Experimental Study On Water And Salt Distribution And Improvement Effect Of Saline Alkali Soil Under Combined Brackish And Fresh Water Irrigation

The severe saline-alkali soil,taken from Dongying city,Shandong province,was chosen as study object.In the laboratory,one-dimension vertical infiltration experiments were conducted on the soil to study under salinity of brackish water,different sequences and combination ratios between brackish and fresh water,the vertical distribution feature of soil water.The applicability of one dimensional algebraic model in simulating water content of soil profile was analyzed.The total salt content and the vertical distribution characteristics of sodium,calcium and magnesium in soil profile under different irrigation patterns were analyzed.The standard desalting rate,the standard desalting depth,SAR,ESP or other indicators were adopted to evaluate effect of saline-alkali soil improvement.Main results showed that:(1)Irrigation with brackish water was beneficial to improve the water content of soil profile after irrigation,meanwhile the effects of different irrigation patterns on soil moisture content were different.Soil water content was maximal after the infiltration of brackish water directly,salinity of 4 g·L-1.Compared with irrigation with freshwater,soil moisture increased by 4% equally in the 10-45 cm soil depth.Under combination irrigation,soil moisture with brackish water irrigation first was greater than freshwater irrigation first.So did the uniformity.While the ratio of freshwater among irrigation quota decided the similarity of moisture content between combination irrigation and freshwater irrigation.The more freshwater accounted for irrigation,the higher the similarity of moisture content of soil profiles.(2)There were some differences in the results of simulation during one dimensional algebraic model simulated moisture content of profiles.Under freshwater irrigation directly,the result of brackish water,salinity of 4 g·L-1,was worst.Its root mean squared error(RMSE)and index of agreement(D)were 2.29% and 0.90.Under combination irrigation with brackish water and fresh water,the simulation results of freshwater irrigation first were better than brackish water irrigation first.The simulation results were significantly affected by the proportion of combination.But all the results of simulation shared root mean squared error(RMSE)lower 2.80% and index of agreement(D)exceed 0.90.So one dimensional algebraic model was suitable for describing the water content of heavy saline alkali soil under the condition of brackish water irrigation(3)Compared with brackish water irrigation directly,brackish water irrigation first was more meaning to leach salt included in upper and middle soil layers.Although freshwater irrigation first was more conducive to leach salt existed in bottom soil layers.The more freshwater among combination irrigation,the greater the leaching range,reaching results of freshwater irrigation directly.When mineralization degree was lower than 3 g·L-1,brackish water irrigation was more conducive to leach sodium ions than freshwater irrigation directly.Meanwhile the more freshwater among combination irrigation,the greater range sodium ions were leached more than freshwater irrigation directly.For example,in the 0-40 cm soil depth range,2 g·L-1 brackish water irrigation first leached more 15% sodium ions than 2 g·L-1 brackish water irrigation directly.Fresh water irrigation first accelerated the leaching of calcium ions in the deep soil,and brackish water irrigation first accelerated the leaching of calcium ions in the upper soil.The more proportion of freshwater,the greater depth of the soil layer,getting more calcium ions leached.With the increase of the proportion of freshwater among combination irrigation,the depths of leaching calcium ions,the same as freshwater irrigation directly,were improved 15 cm,25cm and 35 cm respectively.But brackish water irrigation directly on leaching magnesium ions was better than freshwater irrigation directly.And brackish water irrigation first was more effective to leach magnesium ions in the deep soil layers.Freshwater irrigation first was good for leaching magnesium ions in the upper and middle soil layers.The more brackish water,the greater range magnesium ions leached.(4)The freshwater and brackish water combination irrigation was more conducive than brackish water irrigation to increase the standard desalting depth,increasing the average desalination rate in that range.And the uniformity of desalination was improved in this range.No matter which brackish water irrigation mode,the total salt,SAR and ESP shared peak value.The soil quality tend to getting worse in the depth over 40 cm.When salinity was over 4 g·L-1,the soil quality worsened by brackish water irrigation in the depth range of 5-15 cm.However brackish water irrigation first should solve that problem,and the more freshwater,the more obvious the effect.