The Effect Of Underground Drainage And Sand Mixing On The Physical And Chemical Properties Of Heavy Salinized Soil In Hetao Irrigation Area

Taking the heavily salinized soil in the lower reaches of Hetao Irrigation Area in Inner Mongolia as the research object,aiming at the problems of high silt content,poor physical properties,slow infiltration speed and salt leaching difficulty in this area,the underground pipe drainage and salt drainage project was implemented on the basis of various saline alkali land improvement measures such as desulfurization gypsum,deep furrow in the field and surface sand mixing The monitoring of soil physical and chemical properties,hydraulic properties and nutrients reveals the improvement and regulation mechanism of severe salinized soil in the lower reaches of Hetao irrigation area,puts forward the best buried parameters of concealed pipe,and obtains the appropriate amount of sediment through laboratory test,which provides theoretical basis and technical support for the application and development of farmland drainage system and saline alkali land improvement in Hetao irrigation area.The specific research contents and conclusions are as follows.(1)After two years of repeated irrigation,the soil bulk density of each treatment decreased slightly,the soil field capacity,saturated water content and saturated hydraulic conductivity increased in different extent after each irrigation,and the change of soil structure was not obvious.In the whole leaching process,Na~+and Cl~-were more easily leached ions.The soil desalination rate of four times leaching in 0-40 cm soil layer was between 5%and 63%,and the soil salt content of different treatments decreased from 11.75-28.06 g/kg to 5.28-13.02 g/kg.(2)The decreasing range of salt ions with the buried depth of buried pipe is 0.8 m<1.1m<1.3 m.therefore,appropriately increasing the buried depth of buried pipe in soil can enhance the salt leaching effect of soil.After subsurface drainage,the process of soil alkalization will slow down,which shows that the alkalization degree of soil with different buried depth of subsurface pipe after leaching is lower than that of the initial,indicating that subsurface drainage measures can slow down the trend of soil alkalization to a certain extent.The results showed that the soil nutrients were lost in the process of underground pipe drainage,and the loss degree of soil nutrients under different buried depth of underground pipe was 1.3 m>1.1 m>0.8 m.(3)In the whole leaching process,the contents of Na~+and Cl~-were easier to leach,and the desalination rate of soil was between 5%and 63%after four times leaching in 0-40 cm soil layer.The results showed that the soil desalination rate of different tube spacing treatments was 10 m>20 m>30 m.After irrigation,the soil alkalization index of each treatment showed a trend of first increasing and then decreasing,and the arrangement of small spacing subsurface pipes had more potential to alleviate the degree of soil alkalization.However,the process of subsurface drainage will cause a large amount of soil nutrient loss,and the loss amount of each spacing treatment is 10 m>20 m>30 m.(4)The saturated hydraulic conductivity of soil increased with the increase of sand content.The saturated hydraulic conductivity of 0-30%sand content was between 0.00138-0.402 mm/min.The fitting accuracy of van Genuchten model was better than Gardner model,and the fitting parameter?value under different sand content was significantly different(P<0.05),indicating that the water release performance of soil was significantly enhanced with the increase of sand content.The results show that the proportion of larger pores in soil increases and the availability of soil water is improved by adding sand.(5)With the increase of sand content in the surface layer of soil,the cumulative infiltration amount and wetting peak velocity of soil increase gradually;when the sand content is 18%-24%,the stable infiltration rate of soil is between 0.065-0.091 mm/min;when the sand content is 26%-30%,the stable infiltration rate of soil is more than 0.1mm/min,but the water holding capacity of soil is low.The soil salt content of 30cm soil layer with 18%-24%sand addition for 20 days was more than 90%lower than that without sand addition,which could ensure the desalination capacity and water holding capacity of the soil at the same time.Kostiakov model can well simulate the process of soil water infiltration in this study.