A field comparison of sunflower (Helianthus annuus) and sorghum (Sorghum bicolor) in a long drying cycle. I. Water extraction

Abstract

This paper describes the extraction of water by field crops of sunflower and sorghum from deep podzolic profiles during a drying cycle encompassing most of the growth period. The work was done to evaluate sunflower as a prospective dryland crop for southern Australia, and data are presented for two contrasting years. Sunflower extracted water more rapidly from all levels of the measured profile (0-2 m). Both species drew similar amounts from the top 1 m, but sunflower took two to three times more than sorghum from 1-2 m, the difference being greater in the year of greater evaporative load. Sunflower exhausted the soil of available water to 150-175 cm, sorghum only to 50 cm, taking progressively less from each successive layer down the profile. Extraction of available water from 0-2 m by sunflower was estimated to be 92%, and by sorghum 64%. In the absence of rainfall during its growth period, dryland sunflower is strongly dependent on the presence of water at depth - for example, at 50% flowering the crop was already drawing two-thirds of its water from below 1 m. This dependence limits the frequency with which the crop may be grown in southern Australia and regions similarly characterised by low effective summer rainfall, high evaporative load, and uncertainty in the time needed for a recharge of the profile sufficient to grow sunflower again. In such environments, sunflower may better be regarded as an opportunity crop than as one featuring regularly in farm rotations.