Germination and emergence of irrigated cotton in Pakistan in relation to sowing depth and physical properties of the seedbed

Emergence of cotton (Gossypium hirsutum L.) sown under hot dry conditions can be suboptimal in Pakistan. A field study to assess soil physical factors that can affect germination and emergence was conducted in a silt loam Fluvisol during May and June 1997. The field was irrigated and then cultivated following local practice. There were two main treatments, irrigated and control, with two replicates. Seeds of cotton were sown at 23, 46 and 92 mm depth in each plot in a split plot design. Irrigated plots received 15 mm high intensity (300 mm h(-1)) simulated rainfall 2 days after sowing (DAS). The control plots received no irrigation after sowing. Seedling germination, emergence, growth, soil temperature and matric potential of the seedbed were recorded. Undisturbed cores, equilibrated in the laboratory at a range of water contents, were used to determine penetration resistance (PR) as a function of matric potential. Field germination at all sowing depths in both treatments was similar to laboratory values. However, a hard crust, which formed on the irrigated plots, prevented emergence. Emergence in control plots decreased with increased sowing depth and there was no emergence from 92 mm depth. Emergence occurred from 6 to 9 DAS. Four DAS, the seedbed temperature at 23 mm depth exceeded 42 degreesC (a critical temperature above which germination does not occur) but seeds had already germinated before the seedbed reached this temperature. From 4 DAS onwards there was a combination of high soil temperature (> 38 degreesC), high soil impedance to shoot growth (>1.6 MPa) and low matric potential (< -500 kPa), which is likely to have reduced emergence. Roots were either delayed or prevented from entering the base of the seedbed due to its high PR. Traffic was not controlled during seedbed preparation and the effect of tractor wheels running in the base of the seedbed was partly responsible for its high PR. Use of controlled traffic should reduce this effect. Results also show the importance of preparing and sowing the seedbed as soon as possible after irrigation in order to obtain good emergence and root growth before soil physical conditions become limiting. (C) 2001 Elsevier Science B.V. All rights reserved.