Soil water in pedology refers to the water that can be separated from soil at 105 ℃ under an atmospheric pressure.Soil water is the material basis for plant growth and survival. It not only affects the yield of trees, field crops, vegetables and fruit trees, but also affects the distribution of plants on the land surface.In pedology, there are two soil water classifications according to the different research methods of soil water. One is to study soil water from the point of energy, so as to form the energy classification of water. It mainly studies the energy state and movement of water, mainly used to study the movement of water in layered soil, and the transformation (evaporation, transpiration) of water in different media,The movement of water in the soil plant atmosphere continuum (SPAC) and the availability of soil water for plants.
It refers to the gaseous water absorbed from the air on the surface of soil particles by molecular gravity and kept on the surface of soil particles.
The water existing in the soil pores in the aeration zone and absorbed by the soil particles.Generally, there are four forms as follows: ①Imbibe water。It is also called strong binding water.The soil particles have a great attraction to it. The water molecules close to the particle surface are closely arranged, and the attraction is equivalent to 10000 atmospheres.This layer of water has weak ability to dissolve salts. It is still not frozen at - 78 ℃. It has the property of solid water and cannot flow, but can be converted into gaseous water and move. ②A film of water formed on the surface of adsorbed water.It is also called weakly bound water.The attraction of soil particles to it is weakened, and the suction force is 31~6.25 atmospheric pressure, which is similar to the property of liquid water and can move from the thicker part of the film to the thinner part. ③Capillary water held in soil pores by capillary attraction.The suction force is 6.25~0.08 atmospheric pressure.Capillary water can transfer hydrostatic pressure and be absorbed by plant roots. ④Gravity water, which moves under the action of gravity, has the property of general liquid water.It is not easy to keep in the upper layer of soil except for perched water.Growth, decline, dynamic change and decline of soil water
Water, evaporation, emission and runoff are closely related.
Film water and its movement direction
The generalized soil water is.There are solid water, gaseous water and liquid water.It mainly comes from rainfall, snow, irrigation water and groundwater.Liquid water is generally divided into hygroscopic waterCapillary waterAnd gravity water, respectively, represent soil water under the action of adsorption force, meniscus force and gravity.Soviet scholars also classified the water maintained by the absorption force on the surface of soil particles as hygroscopic water and bound water, the latter being divided into tight bound water and loose bound water;The capillary water is divided into capillary support water, capillary suspended water and capillary ascending water;Gravity water permeates free gravity water, free gravity water, etc.Soil water is an important component of soilsoil fertilityAnd self purification ability.
Solid water——Ice crystals formed when soil water freezes.
Vapor water——Water vapor in soil and air.
Bound water——It is also divided into hygroscopic water (tight bound water) and membrane water (loose bound water)
Free water - also divided intoCapillary water、Gravity waterandgroundwaterThe capillary water is divided into suspended water and supporting capillary water.
Hygroscopic water
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Simulation of soil water and nitrogen behavior in regional farmland
The soil dried indoors seems dry, but actually contains water.If this kind of air dried soil sample is placed in an oven and baked at 105 ℃, or it is placed in a desiccator with a moisture absorbent (such as phosphoric anhydride) and weighed every other period of time, it will be found that the weight of the soil sample is gradually reduced until it is weighed to constant weight, then the soil is dry, called dried soil.If the dried soil is placed in the atmosphere at normal temperature and pressure again, the weight of the soil will gradually increase until it reaches the balance with the air humidity at that time, and it will change correspondingly with the change of air humidity.The above phenomenon indicates that the soil has the ability to absorb water vapor molecules.The water absorbed in this way is called hygroscopic water.The hygroscopicity of soil is determined by the molecules on the surface of soil particlesgravitationIt is caused by the interaction of charged ions and charged solid surface electrostatic attraction in the soil colloid double electric layer with water molecules. This attraction attracts the dipole water molecules to the soil particle surface, and the process of adsorption of water molecules releases energy (heat energy).Therefore, the more viscous the soil texture and the larger the specific surface area, the greater its moisture absorption capacity.Figure 6-1 shows the relationship between moisture absorption water content and air relative humidity in different particle size ranges of soil.The action distance of causing moisture absorption is very short, which is only equal to the diameter of several water molecules, but the action force is large, so it can not only absorb water vapor molecules, but also make water molecules dense on the surface of soil particles, and the density of moisture absorption water can reach about 1.7.Therefore, this kind of water cannot be absorbed by plants, which is invalid water for plants.Gravity can not make the hygroscopic water move, and it can only move under the precondition of absorbing energy and transforming it into vapor state, so it is calledTight bound water。
1. Fractions smaller than 0.002mm
2. 0.002-0.006 mm size fraction
3. 0.006-0.02 mm size fraction
4. Particle size greater than 0.02 mm
Membranous water
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After the soil particle has fully absorbed the hygroscopic water, there is still residual absorption force. Although this force can no longer absorb the water vapor molecules with high kinetic energy, it is still enough to attract a part of liquid water. A thin water film is formed around the hygroscopic layer around the soil particle. The water in this state is called membrane water.Although gravity cannot move the membrane water, it can move from the thicker part of the water film to the thinner part, as shown in Figure 6-2, but the speed of movement is extremely slow.Therefore, compared with hygroscopic water, this water is also called loose bound water.Due to the attraction of some membranous waterPlant rootBecause the membrane water moves too slowly and cannot be replenished in time, higher plants can only use all the water in the soilMembranous waterPart of.When the soil also contains all moisture absorption water and part of membrane water, higher plants have been permanently wilted.
Capillary water
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brief introduction
Soil water
Water has obvious surface tension due to its molecular gravity;After absorbing enough membrane water, soil particles still have extra gravity;The pore system of soil is a complex capillary system.Therefore, the soil has capillary force (potential) and can absorb liquid water.Capillary water refers to the absorption and preservation of liquid water in the soil pore system with the help of capillary force (potential). It can move from the direction of small capillary force (potential) to the direction of large capillary force, and can be absorbed and utilized by plants.If the soil texture is sticky and the capillary radius is small, the capillary force (potential) will be large.Because the soil pore system is complex, the large and small pores are interconnected in some places and blocked in others, there are several kinds of capillary water in the soilstate, can be briefly classified into two categories: suspended water and supporting capillary water.
Suspended water
Suspended water refers to the capillary water that is not affected by the recharge of underground water sources, that is, the liquid water absorbed by the soil after atmospheric precipitation or irrigation.The capillary system of loam and clay is developed, and the suspended water is mainly in the capillary pores, but some is in the non capillary pores blocked at the lower end;The capillary system of sandy soil and gravelly soil is underdeveloped, and there are many macropores. The suspended water is mainly around the places where soil particles or gravel contact each other. Sometimes the water rings are fused together, sometimes they are not connected with each other, collectively referred to asContact water(Figure 7-4P142).In homogeneous soil, when the suspended water is in equilibrium, the water content of the soil is basically the same from top to bottom.
Support capillary water
Support capillary waterIt refers to the capillary water in the soil that is supported by the groundwater source and rises to a certain height, that is, the part of the groundwater that rises along the soil capillary system and remains in the soil.The content of this water in the soil is gradually reduced from bottom to top within the range of capillary rise height until a certain limit.The reason for this phenomenon is that the soil pores are large and small, and the formed ascending pipes are thick and thin. In the thick pipes, the height of water rise is small, while in the thick and thin pipes, the height of water rise is large. Therefore, the supporting capillary water near the groundwater saturation area almost fills all the pores, and the farther away from the water saturation area, the less supporting capillary water.
The rising height of capillary water in coarse particle spacing is small, while that in fine particle spacing is large (Table 7-1).If the soil particle with a diameter of 0.001 mm is taken and calculated according to the above formula, theoretically the height of capillary water rise should be 75 m, but from the observation results of nature, this value has never been confirmed.Even in clay, the height of capillary prosperity on water rarely reaches 5~6 meters, and generally does not exceed 3~4 meters.This may be because when the capillary diameter is too small, the pores are easily blocked by membrane water.
Gravity water
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WhenatmosphereWhen the precipitation or irrigation intensity exceeds the capacity of the soil to absorb water, the residual gravity of the soil is basically saturated, and the excess water is lost through the macropores due to gravity. This form of water is called gravity water.Sometimes, because the soil is sticky, gravity water is not easy to be discharged for a while, and it is temporarily detained in the soil pores, which is called perched water.Although gravity water can be absorbed by plants, it is lost quickly, so there are few opportunities to use it;When the gravity water is temporarily detained, it will occupy the soil macropores, hinder the supply of soil air, and adversely affect the water absorption of higher plant roots.
groundwater
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IfsoilorParent materialThere is an impermeable layer in the. The downward leaking gravity water will accumulate in the soil pores above it to form a water saturated layer of certain thickness. The water in the layer can flow and is called groundwater.From the above concept of supporting capillary water, it can be seen that there is no obvious upper limit for the saturated water layer of soil.However, if a well is dug in this soil, the outflow of groundwater will form a free water layer in the well.The depth of the water level from the surface is called the groundwater level.Groundwater can supply the needs of higher plants by supporting capillary water.Under drought conditions, due to the lack of water in the surface soil, the roots of some drought tolerant trees, such as Populus euphratica, can be as deep as 3-5 meters to use groundwater. If the groundwater level is high (that is, too close to the surface), water soluble salts will be concentrated into the surface soil along with the evaporation of water. Especially when the salinity of groundwater is high (that is, there are many salts), this upwardmotionWill increase the salt content of the soil surface to a harmful degree, which is called salinization.In humid areas, if the groundwater is too high, the soil will be too wet, and there will be seasonal ponding on the surface, making most higher plants unable to grow, and soil organic residues difficult to decompose. This is swamping, and attention must be paid to prevention and control.In addition, the high distribution of groundwater level and seasonal changes are unfavorable to the growth of trees.In recent years, groundwater resources have been overexploited, which has led to the continuous decline of groundwater level in some water poor areas (such as northwest China), bringing severe challenges to the survival of human beings, animals and plants.
Soil water and plants
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The roots of plants absorb water from the soil, transport it into the leaves through the stems, and then dissipate it into the atmosphere through transpiration. A part of atmospheric precipitation enters the soil.Therefore, soil water, atmospheric water and water in plants form a continuum.In a certain range, the more atmospheric precipitation, the higher the content of soil water.
Before seed germination, soil moisture begins to affect plants. For example, poplar and willow seeds must contact with wet soil within a few days after maturity, or they will lose the ability to germinate.The seeds need water during germination. One is to promote the softening of seed coat, and the other is to change the gel protoplasm into sol.
Because the root system of the plant grows directly in the soil, the amount of soil water content directly affects the development of the plant root system.Plants on dry soil, especially in grassland and desert areas, are mostly deep rooted plants;Most of the plants growing on wet soil are shallow root plants, and most of the roots of these plants walk in the soil layer several inches below the surface soil.
Water also has a maximum, minimum and optimum point for plant growth.Below the lowest point, the plant wilts and stops growing.When it is higher than the highest point, the plant roots suffer from hypoxia, suffocation and rotting.Only in the most suitable range can the water balance of plants be guaranteed.The optimum water content required by different plants is different（Ipomoea）For example, several species of plants (Fig. 3-3), horseshoe grass growing on sand dunes（I. pescaprae）The optimum soil water content is about 60%, and the sweet potato grows in a mesogenic environment（I. batatasvar.edulis）The optimal water content is about 85%, while the water spinach growing on the wetland（I. aquatica）The optimum soil water content can reach more than 110%.
Soil moisture also affects the quality of plant products.With the increase of water content, the synthesis of plant nitrogen and protein decreases, the starch content increases correspondingly, and the fiber quality of cotton and jute becomes worse.[1]
