A survey of the aeration status of drip-irrigated orchards

We extensively surveyed the soil aeration status in 35 commercial, drip-irrigated Israeli orchards, mostly in 2007 and 2008. The main objective of the survey was to evaluate the extent and severity of soil hypoxia in drip-irrigated orchards. The survey involved measuring soil gaseous O-2 concentrations at depths of 0-60 cm, 20 cm to the side of the emitter. Oxygen concentrations at active root depths were usually higher than 15% (vs. 21% in the atmosphere) and decreased approximately linearly with increasing depth. During the cold, rainy winter the soil O-2 concentrations were usually higher than in the warm irrigation season, but after heavy rain they usually dropped for a few days. Low O-2 concentrations were mostly found in intensively irrigated. clayey soils. The negative gradients of O-2 concentration vs. depth were highly correlated with soil water content which, in turn, was highly correlated with the soil clay content. Thus, the concentration gradients were also higher in orchards irrigated with a single drip line per tree row than in those with two lines per row. The O-2 concentrations decreased with increasing temperature. In a few sites those in plots irrigated with recycled effluent water were similar to or slightly lower than those in plots irrigated with fresh water at similar rates. Within each irrigation cycle the O-2 concentrations decreased after water application and increased as the soil dried. A few observations showed that O-2 concentrations near mature trees were lower than those near young trees or in uncultivated soil. Rough evaluation of the diffusive vertical O-2 flux, averaged over all orchards, based on the mean O-2 concentration gradient and on the mean O-2 diffusion coefficient yielded a value of 15 g m(-2) day(-1), which is consistent with reported respiration rates of cultivated soils at 25 degrees C. It is likely that in some circumstances this O-2 diffusion rate may be a limiting factor with regard to root respiration, photosynthesis, water and nutrient uptakes, plant growth and yield, especially under intensive irrigation and fertigation and at elevated soil temperatures. (C) 2012 Elsevier B.V. All rights reserved.