Ecological aspects of weed control in cotton in the Ord River valley, W.A. 2. The effect of soil depth on emergence of weed seedlings

Abstract

A study was made of the emergence from various soil depths of cotton, six grasses and six broad-leaved weeds commonly occurring in the irrigated area of the Ord River valley, W.A. The experiments were conducted in a glasshouse under temperatures fluctuating between 20¦ and 35¦C, under conditions of adequate aeration and soil water in Cununurra clay, the main soil type of the Ord irrigation area. Seedling emergence was inversely related to seed depth, and, in general, the ability of seeds to emerge from various soil depths was positively associated with the average seed weight of the species. Of the grasses, only Echinochloa colonum and Iseilema vaginiflorum emerged from depths exceeding 3 inches. The maximum depths from which broad-leaved species emerged varied from 3 to 7 inches, depending on the species. The emergence pattern of cotton was almost identical with that of Hibiscus ficulneus and Operculina turpethum, but cotton and Operculina were the only species to emerge from a depth of 7 inches and then only in small numbers. The practical implications of these findings on depth of cultivation and herbicide placement are discussed.