). Observations on the spatial distribution of C. petiolare elsewhere support our experimental results.