Abstract
Available N, P and exchangeable K, Fe and Mg in soil and subsequent N uptake by rice plants (Oryza sativa), as well as biomass production, were studied during one season in non-fertilizer farmers’ fields. Soil organic matter increased during the season from 2.5% to 3.0%, probably due to rice litter, root biomass production and dead weeds and cyanobacteria. Soil NH4+- N decreased during the first 1.5 months from 24 to 3 kg ha-1 and was at harvest time 1 kg ha-1. N- uptake by plants ranged 54-107 kg ha-1. Mineralization of soil organic matter is the major source for N in this traditional farming system. P, K, Fe and Mg decreased only marginally during the growing season and the amounts were not growth limiting; N seems to be the yield limiting soil nutrient.
