Soil Water Movement Under The Soil Electric Field And The Coupling Effect Of “soil Electric Field-humus”

Soil water movement is closely related to many aspects such as agricultural production and environmental protection.Therefore,soil water movement has always been a hot issue in academic research.Since the introduction of Darcy’s law,although domestic and foreign scholars have established a large number of soil water infiltration models and tried to achieve an accurate description of soil water movement,it is quickly found that it is difficult due to the complexity of the soil itself and soil water movement.Find a universally applicable mathematical model to achieve an accurate description of soil water movement.Therefore,people began to study the internal mechanism of soil in the process of water infiltration,and tried to explore the law of soil water movement from the factors and mechanisms that affect soil water movement.Recent studies have found that soil electric fields play an important role in soil water movement.The stronger the soil electric field,the greater the repulsive force between the soil colloidal particles,the worse the stability of the soil agglomerate and the stability of the soil void,and the slower the water movement,and vice versa.It is well known that soil humus can improve the stability of soil aggregates and soil pores,thereby promoting soil water infiltration.However,from the perspective of colloidal chemistry,on the one hand,humus is also a kind of colloidal particles with a large amount of negative charge on the surface,therefore,increasing the humus content in the soil must strengthen the electric field of the soil,thereby reducing the stability of soil aggregates and soil pores,and thus reducing soil water movement;on the other hand,as a macromolecular organic substance,humus can increase the long-range molecular attraction between soil particles and improve the stability of soil aggregates and soil pores,thereby promoting water movement.This indicates that there is a complex coupling between soil electric field and soil humus during soil water movement.However,the current research in this area is still less involved.Based on this,this study will focus on the coupling between soil electric field and humus during soil water movement,and clarify the effect,strength and mechanism of soil water movement.In this study,different concentrations of NaCl,KCl,and CaCl₂ electrolyte solutions were used to control the electric field.The purple soil samples with different HA（humus）content were used as test materials.The indoor one-dimensional vertical soil column infiltration test was used to determine different electric field strengtHA and different HAs.The rate of soil water infiltration at the time of the study,the study found that:（1）The water infiltration rate increases monotonously with the increase of electrolyte concentration under 1:1 electrolyte（NaCl,KCl）system,and increases with the electrolyte concentration from 10^-5mol L^-1 to 1mol L^-1.The water infiltration rate of KCl and NaCl electrolyte system increased by about 2 times and 21 times respectively;and when the electrolyte concentration is the same,the water infiltration rate is related to the type of electrolyte,which is expressed as:KCl>NaCl,for,the water infiltration rate under KCl electrolyte system is about 1.2-12.2 times of the water infiltration rate of the NaCl electrolyte system.（2）The water infiltration rate of 2:1 electrolyte（CaCl₂）system is 10^-2 mol L^-1 as the critical point.The water infiltration rate increases with the electrolyte concentration when the electrolyte concentration is lower than 10^-2 mol L^-1;and when the electrolyte concentration is 10^-2 mol L^-1,the mean rate of infiltration is increased by 56%and17%.compared with the electrolyte concentration of 10^-5 mol L^-1 and 10^-3 mol L^-1,respectively.The water infiltration rate decreases with the increase of electrolyte concentration when the electrolyte concentration is higher than 10^-2 mol L^-1;and when the electrolyte concentration is 10^-2 mol L^-1,the mean rate of infiltration is decreased by8%and 39%.compared with the electrolyte concentration of 10^-1 mol L^-1 and 1 mol L^-1,respectively.（3）When the electrolyte concentration is the same,for NaCl electrolyte system,the infiltration rate is the highest at 1%HA,followed by 0%HA or 5%HA,and the minimum infiltration rate at 2.5%HA;for KCl electrolyte system,0%HA infiltration The rate is the highest,followed by 2.5%,and the smallest is 1%HA or 5%HA.Based on the above results,we can get the following conclusions.（1）In the presence of a 1:1 electrolyte and a 2:1 electrolyte,the electric field plays a different role in the movement of water.In the presence of a 1:1 electrolyte,the infiltration of water in the purple soil is determined by the soil electric field.The stronger the soil electric field,the stronger the electrostatic repulsion between the soil particles,the lower the stability of the soil agglomerates,and the worse the soil pore structure,resulting in slower water infiltration.Since K⁺is more capable of shielding soil electric fields than Na⁺,the interaction force between soil particles is smaller,so water infiltration is faster under K⁺system.The critical concentration of water infiltration in purple soil is 2:1 type electrolyte system;the critical concentration is 10^-2mol L^-1 for CaCl₂;and 10^-2 mol L^-11 CaCl₂ is also the critical concentration of relative total water potential（sum of matrix potential and osmotic potential）.When the electrolyte concentration is low,such as when the CaCl₂ concentration is less than 10^-2 mol L^-1,the matrix potential based on the electrostatic field dominates the soil water movement;when the electrolyte concentration is higher,such as the CaCl₂ concentration is higher than 10^-2 mol L^-1At this time,the electrostatic field is fully shielded by Ca²⁺,and the osmotic potential dominates the soil water movement.（2）The soil electric field and HA has a strong coupling effect on the water infiltration in purple soil.Due to the coupling effect,the water infiltration rate under NaCl electrolyte system is 1%HA>0%HA or 5%HA>2.5%HA;the water infiltration rate under KCl electrolyte system is 0%HA>2.5%HA>1%HA or 5%HA;this may be related to the increased soil electric field of HA and the cementation and water repellency of HA.In addition,the coupling strength of“soil electric field-HA”is related to electric field size and HA content,and there is obvious ion-specific effect;that is,when the electric field and HA content are the same,the coupling strength is related to the electrolyte type,and it’s KCl>>NaCl.