In Canada, most lettuce (Lactuca sativa L.) is produced on cultivated organic soils, which can be very productive but are also very sensitive to degradation and compaction. The objective of this work was to evaluate the effect of soil compaction, irrigation thresholds, and transplant type on the growth and water-use characteristics of Romaine lettuce that is grown in organic soil. The experiments were conducted in greenhouses at Laval University. Tensiometers and time-domain reflectometer probes were used to characterize the water-use characteristics of the Romaine lettuce. Most of the growth characteristics of the Romaine lettuce, with the exception of the dry weight, were significantly influenced by the available rooting depth (soil column height) and by the irrigation threshold used. Lettuce water uptake decreased significantly as the depth increased. In addition, in drier conditions, the deeper soil layers contributed more to the total water uptake than the surface soil layers. The water productivity was lower in the presence of a compacted layer combined with a direct seeding treatment, compared with all of the other treatments. First, it is concluded that the irrigation method should allow a certain degree of dryness by use of a lower irrigation threshold (ideally between -20 and -30 kPa) to stimulate deep rooting. Second, the use of small lettuce plant preseeded in small block of peat substrate instead of direct seeding in the field can compensate for a possible compaction effect.
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