Relationships between soil, fallow period, weeds and rice yield in slash-and-burn systems of Laos

Abstract

Decline in soil fertility accelerated by shorter fallow periods was expected to be a major constraint in slash-and-burn rice production systems in northern Laos. In this paper we describe relationships between fallow period, soil fertility parameters, weeds and rice yield. Soil infertility is not perceived a major yield constraint by the farmers. Of the various soil parameters observed only soil organic matter showed consistent association with rice yield (r=0.42, p<0.01). Fallow period and rice yield showed no association and the relationship between fallow and organic matter was very weak (r=0.16, p<0.01). Rice yield was negatively related to densities of Ageratum conyzoides and Lygodium flexuosum. Soil loss during the cropping period ranged from 300–29.300 kg ha⁻¹. For the same period organic matter, total N, available P and available K content in the top 0–3 cm decreased by 11,12,17, and 17%, respectively, and loss of total N for the soil depth of 0–25 cm was estimated at 400 kg ha⁻¹. Soil physical properties, moisture stress and available N are the most likely detriments to rice yields. Further attempts to relate soil properties to rice yield should include repeated measurements during the cropping season and observations on soil physical properties.