Effect of composite soil improvement agents on soil amendment and salt reduction in coastal saline soil

The aim of this study was to develop a rapid, low-cost method to desalinate muddy coastal saline soil to facilitate its use for economic crops. We used composite improvers (phosphogypsum, dung, humic acid, and corn straw), comprehensive agronomic measures (deep tillage, shallow soil improvement, high ridge cultivation, and drip irrigation), biological measures (step planting of salt-tolerant plants), and adopted L₁₆(4⁵) orthogonal design, and fuzzy mathematic evaluation to systematically analyze the effects of composite improvers on soil amelioration, salt reduction, and Abelmoschus manihot (L.) Medic yield. The total effect of each composite improver was comprehensively evaluated, and the preferred improvers suitable for application on coastal muddy saline soil were determined. Results showed that composite improvers with a capital cost of 2.55-6.01 ￥·m^-2 significantly reduced soil salt content when used with comprehensive agronomic measures consisting of 40 cm of tillage, 30 cm of soil improvement, 15 cm of ridge, drip irrigation controlled to -10 kPa of soil potential, and Sesbania cannabina (Retz.) Poir used as the fore-rotating plant. Soil salt content decreased from 10.86 g·kg^-1 to < 2.0 g·kg^-1 in the 0-10 cm soil layer and to < 2.5 g·kg^-1 in 10-20 cm layer. Soil organic matter content increased to >12 g·kg^-1; there were significant increases in alkali-hydrolyzed nitrogen and available phosphorus, and the soil macroaggregate content was 2.41-7.62 times higher than that of the control. The proper combinations of composite improvers for heavy saline silt soil were screened based on the comprehensive evaluation of soil salt content, pH, organic matter, alkali-hydrolyzed nitrogen, available phosphorus, available potassium, and micro-aggregates. A combination of 22 500 kg·hm^-2 phosphogypsum + 105 m³·hm^-2 organic fertilizer + 3 750 kg·hm^-2 humic acid + 45 m³·hm^-2 maize straw provided the optimum effect on the improvement of coastal saline soil. Soil organic matter and alkali-hydrolyzed nitrogen increased by 181.87% and 130.52%, respectively, whereas the capital cost was only 4.05 ￥·m^-2. Two other combinations provided suboptimal results:15 000 kg·hm^-2 phosphogypsum + 75 m³·hm^-2 organic fertilizer +3750 kg·hm^-2 humic acid +135 m³·hm^-2 maize straw (capital cost 4.48￥·m^-2); and 30000 kg·hm^-2 phosphogypsum +5 m³·hm^-2 organic fertilizer +2250 kg·hm^-2 humic acid +135 m³·hm^-2 maize straw (capital cost 5.02 ￥·m^-2).