Agriculture is the foundation of human beings. Countries spend a lot of money every year on soil moisture monitoring. Many foreign companies are engaged in lyrical, meteorological and irrigation-related services for agriculture. The entire industrial chain, commercialization and marketization are relatively mature.
In recent years, among the professional enterprises related to soil moisture and ecological environment monitoring products and services in China, the annual turnover of several companies has exceeded 100 million yuan, and has become a sought-after enterprise in the capital market.
In the context of imbalanced water resources (partially scarce), increased climate change, the emergence of intelligent monitoring of ecological environment, the lack of agricultural labor, and the shift of large-scale investment to the agricultural industry, this has actually become a rare occurrence. tuyere.
Xiao e has been engaged in lyrics for many years, standing in the air, feeling the wind is getting bigger and bigger. More and more recently, these questions have been consulted:
What is the difference between the volumetric water content, the weight and water content, and the relative water content of the soil?
Does the soil moisture sensor measure volumetric water content or weight water content?
How can I measure the relative water content of the soil?
Xiao e originally wanted to write the answers to these questions on the mobile phone memo. Someone asked to copy and paste WeChat. Later, I thought that this efficiency is still not high, and I wasted a lot of time. The most important thing is: I need to write a self. The article of the system comes out, and the ins and outs of things are clearly explained.
This article is divided into three parts:
The first part is the definition of several basic concepts, put together, to avoid new friends who contact the industry to go to Baidu search.
The second part is the core dry goods of this paper, about 3000 words. From the perspective of the historical development of soil moisture monitoring, the development history of soil moisture sensors that can be used commercially is introduced. The basic concepts of volumetric water content and relative water content are explained. .
The third part is about the future of soil sensation. For those who have stepped in or are ready to step into the industry, they must think about what will happen in the future and have the opportunity to stand or approach this special storm.
First, pure concept definition (do not like to see the definition of the concept, please read the following dry goods)
1. Soil water content: Generally refers to the absolute water content of the soil, that is, 100g of dry soil contains several grams of water, also known as soil moisture content.
2. Weight and water content: refers to the ratio of the weight of water in the soil to the weight of the corresponding solid phase material.
3. Volumetric water content: refers to the ratio of the volume occupied by water in the soil to the total volume of the soil.
4. Saturated water content: refers to the water content when all pores between soil particles are filled with water.
5, relative water content: generally refers to the percentage of soil water content in the field water holding capacity.
6. Water holding capacity in the field: refers to the relatively stable soil water that can be maintained by the soil profile after a sufficient time to fully infiltrate and prevent the water from evaporating after sufficient water or precipitation on the deep groundwater and well-drained land. Content, field water holding capacity is the upper limit of soil water available to most plants.
Second, the ins and outs of the soil "sentimental"
墒, refers to the soil moisture suitable for plant growth and development. Therefore, agricultural production often has the saying of robbing, protecting and running. Whether the soil moisture is suitable for plant growth and development is like whether people have food to eat, people can eat solid, liquid, all kinds of strange foods, and plants can only absorb water from the soil by the huge root system, and dissolve it in the soil moisture. Nutrients are absorbed into the body. Great photosynthesis has created the earth, and plant-specific photosynthesis can only be accomplished with the participation of water molecules. Therefore, the measurement and reporting of soil moisture is extremely important to human beings.
Then, the question is, how to measure the soil moisture to reflect whether the soil moisture is in a state suitable for plant growth and development?
As a result, a large wave of concepts related to soil moisture conditions has been developed (it is recommended to skip, do not read the N-concepts created by people in this paragraph), such as: volumetric water content, weight water content, relative water content, saturated content Water quantity, field water holding capacity, crop water stress, wilting point, wilting coefficient, irrigation water utilization coefficient, soil gravity water, soil capillary water, soil moisture absorption, soil membranous water (film water), soil bound water, soil freedom Water, chemical combined water, groundwater, drought, locust disaster, extreme drought, severe drought, moderate drought, mild drought, soil water characteristic curve, soil hydraulic conductivity, soil water diffusion rate, soil particle size...
Isn't it a soil moisture? It is necessary to correctly express the relevant knowledge framework of human beings, and to understand the relevant concepts at least twenty-eight, scare the baby! Not afraid, this article will make the soil lyrical things clear and understand!
First of all, how has people judged the identification of soil moisture since history?
It can be seen with the eyes, but the eyes cannot form a standard of communication between different people. You can pinch the soil by hand, and then put your hands on the soil and let go of the soil to see the looseness of the soil. However, the degree of exertion of the soil between different people is different, and it cannot form the common standard.
Since the eyes and hands are not reliable, can you see how much water is in the soil by direct heating to volatilize the water (compared to the weight difference of the soil before and after heating)? The oven drying method, the alcohol burning method, and the infrared drying method are all used. The oven drying method has always been considered the most classic and accurate standard method.
Obviously, the dry weighing method is the weight water content.
The advantage of the drying method is that the results are reliable with respect to the sample itself, but its disadvantages are also obvious. When sampling, the soil will be destroyed, and deep sampling is difficult. When the fixed point measurement is made, it is inevitable that the sampling will be misplaced and bring errors. In many cases it is difficult to perform long-term in situ monitoring and is also affected by soil spatial variability. It takes time and effort to take the soil drying operation. It takes at least 8 hours. It also needs to dry the drying box and the power supply. It is completely unsuitable for field work and it is impossible to scale the application.
Inventing the machine is a human specialty. Since the shortcomings of the weighing drying method are so serious, what mechanical instruments have been invented based on scientific principles to measure the moisture in the soil? (In advance, in all methods of measuring soil moisture, only the drying method is the weight and water content, because the tool must be used in the drying method: weighing the weight.)
In the 1950s, the most cattle soil moisture instrument was born - the neutron instrument. Since each water molecule contains two hydrogen atoms, the fast neutrons radiated by the neutron source are slowed down to thermal neutrons when they encounter hydrogen atoms, and the correlation between the number of thermal neutrons and the soil water content is Determine how much soil moisture is.
Many countries in the world have studied neutron neutrons, making the neutron law more and more perfect. The neutron method is very suitable for monitoring soil moisture dynamics in the field. After the casing is permanently placed, it does not damage the soil. It can be continuously measured for long-term positioning, and is not affected by hysteresis. The sounding depth is not limited. The neutron meter can also be connected with the automatic recording system and computer. Therefore, it has become a better method for in situ determination of soil water content in the field and has been widely used.
However, the need for field calibration is one of the main shortcomings of the neutron method. In addition, the equipment is expensive, one-time investment is large, and a single instrument is more than 100,000 RMB. The soil sampling range of the neutron method is a sphere, which makes the measurement results deviate in some cases. For example, if there is a radish near the neutron instrument, the water content of the radish is also regarded as the soil moisture data. In addition, the soil is at Data deviations can occur during dry or wet cycles, layered soils, topsoil, etc. In addition, there are potential radiation hazards in neutrons.
In the 1970s, the most widely used soil moisture meter was born today – the dielectric moisture meter.
The dielectric method utilizes the dielectric constant characteristics of a material material (the material material here is soil). The principle that the medium generates induced charges and weakens the electric field when an external electric field is applied (junior middle school physics)
From the electromagnetic point of view, the free water has a dielectric constant of 80 at normal temperature and pressure, about 3 solid particles in the soil, and 1 in air. Therefore, the dielectric constant of water is dominant in the soil compared to solid particles and air in the soil. The higher the soil moisture content, the greater the dielectric constant value. Many experiments have shown that regardless of the difference in composition and texture of soil, soil dielectric constant and volumetric water content are always nonlinear single-valued functions. The well-known TDR method , FDR method , FD method, and capacitance method soil moisture meter are all dielectric moisture meters. After the 1990s, the dielectric moisture meter was used as the basic equipment for studying soil moisture.
There is no single product that can dominate the world. There is another type of moisture meter on the market that has a certain market - the tensiometer.
The tensiometer measures the matrix potential of the soil. The advantage of the tensometer method is that it is accurate to measure the soil matrix potential when the soil is relatively moist, suitable for monitoring irrigation and water stress. The tensiometer method is less affected by soil spatial variability than the method of measuring soil volume or weight water content. It is also a direct measurement method for low-cost devices that can measure continuously.
The main disadvantage of the tensiometer method is that it is slow to react and requires a long time to balance before reading; the measurement range is small and not suitable for very dry soil. If the porcelain cup is not in close contact with the soil (such as when it is placed in the root activity range or where the organic fertilizer decomposes to generate gas or when the soil shrinks due to water shrinkage), the reading may be slow or stagnant. During the measurement process, especially during the hot and dry seasons, it is necessary to maintain and replenish the porcelain cups frequently. Porcelain cups are fragile and require regular maintenance and replacement, labor and time consumption, and high operating costs.
It should be emphasized that in the above-mentioned numerous methods, in addition to using the drying method to obtain the weight water content and using the tensiometer to measure the soil water potential, various other soil moisture instruments present on the earth currently measure the volumetric water content.
Volumetric water content refers to the proportion of water in a certain volume of soil. For example, measuring 20% ​​of the volume of water means that a volume of 100 ml of soil contains 20 ml of water.
In fact, the expression of volumetric water content is more scientific. The moisture in the soil of nature is distributed in the soil in a volume rather than the weight. The roots of plants can only absorb water from the roots within the spatial distribution of the soil.
However, the problem has arisen: some data on the soil moisture content have been measured with the instrument, and the soil moisture is still a hundred thousand miles away. Because the measured soil moisture content is a single point of static data, the response to plant soil moisture is a dynamic soil moisture range. This dynamic range of soil moisture is related to a range of factors such as soil type, plant type, plant growth stage status, and weather conditions.
How to do it? A concept of NB was created using: relative water content.
The concept of relative water content puts aside the above-mentioned complex correlation factors, considering only one key dimension: the difference in soil water absorption capacity. For example, the water absorption capacity of a sandy soil is 18% of the volumetric water content (when the water content is higher than 18%, the water is infiltrated by the action of gravity, anyway, and finally the sandy soil can only hold and hold the soil volume. 18% moisture). Thus, if the water content is measured at 9% at a certain time, the relative water content at that time = 9% / 18% = 50%, which is simple and clear: the current water content state is at 50 relative to the maximum water absorption capacity of the soil. %s level.
Relative water content is most widely used in government public opinion reports. Among them, there are drought grade indicators based on soil relative water content (R): 60
The relative water content is good, and the problem comes along: How to accurately determine the maximum water absorption capacity of the soil at the monitoring point?
This involves two concepts: field water holding capacity and saturated water content.
Field water holding capacity (abbreviation: Tian hold) refers to allowing sufficient water to infiltrate after sufficient water or precipitation on deep groundwater and well-drained land, and prevent its water from evaporating. After a certain period of time, the soil profile can be maintained more stable. The soil water content is the upper limit of the soil water available to most plants.
Saturated water content, also called saturated water holding capacity, refers to the maximum water content when the soil pores are full of water.
The relative water content is the ratio of the current measured water content to the field water holding capacity or saturated water content. For places where recurrent disasters occur, the current water content is more than the saturated water content, which directly reflects the disaster relief state. For areas where frequent droughts need irrigation, the current water content is compared with the upper field water holding capacity, which directly reflects the state of the field drought.
However, from the concept of field water holding capacity and saturated water content, it can be seen that this is a value that can be obtained by actual measurement. Different soil types and different soil structures differ greatly, even if the soil in the same location is due to agricultural land cultivation and agronomic measures. The difference will lead to differences in field water holding capacity and saturated water content.
The problem has arisen: the cost and time cost of measuring the water holding capacity and saturated water content in the field are extremely high. Moreover, the inconsistency of the soil determines that the relative water content obtained is scientific only when the moisture meter measurement position is consistent with the measured field water holding capacity and saturated water content.
Therefore, obtaining objective and accurate relative water content is not an easy task.
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