Seedbed factors affecting establishment of summer crops in a Vertisol

Abstract

Poor establishment and associated low yields of summer crops are problems in the self-mulching Vertisols of tropical and subtropical Australia. To investigate this problem, an experiment was conducted on Mywybilla clay where sorghum ( Sorghum bicolor (L.) Moench cv. ‘Goldrush II’) and sunflower ( Helianthus annuus L. cv. ‘Hysun 33’) were sown on 25 September and 5 November 1986, and on 12 February and 30 April 1987, at 3, 7 and 12 cm depths, with and without a presswheel. Different levels of post-sowing irrigation were applied. After sowing, soil physical properties (water content, bulk density, penetration resistance, aggregate size, temperature) of the seedbeds were monitored. Relationships between soil properties and attributes of crop establishment (establishment percentage, rate of emergence and seedling yield) were examined. Shallow sowing increased the diurnal range in seedbed temperature and increased the mean temperature experienced by the seed. Soil temperatures were reduced by increased soil water content. Mean soil temperature was correlated with mean air temperature and soil water content. Bulk density was increased by presswheels, but not consistently, and decreased with increasing soil water and with depth. Penetration resistance was low in all seedbeds, with values highest in dry soil after deep sowing in the first seedbed. The proportion of small aggregates (less than 5.6 mm) after soil drying was highest when seedbed water content was less than about 0.22 g g −1, or −15.0 bar water potential. Levels of establishment, time to median emergence, and seedling weight of both species increased with increasing post-sowing soil moisture content. Time to median emergence was delayed and seedling weight was decreased with increasing depth of sowing. Establishment was linearly correlated with rate of emergence in suboptimum conditions. As soil moisture decreased, establishment was better at higher soil temperatures. For 80% establishment, soil water content varied from 0.25–0.30 g g −1 in cooler early season seedbeds, to 0.20–0.25 g g −1 in warm late sown seedbeds. Without moisture stress, rate of emergence increased with soil temperature but establishment peaked between average maximum soil temperatures at seed level of 24–32°C. The peak temperature increased as sowing depth became more shallow. Establishment and rate of emergence were only fairly described ( R 2 = 0.65 and R 2 > 0.50, respectively) by considering soil temperature, moisture content and depth of sowing in simple linear multiple regression models. Seedling weight usually increased in conditions suited to rapid emergence and high establishment. The exception was in a dry seedbed when moisture was available below seedbed depth. Then, deeper sown seeds produced fewer but heavier seedlings. Any benefits of presswheels to establishment were associated with intermediate soil moisture content and shallow sowing. The practical implications of the results are considered.