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 c an be suboptimal in Pakistan. A field study to assess soil physical factors that can affect germination and emergence was conducted in a silt loam Flu visol during May and June 1997. The field was irrigated and then cultivated following local practice. There were two main treatments, irrigated and co ntrol, with two replicates. Seeds of cotton were sown at 23, 46 and 92 mm d epth in each plot in a split plot design. Irrigated plots received 15 mm hi gh intensity (300 mm h(-1)) simulated rainfall 2 days after sowing (DAS). T he control plots received no irrigation after sowing. Seedling germination, emergence, growth, soil temperature and matric potential of the seedbed we re recorded. Undisturbed cores, equilibrated in the laboratory at a range o f water contents, were used to determine penetration resistance (PR) as a f unction 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 pl ots decreased with increased sowing depth and there was no emergence from 9 2 mm depth. Emergence occurred from 6 to 9 DAS. Four DAS, the seedbed tempe rature at 23 mm depth exceeded 42 degreesC (a critical temperature above wh ich germination does not occur) but seeds had already germinated before the seedbed reached this temperature. From 4 DAS onwards there was a combinati on of high soil temperature (> 38 degreesC), high soil impedance to shoot g rowth (>1.6 MPa) and low matric potential (< -500 kPa), which is likely to have reduced emergence. Roots were either delayed or prevented from enterin g the base of the seedbed due to its high PR. Traffic was not controlled du ring seedbed preparation and the effect of tractor wheels running in the ba se of the seedbed was partly responsible for its high PR. Use of controlled traffic should reduce this effect. Results also show the importance of pre paring and sowing the seedbed as soon as possible after irrigation in order to obtain good emergence and root growth before soil physical conditions b ecome limiting. (C) 2001 Elsevier Science B.V. All rights reserved.