Decreasing soil pH value to promote spinach growth by application of urea phosphate under alkaline stress

Abstract: Soil alkalization is one of major environmental problems which adversely affect growth and development of plant and crop yield in agricultural production all over the world. Effective measures should be taken to reduce the impact of alkaline stress on plant growth. Urea phosphate is a new-type compound fertilizer in the drip-irrigation. It can generate acid after hydrolysis, but it is unclear if it can improve soil pH value and plant growth after applying to alkaline soil at present. Therefore, this study was conducted to test the effects of urea phosphate solution on soil pH value, plant growth and physiological characteristics of large-leaf spinach. The alkaline stress was simulated by irrigating 5 mL/d solution which was a mixture of 300 mmol/L NaHCO3 solution and 300 mmol/L Na2CO3 solution at 1:1 molar ratio for 6 d in a greenhouse. Four treatments were set in a random complete block design in this paper, including CK (Non-alkali stress), AS treatment (adding 5 mL deionized water after alkali-stress), AS+FT treatment (adding 5 ml inorganic fertilizer solution after alkali-stress which was mixed with diammonium phosphate and urea solution and had a concentration equal with urea phosphate solution) and AS+UP treatment (adding 5 ml urea phosphate solution after alkali-stress). The results showed that the soil pH value was significantly increased by 44.19% after alkali-stress, but it were significantly decreased by 2.69 and 0.86 when applying urea phosphate solution and inorganic fertilizer solution to soil 6 days after alkaline stress, and urea phosphate solution had better effect than inorganic fertilizer solution on improving soil pH value. Compared to AS treatment, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities in AS+UP treatment were significantly increased by 6.42%, 42.08% and 55.13%, respectively, malondiadehyde (MDA) content was significantly reduced by 10.62% and then the extent of membrane lipid was obvious alleviated. Meanwhile, photosynthetic pigment content was greatly improved after applying urea phosphate solution. Chlorophyll a, chlorophyll b and carotenoid concentrations of AS+UP treatment were increased by 22.36%, 32.06% and 10.88%, respectively. Improvement effect on spinach seedlings growth in AS+UP treatment was superior to AS treatment, and plant height, whole leaf area and dry substance of spinach of AS+UP treatment were respectively 23.55%, 21.03% and 33.84% higher than AS treatment. However, the seedling growth in AS+FT treatment was inhibited and physiological characteristics of spinach leaves were decreased. Compared to AS treatment, SOD, POD and CAT activities in AS+FT treatment were reduced by 15.33%, 29.06% and 51.15%, respectively, and its MDA content was higher than that of AS treatment, so chlorophyll a, chlorophyll b and carotenoid contents were decreased by 19.84%, 37.15% and 14.15%, respectively. Moreover, leaf area, dry matter weight and root length were also respectively 26.47%, 26.16% and 1.65% lower than AS+UP treatment. This research indicates that urea phosphate solution has positive effects on improving soil condition and enzyme activity under alkaline stress. Therefore, it can be promoted as a kind of soil amendment to reduce soil pH value in alkaline soil, enhance the ability to adapt to alkali stress and ensure the normal growth and development of crop plants, and it may be a valid method to increase land use efficiency and mitigate the pressure of land demand.