Weed competition below ground: a three-year study

Abstract

In the semi-arid region of the inland Pacific Northwest (PNW), Russian thistle (Salsola tragus), kochia (Bassia scoparia), and prickly lettuce (Lactuca serriola) are three important weed species that can regrow after harvest and use water. However, the way in which these species compete for soil water, the most limited resource, has not been explored adequately. In a 3-yr field study at the Columbia Basin Agricultural Research Center at Adams, Oregon, individual Russian thistle, kochia, prickly lettuce and spring wheat plants separated 4 m from each other were grown in fallow fields. The experiment was a randomized complete block design with five replications. Soil samples, 150 cm deep, were taken at seeding time (March), wheat harvest time (July), and when the plants were about to die (early October). The results varied depending on the annual precipitation and plant biomass, but on average and from March to October, the soil under a single Russian thistle or kochia plant was drier than the control (soil without any plant) by 0.04 kg water/kg soil, and the soil under a prickly lettuce or spring wheat plant was 0.028 kg water/kg soil and 0.008 kg water/kg soil drier than the control, respectively. In general, these differences were lower in the top 60 cm of soil and greater in the deeper soil profile. Russian thistle and kochia roots dried deeper soil more than prickly lettuce and spring wheat.