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 O2 concentrations at depths of 0–60cm, 20cm 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 O2 concentrations were usually higher than in the warm irrigation season, but after heavy rain they usually dropped for a few days. Low O2 concentrations were mostly found in intensively irrigated, clayey soils. The negative gradients of O2 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 O2 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 O2 concentrations decreased after water application and increased as the soil dried. A few observations showed that O2 concentrations near mature trees were lower than those near young trees or in uncultivated soil. Rough evaluation of the diffusive vertical O2 flux, averaged over all orchards, based on the mean O2 concentration gradient and on the mean O2 diffusion coefficient yielded a value of 15gm−2day−1, which is consistent with reported respiration rates of cultivated soils at 25°C. It is likely that in some circumstances this O2 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.