Impacts of a gravel–sand mulch and supplemental drip irrigation on watermelon (Citrullus lanatus [Thunb.] Mats. & Nakai) root distribution and yield

We investigated the root distribution and yield of watermelon grown with a gravel–sand mulch, a plastic-film mulch, or a combination of the two mulches, and in a non-mulched control under natural precipitation and under three levels of drip irrigation (23, 45, and 68 mm). Roots reached depths of 1.5 m in mulched fields, but most roots (more than 85.3% in 2001 and 92.5% in 2002) in the non-irrigated control were found within the top 100 cm of soil. The gravel–sand mulch increased watermelon root length density (RLD) by 75% compared with the control. In the mulched field, RLD was higher in the top 30 cm of soil in the control (non-irrigated) treatment in 2002 than in 2001, a relatively dry year. Root growth was densest at 68 mm of irrigation in 2001 and without irrigation in 2002. RLD reached a maximum of 1.95 cm cm−3 when irrigation and rainfall totalled 194 mm, but decreased at higher levels. The relationship between RLD and total irrigation plus rainfall followed a quadratic curve. In fields mulched with the gravel–sand mixture, roots grew more in the absence of the plastic-film mulch, except in the 68-mm irrigation treatment in 2002, but there was little difference among treatments in 2001. In the mulched field, watermelon yields decreased significantly with decreasing irrigation in 2001, but the yields did not differ significantly from those in 2002. Watermelon extracted more water from soil in 2001 than in 2002. These results demonstrated that rainfall and irrigation affect both the horizontal and vertical distribution of roots and yield. Moderate water deficits increased root length.