RU2239832C1 - Method of measurement of soil moisture potential - Google Patents

Abstract

FIELD: farming and soil science; estimation of physical properties of soils.

SUBSTANCE: proposed method includes placing the porous probe connected with reservoir filled with liquid in soil. Soil solution is poured into reservoir and cathode is introduced in it; anode is embedded in soil. Then, rate of escape of liquid from reservoir into soil is measured at different current intensity and curve "liquid escape rate-pressure" is plotted. Potential of soil moisture is determined by point of this curve corresponding to zero rate of escape of liquid. Besides that, preliminary calibration is performed for determination of correspondence of pressure to current intensity. Probe is introduced in soil in new point for each subsequent measurement.

EFFECT: reduction of time required for single measurement; increased productivity in conducting series of measurements.

3 cl, 2 dwg, 1 ex

Description

The invention relates to agriculture and soil science, and in particular to methods for determining the water-physical properties of soils.

A known method for determining the potential of soil moisture [1], based on the use of a membrane press. The soil is placed on a porous membrane that passes water that is not permeable to particles of soil and air. After that, excess pressure is created above the soil. By varying the pressure, the onset of soil moisture is determined.

The main disadvantage of this method is the inability to conduct experiments in natural conditions.

Closest to the claimed is a method for determining the potential of soil moisture [2], which consists in the use of a tensiometer. A tensiometer is a hydrophysical device for measuring capillary-sorption pressure of moisture in soil in the range from 0 to - 90 kPa. It consists of a glass tube filled with deaerated water, connected to a finely porous ceramic probe (pore size 1-2 microns) and a vacuum gauge. The device must be tight. The ceramic probe is immersed in the soil, and due to the difference in the thermodynamic potentials of the water in the device (free water) and in the soil, moisture is transferred from the device to the soil until equilibrium is established. A rarefaction occurs in the device, which characterizes the suction power of the soil (potential of soil moisture), which is recorded by a vacuum gauge. Various tensiometer designs are known which differ from each other primarily by a manometric device.

The main disadvantage of this method is the length of the equilibrium between the tensiometer and the soil. For rarefaction in the device, a certain amount of water must come out of it, and this water, once in the soil, should not significantly change the potential of soil moisture in the probe area. Consequently, the size of the soil sample must be large enough, and it is necessary to wait for the excess moisture of the tensiometer to leave the vicinity of the probe. The time to establish equilibrium takes hours, and for some soils poorly permeable to water, even a day.

In addition, the use of devices operating under reduced pressure is very inconvenient. The slightest violation of the tightness makes them inoperative.

The aim of the invention is to reduce the time of a single measurement and increase productivity in mass measurements, as well as the rejection of work at reduced pressure.

An object of the invention is to establish the dependence of the suction pressure of the soil on the rate of fluid exit from the device.

The problem is solved by placing a porous probe connected to a container filled with water or soil solution in the soil, placing it in a container with a soil solution of the cathode, and in the soil of the anode and measuring the rate of liquid exit from the container at various currents, a curve is constructed from coordinates “fluid exit velocity - pressure”, the value of the potential of soil moisture is determined by the point of this curve corresponding to the zero fluid exit velocity, and also pre-conduct calibration to establish If the pressure value corresponds to the current strength, as well as for each measurement of the rate of liquid exit from the reservoir to the soil corresponding to a certain current strength, the porous vessel is placed in the soil at a new point in the area selected for study.

In this case, the liquid in the ceramic filter will move under the action of the resulting two forces. In the direction of the soil, the movement will be determined by the potential of soil moisture (soil suction force), and in the opposite direction, the soil solution will move due to electroosmosis, since the ceramic filter is negatively charged. In dense dry soils, which have a large “suction power”, it is technically difficult to achieve equilibrium between flows in the field, since the potential difference between the electrodes should be 250-350 V. In these conditions, it is more appropriate not to completely exclude the liquid from the device to the soil, but only reduce the output speed. Placing the device every time in a new point of the investigated area, the dependence of the fluid exit rate on the suction pressure is removed. By calibrating the device on the bench, we get the opportunity to determine the magnitude of the suction pressure by the magnitude of the current. Thus, for each current strength, we know how much the suction pressure decreases. Having built a graph, we can determine the value of the real suction pressure at zero output of liquid into the soil. A similar method allows measurements to be made in cases where the rate of liquid exit from the device exceeds the time of liquid resorption in the soil, since the graphical method allows you to circumvent the difficulties associated with the presence of two variables - the moisture conductivity coefficient and the suction pressure itself.

The technical essence of the invention consists in placing a porous probe connected to a container filled with liquid into the soil, introducing the soil solution and cathode into the container, and into the soil - the anode and measuring the rate of liquid exit from the device into the soil at various currents. According to the data obtained, a curve is constructed in the coordinates: the fluid exit velocity is pressure, and the soil moisture potential value is determined by the point of this curve corresponding to the zero fluid exit velocity from the device. The device is preliminarily calibrated to establish the correspondence of the pressure value to the current strength. At each measurement of the rate of liquid exit from the device to the soil, corresponding to a certain current strength, the device is placed in the soil at a new point in the area selected for study.

The proposed method allows you to refuse to work under reduced pressure and significantly reduces the measurement time. In addition, it allows you to move from measuring pressure to measuring current, which is much more accurate and simpler.

The following examples reveal the essence of the invention.

Example 1

A porous probe connected to a container filled with soil solution is placed in a glass with soil solution. The cathode is located inside the probe, and the anode, which is a few turns of copper wire with a diameter of 1 mm, is outside, adjacent to the outer surface of the probe. The soil solution tank is connected to the probe using a flexible hose. It can be raised to different heights, creating a different, excess pressure inside the probe. From a set of power batteries, a voltage of 12, 24, 36, 48, 60, 72 V is applied to the electrodes and the device is calibrated — the pressure value is set to the value of the current strength, which keeps the meniscus in a stationary state. After calibration, the device is placed in the soil, at various points when measuring at a given current strength, and the magnitude of the rate of liquid exit from the device corresponding to a certain current strength is determined. On the graph, the points of the fluid exit velocity are plotted, each of which corresponds to a certain effective soil suction pressure. The true value of the suction pressure of the soil corresponds to a zero rate of fluid exit from the device.

The experiments showed that the accuracy of measuring the potential of soil moisture is not lower than the accuracy of determination using conventional tensiometers, and the speed of determination increases many times, since you do not have to wait for the establishment of equilibrium (absorption of moisture through the soil) after it leaves the tensiometer. A decrease in the amount of liquid discharged into the soil from the device determines a decrease in the measurement time. Moreover, the denser and less moisture-permeable soil, the more time is required to establish equilibrium, and the correspondingly greater is the time gain during measurements. The proposed method allows one measurement in 7-8 minutes, which significantly increases productivity and reduces costs when conducting mass surveys of soils. In addition, the method has greater reliability, since if a conventional tensiometer is not tight enough, it will become clear only after measurements, which forces several parallel measurements to increase reliability. The proposed method is devoid of this disadvantage.

Thus, the proposed method reduces time, improves productivity and reliability of measurements.

Sources of information

1. Vadyunina A.F., Korchagina Z.A. Research methods for the physical properties of soils and soils. M.: Higher School, 1973. S. 185-187.

2. Shein E.V., Kapinos V.A. Collection of problems in soil physics. M .: Moscow State University, 1994. P.48-50.

Claims (3)

1. The method of measuring the potential of soil moisture, which consists in placing a porous probe connected to a tank filled with liquid in the soil, characterized in that the soil solution is placed in the tank and the cathode is introduced, and the anode is inserted into the soil, and the rate of liquid exit from the tank into the soil is measured at various currents, according to the data obtained, a curve is constructed in the coordinates “fluid exit velocity - pressure”, and the soil moisture potential value is determined by the point of this curve corresponding to the zero fluid exit velocity. 2. The method according to claim 1, characterized in that the calibration is preliminarily performed to establish the correspondence of the pressure value to the current strength. 3. The method according to claims 1 and 2, characterized in that for each measurement of the rate of liquid exit from the reservoir to the soil corresponding to a certain current strength, the porous probe is placed in the soil at a new point in the area selected for study.