The liquid phase part of soil, in which various inorganic and organic compounds are dissolved.Their composition and properties often change due to the constraints of soil parent material, climate, topography and biology, which in turn affects the formation, development of soil and plant growth.Some substances in the soil solution can reduce the surface tension of the solution in varying degrees to produce positive adsorption, while others may increase the surface tension of the solution to produce negative adsorption.Therefore, the composition and concentration of soil solution at different distances from the surface of soil particles are different, so as to provide an optional liquid phase location for root hairs.If the concentration is too thin, the plant cannot get sufficient nutrition;Too high concentration will prevent plants from absorbing water;When it contains more harmful salts, it can causesoil salinization。[1]
Chinese name
Soil solution
Foreign name
Soil solution
Role
Foundation of soil material migration and transportation
Definition
Soil interstitial water containing solute and dissolved gas
Soil solution is the general term of soil water and its solutes and suspended substances[2]。Its solutes include various soluble salts and nutrients, as well as soluble pollutants, which generally include inorganic colloids such as iron and aluminum oxides;Inorganic salts such as carbonate, bicarbonate, sulfate, chloride, nitrate, phosphate, etc;Organic compounds such as humic acid, organic acidcarbohydrate, protein, etc.
Soil moisture is an important part of soil and the "blood" of soil.It mainly comes from precipitation, irrigation and groundwater 2~3m near the ground. It is not only the main source of plant nutrients, but also the medium for pollutants entering the soil to migrate to other environmental spheres.Different soils have different water holding capacity, and the water in sandy soil is easy to lose, while the water in clay is not easy to lose.
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The role of soil solution is mainly manifested in that it is the carrier of material transformation and transportation[3]。
First, soil solution is the place where most soil chemical reactions and soil formation processes take place, the carrier of material exchange between soil and environment, and the basis of material migration and movement.
Second, soil solution is the source of nutrients for plant roots.Plant roots directly absorb nutrients from soil solution, which is the beginning of nutrient cycle.The nutrients in the soil solution are constantly moving to the root system, which changes the composition of the soil solution. The solid nutrients in the soil are constantly released and added to the soil solution, realizing the conversion, transportation, renewal and balance of nutrients. The soil solution can reflect the latest dynamics of the soil.
Composition and concentration
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Soil solution consists of water and solute.The chemical composition and concentration of solute depend on the composition of soil and the material exchange between soil and biology, atmosphere and water, which is the result of a series of physical, chemical, physicochemical and biochemical processes.For example, water dissolves soluble substances and gases in soil and air;Substitution and absorption of solution and soil colloid;Dissipation of solution on soil colloids;Interaction between solution and plant root hairs and soil microorganisms;The interaction between soil water and phreatic water.Because of these effects, there are molecular, ionic and colloidal dissolved substances in soil solution.There are inorganic compoundsOrganic compoundOrganic inorganic complex, gas and the smallest colloidal hydrosol.
Organic solutes include soluble cellulose, humic acid, sugar, organic acid, amino acid, pesticide, organic metal complex and chelate;Inorganics including Ca2+、Mg2+、K+、Na+、NHfour+、NOthree-、COthree2-、HCOthree-、CI-、SOfour2-Trace elements and heavy metals.The ratio of organic part to inorganic part in the solution is obviously different in different soils and different soil layers. Generally, the organic part is dominant in grassy podzolic soil and gleyed podzolic soil.In black soil, the two parts are roughly equal.In chestnut soil, brown calcium soil and gray calcium soil, most of them are inorganic parts.
The content of inorganic part in soil solution varies greatly in various soils.In general non saline and non alkaline soil, most of them are a few ten thousandths;In saline soil and alkaline soil, it can reach several thousandths to several percent.Inorganics mainly include calcium, magnesium, potassium, sodium, ammonium and other nitratesnitrite, bicarbonate, carbonate, chloride, sulfate and phosphate.There are ferrous and manganese compounds in swamp soil and acid soil.In alkaline soil and saline soil, sodium chloride, sodium sulfatesodium bicarbonateSodium carbonate is dominant.
Soil colloid is mainly composed of silicic acid, iron and aluminumhydroxideAs well as some organic compounds, their content in the soil solution is very small. According to the research of Gaidroitz, the colloidal substances in the soil solution account for 1/4~1/10, or even 1/20 of the dry residue, and only the colloidal substances in the alkaline layer of alkaline soil are dominant.
The chemical composition of soil solution in different soils is different to some extent, and shows obvious zonality.
For plants, the suitable range of total concentration of soil solution is 200~1000 mg/kg.Whether plant roots can absorb water from soil depends on the concentration of root hair cell fluid and soil solution. Generally, the concentration of cell fluid is greater than that of soil solution, so they can absorb water from soil.Otherwise, it cannot.When the concentration of soil solution is greater than 1000mg/kg, it is difficult for roots to absorb water and nutrients.Too much fertilization at one time makes the concentration of soil solution too high, and the plant roots can not absorb water but lose water, resulting in "burning seedlings".When the concentration of the solution is less than 200mg/kg, the concentration is too thin, lacking nutrients, and cannot meet the needs of crops.
Factors affecting the change of soil solution
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The substances in the soil solution interact continuously with the solid and gas phases of the soil, the roots of plants, and the small animals and microorganisms inhabiting the soil.Soil parent material, climate, groundwater, precipitation, irrigation, fertilization, pesticides, crop varieties and growth periods, farming measures, etc. all affect the composition and content of soil solution, so the composition and concentration of soil solution are often changing.The factors affecting the change of soil solution mainly include the following aspects:
soil temperature
The change of soil temperature is the main factor causing the change of soil solution, because the increase of temperature increases the solubility of many inorganic salts.The solubility of various inorganic substances increases with the increase of temperature, and the final increase degree is different for different substances.
soil moisture
The change of soil moisture strongly affects the change of soil solution. With the increase of soil moisture, the concentration of soil solution decreases, while at high temperature and strong evaporation, the soil becomes dry and the concentration of soil solution rises sharply.When the concentration of soil solution is low, all kinds of elements contained in soil solution exist in ionic state, otherwise, they exist in molecular state.Even in the case of drought and serious water shortage, some substances will precipitate from the solution.The first is calcium carbonate, the second is gypsum, and the last is soluble salts, such as chlorides.
Soil pH
Soil pH affects the change of soil solution, which is due to the different solubility of various substances under different pH conditions.In addition, pH also affects the activity of microorganisms, thus affecting the transformation of substances in soil solution.
Interaction between components of soil solution
When the concentration of some elements in the soil solution reaches supersaturation under certain conditions, precipitation will occur, such asalkaline-earth metalIons are easily mixed with acid radical (such as COthree2-、SOfour2-Some substances can also reduce or increase the solubility of other substances, such as NatwoSOfourLower CaSOfourSolubility of.
Soil biological activity
The substances in the soil solution interact continuously with the solid and gas phases of the soil, the roots of plants, and the small animals and microorganisms that stalk the soil.CO release from soil biological respirationtwoPlant roots secrete organic matter, and microorganisms participate in the transformation of carbon, nitrogen, phosphorus, sulfur and other elements in the soil, which are directly related to the composition and concentration of soil solution.For example, microorganism decomposes organic matter to make CO in soiltwoAs the content increases, the concentration of carbonic acid in the soil solution also increases. Different plants have different roles in the utilization and enrichment of various elements, which can also cause differences in the content of elements in the soil solution.
