Drip irrigation scheduling for annual crops in an impermeable saline-sodic soil with an improved method

Abstract

A two-year field experiment was conducted to reclaim a highly saline-sodic soil for growing annual crops in the Ningxia Plain, northwest China. The surface layer of this highly saline-sodic soil (known as “takyric solonetz”) had an electrical conductivity (EC_e) of 12.3 dS m⁻¹, a sodium (Na) adsorption ratio (SAR) of 44.1 (mmol L⁻¹)^0.5, and a saturated soil paste pH (pH_s) of 9.3, with a very low saturated hydraulic conductivity (K_s < 0.1 mm d⁻¹ [0.004 in day⁻¹]). To increase the soil infiltration rate, very low-flow drip irrigation was used, and the original soil beneath the drip tape was replaced with sand in grooves that were 0.15 m (0.50 ft) wide and 0.15 m deep. The drip lines were placed on beds covered with straw into which oil sunflower (Helianthus annuus L.) seeds were planted in 2013 and tomato (Lycopersicon esculentum Mill.) seedlings in 2014. Five water treatments were based on the soil matric potential (SMP), which was measured with tensiometers installed 0.2 m (0.7 ft) beneath an emitter. The tensiometers triggered an irrigation of 2 mm (0.08 in) at a SMP of −5, −10, −15, −20, or −25 kPa (−0.73, −1.45, −2.18, −2.90, or −3.63 psi). The salinity and SAR of the applied water were 2.03 dS m⁻¹ and 5.89 (mmol L⁻¹)^0.5, respectively. Considerable reclamation was achieved in two years. The low-salinity, low-pH, and low-SAR zone that formed under the drip line during the first year enlarged in the following year. The average EC_e within the 0 to 40 cm (0 to 16 in) depth in the ridge zone of the bed decreased with increasing SMP, and the final EC_e for all five treatments was less than 3.0 dS m⁻¹. The final pH_s decreased to approximately 8.2 beneath the drip line, and the low-pH region of the −5 and −10 kPa treatments were larger than those of the −15, −20, and −25 kPa treatments. The final average SAR at the depth of 0 to 30 cm (0 to 12 in) beneath the emitters was in the range 10.2 to 12.8 (mmol L⁻¹)^0.5 except for the SMP treatment of −25 kPa, for which the average SAR was 19.5 (mmol L⁻¹)^0.5. The maximum yield for both crops occurred when SMP was −10 kPa; however, there were no significant differences among treatments, and the yields were close to those achieved in local farmland. The findings indicated that a sand-filled groove in the soil surface beneath drip emitters could facilitate the reclamation of saline-sodic wasteland with farming annual crops, such as oil sunflower and tomato. Furthermore, a SMP of −10 kPa could be used effectively to trigger drip irrigation in the first two years.