Study On Water And Salt Transport In Saline Soil Based On The Adjustment Of Electric Field,Mechanical Composition And Osmotic Pressure

The main distribution area of saline soil in China is relatively wide,in which the available saline soil area accounts for a large proportion.In recent years,researchers have done a lot of research on the improvement of saline soil,and put forward techniques and methods to improve the structure of saline soil and realize rapid salt washing.Soil water has a profound influence on soil salinization and salinization.Previous studies mostly change the movement rate of soil moisture by adjusting the pore condition of soil,mainly to improve the pore condition of saline soil,but the mechanism of its adjustment is not deep enough.In recent years,the research shows that the soil electric field affects the cohesion and dispersion of soil particles by regulating the interaction between soil particles,which determines the pore structure of soil and ultimately controls the process of soil water movement.If the repulsion between particles increases,the soil is easy to disperse,the macropores decrease and the water movement slows down.Saline soil contains a lot of Na⁺with low polarizability,which makes the soil under the condition of strong electric field,and the strong electric field not only greatly reduces the potential energy of soil water,but also causes soil aggregates to break,which leads to"muddy when wet and hard when dry".At the same time,a large number of base ions make the electrolyte in the soil solution in a high concentration state,and the osmotic repulsion pressure between soil particles increases,which makes soil aggregates more easily broken and hinders the movement of soil moisture.Based on this,this paper,by adding three modifiers of different proportions of sand,calcareous soil and biochar,uses indoor one-dimensional vertical soil column water movement test and calculation of interaction force between saline soil particles to explore the improvement efficiency of different modifiers and its internal mechanism.The main results are as follows:（1）Sandy soil regulates the mechanical composition and calcium layer soil regulates the soil electric field,which affects the water movement of saline soil.In the alien soil improvement system,the wetting front movement speed of saline soil（CK）is the slowest.Adding 10%calcium layer soil and 10%sand soil respectively promotes the water infiltration of saline soil,but adding 10%sand soil increases the speed even more.Compared with adding sandy soil alone,adding sandy soil and calcareous soil at the same time has no obvious growth rate of water movement in saline soil,that is,there is no synergistic effect between sandy soil and calcareous soil.After 120 h of seepage,the cumulative seepage amount and the cumulative percentage of Na⁺excretion are in the same order as the wetting front migration.Sandy soil has little contribution to the regulation of electric field in saline soil,so the mechanism of sandy soil promoting water movement in saline soil is not to change the inter-particle force.The addition of sand increases the content of sand and silt in saline soil and increases the porosity of soil,that is,sand mainly changes the mechanical composition of saline soil and affects the soil water movement.The calcium layer soil mainly shields the surface electric field of saline soil particles through the ion exchange of Ca²⁺,reduces the repulsive force between particles,improves the soil cohesion,and finally accelerates the soil water movement.（2）The amount of biochar has a profound influence on the water movement of saline soil.The change order of wetting front velocity with biochar addition is 1%>3%>CK>5%.After 50 h of seepage,the order of cumulative seepage quantity,porosity,seepage rate and cumulative Na⁺excretion is:3%>5%>1%>CK,which keeps the opposite trend to the order of total repulsive pressure（electrostatic repulsive pressure+osmotic repulsive pressure）among soil particles in each treatment.Biochar plays a significant role in supporting the pore structure of saline soil because it will collapse and reduce its porosity after the salt is washed out in the process of soil leaching.With the increase of biochar addition,the collapse is weakened,and there is no obvious difference between 3%and 5%treatments on the pore support of saline soil.However,in the wetting front stage,the water repellency of biochar itself is extremely slow,so the salt discharge order of each treatment is 3%>5%>1%>CK.（3）The salt washing efficiency of each treatment after one leaching is quite different.The seepage rate and cumulative salt discharge percentage of each treatment increased slowly or stagnated at about 30 h,and it was difficult to wash salt at this time.The maximum cumulative Na⁺excretion of saline soil with 10%sandy soil is only 60%,and it can reach 90%with 3%biochar treatment,but there are still a lot of base ions in the soil,and the large osmotic pressure between particles still hinders water transport.（4）Simulated secondary leaching after ploughing can solve the problem of low salt washing efficiency in the later stage.In the second leaching,the order of cumulative seepage,seepage rate,cumulative salt discharge,porosity and total repulsive pressure between soil particles is the same as that in the first leaching.After the first washing,the cumulative percentage of Na⁺excretion in each treatment was 48.75%（CK）,71.82%（1%）,91.02%（3%）and 73.17%（5%）,and after the second washing,the cumulative percentage of Na⁺excretion was 65.61%（CK）and 82.2%（1%）,95.36%（3%）and 84.49%（5%）.The results show that the efficient period of salt washing in saline soil is within the first 30 h of leaching,and repeated leaching can achieve the goal of rapid salt washing.in conclusion:The mechanism of different modifiers regulating the water movement of saline soil is different.Sandy soil and biochar mainly regulate the mechanical composition of saline soil,while calcium soil mainly regulates the soil electric field,which together with osmotic pressure in saline soil affects the total repulsion between particles,controls the soil porosity,and finally controls the water and salt movement of saline soil.In practical production,the best improvement effect can be achieved by adjusting the water movement of saline soil by making compound improver.