Study On The Rules Of Mixing Straw In Sand Cover Measures On Soil Water And Salt Movement In Loess Region Under Brackish Water

As a kind of backup water source,brackish water can alleviate the shortage of irrigation water and increase crop yield,but soil salinization may occur due to misuse.Proper farming measures and irrigation system can ensure that it benefit in a green way.Sand covering is helpful for heat preservation,water storage,soil moisture conservation and salt suppression.Straw returning can improve soil structure,increase soil nutrient and water content and inhibit salt accumulation on soil surface.In this paper,two kinds of soil in Yongjing County of Gansu Province were studied.Three kinds of sand covering thickness(3,6,9 cm)were set on the soil surface.Different proportions of powdery corn straw(0,1%,3%,5%)were mixed in the sand covering layer.One dimensional infiltration test of soil column was carried out.The soil water characteristic curve and several water constants of soil and overburden were measured to study the influence of different mulching measures on the water and salt migration of two kinds of soil under the use of brackish water.The main contents and conclusions are as follows:(1)The moisture constants of two kinds of soil and the sand samples mixed with different proportion of straw powder was measured.The result shows:compared with idle soil,farmland soil has stronger water holding capacity,and the former has higher capillary porosity and field water holding capacity,while the latter has higher saturated water content and water conductivity.The moisture absorption,water holding capacity of sand were increased by mixing straw powder.The ratio of mixed straw powder in sand layer is positively correlated with its maximum moisture absorption,water content of capillary fracture and capillary water holding capacity.The saturated hydraulic conductivity has a negative correlation with the mixing ratio.(2)The water characteristic curves of soil and sand samples mixed with different proportions of straw powder were measured by centrifuge.The result shows:VG-M model,BC model and LND-M model all fit the farmland soil water characteristic curve(R~2>0.99);VG model is suitable to describe the relationship between the variables of soil water characteristic curve of idle soil.BC Model can fit the soil water characteristic curve of pure sand and sand mixed with straw powder(R~2>0.99);And with the increase of straw ratio,the smaller the fitting parameter n is,the larger the fitting parameter?is.(3)One dimensional infiltration test of soil column under different thickness of sand cover was carried out.The result shows:covering at 3cm can increase the infiltration rate of water in the two soils,increase irrigation and promote wetting front advance,and have effect of salt leaching on farmland soil;sand mulching at 6cm can increase the salt accumulation rate(?S/Sa)of farmland soil,but it can reduce the salt input of idle soil and inhibit salt accumulation on the soil surface;sand mulching at 9cm has higher salt adsorption capacity,and can improve the soil water content;the relationship between time and cumulative infiltration of pure soil can be fitted by Philip model,while the relationship between time and cumulative infiltration of sand-covered soil surface is more consistent with Kostiakov model.(4)The infiltration test was carried out on the soil column with different proportion of straw powder in the sand layer.The result shows:treatment 1S9J1 can increase the irrigation of farmland soil;straw mixing ratio can reduce the infiltration rate and actual infiltration of farmland soil,slow down the advance of soil-1 wetting front,but it can promote wetting front advance of idle soil.In this paper,all the mulching measures can increase the irrigation and the average soil water content of idle soil.The relationship between infiltration rate and time of the two kinds of soil basically conforms to the Kostiakov model,and the infiltration rate of idle soil is more stable,and the imitative effect is better than that of farmland soil.The 5% straw powder is mixed in the sand to reduce the surface water content of farmland soil when water is stopped.The larger the overburden thickness is,the lower the soil surface water content is.The deep salt drainage of farmland soil and idle soil is promoted.After redistribution,the average water content of soil in1S3J1?1S6J5?1S9J3 and 2S3J5?2S6J5?2S9J3 treatments is higher,and the water holding capacity is better.In the redistribution process of 2S9J1 treatment,the water loss was the smallest,the water holding capacity was better,and the salt adsorption capacity of the covering layer was the largest.The salt adsorption capacity of different straw powder proportion in the sand layer of farmland soil is1%>3%>5%>0,and the thicker the overburden is,the more the adsorbed salt is.Under the treatments of 2S3J5?2S6J5 and 2S9J5.The accumulation rate of idle soil(?S/Sa)and?S/Is is the highest under the same thickness.The salt drainage is better under this treatment.