A field experiment was carried out for two years to investigate the benefits of negative pressure water supply on surface soil water content, nitrate-nitrogen (NO3−-N) distribution in the soil profile, economic yield and water and fertilizer use efficiency of tomato and cucumber under greenhouse cultivation in the North China Plain. The experiment included two irrigation treatments: drip irrigation with nutrient solution (DIN) and negative pressure irrigation with nutrient solution (NIN). The results showed that the NIN treatment had a relatively stable soil moisture (about 87% of field capacity), and the fluctuation of soil water content in the 0–20 cm soil layer was 20.6%–25.0% during the experiment period in 2014–2015, which was less than the range of 19.2%–28.1% in the DIN treatment. In both the DIN and NIN treatments, the NO3−-N at the end of the four growing seasons was mainly distributed in the 0–40 cm soil layer and showed a gradually increasing trend as the number of cultivation years increased. Compared with the DIN treatment, the NO3−-N content in the 0–60 cm layer of the NIN treatment was significantly decreased by 19.7%–28.0% after the fourth growing season. The NIN treatment produced the highest economic yield with lower water and nutrient input than the DIN treatment, however, no significant difference was observed in tomato and cucumber yield in the two years. Average irrigation water use efficiency (WUEi) and partial factor productivity of fertilizer (PFPf) over the study period were all significantly improved under the NIN treatment relative to the DIN treatment, with increases of 26.2% and 25.7% (P < 0.05), respectively. Negative pressure water supply not only maintained a high fruit yield, but significantly increased WUEi and PFPf, indicating a great advantage in water and fertilizer saving compared with drip irrigation.
It is critical to identify effective buffer chemicals which are capable of regulating the pH of nutrient solution to a desirable level for best hydroponic production of crops. Greenhouse experiments were conducted to examine the pH dynamics of nutrient solutions amended with different inorganic acids during hydroponic production and the yield and nutritional quality of the resulting crops. A typical nutrient solution (pH 8.2) was adjusted to a pH of 5.6±0.2 with 1 M HNO3, 1 M H3PO4, 1 M H2SO4, and a 3:1:1 (v/v/v) mixture of all three acids, respectively. Water spinach (Ipomoea aquatica Forsk) was hydroponically grown for 25 d in the unadjusted control and the pH-adjusted nutrient solutions. The different treatments were monitored for pH changes of the nutrient solution, and measured for shoot yield and nutritional quality of the grown water spinach. It showed that the solution pH adjustments introduced additional anions but did not significantly increase the electrical conductivity (EC). The HNO3-H3PO4-H2SO4 mixture was able to achieve an optimal solution pH ranging from 5.5 to 6.5, while any of the acids only failed to maintain the solution pH within optimal range for 48 h after each adjustment. The shoot fresh weight, dry weight, and height of water spinach grown in the mixed acids-treated solution were the greatest among the five treatments. Relative to the control, the acid mixture treatment also increased the vitamin C, soluble sugar and crude protein contents in plants. Thus, the mixed 3:1:1 (v/v/v) HNO3-H3PO4-H2SO4 is recommended for regulating the pH of nutrient solution in hydroponic production of leafy greens.
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