How To Measure Soil Water Potential

Water content and potential water value in solid are essential for plant growth, agricultural production, yield, and quality. Therefore, they are critical indicators to ensure plant better growth. So, how to measure and analyze soil water content and soil water potential?

Difference between soil water content and soil water potential

Soil water content and soil water potential are two different concepts. These two indicators represent the state of water in the soil, the water holding capacity in solid, and the energy required for water-holding in solid. Soil moisture content is the water holding capacity of unit soil, which refers to the volume of water in a unit volume of soil or the weight of water in a unit weight of soil;

Soil water potential is the energy state of unit soil, which is the energy required to extract unit water from soil under isothermal conditions. The unit is bar (bar, 1 bar = 100KPA).

The size of soil water suction is closely related to the flow of soil water. Soil water content is the capacity index of soil water. There is a certain relationship between soil water potential and soil water content. Generally, the greater the soil water potential, the higher the water content; The smaller the soil water potential, the lower the water content, so we can use the negative pressure soil tensiometer to reflect the soil water content roughly.

What is soil water potential?

Soil water potential is the energy required to extract unit water from the soil under isothermal conditions. If the soil is saturated with water and the water potential is zero, The water content is lower than the saturation state, and the water potential is negative.

The more arid the soil is, the greater the negative value is. The survival range of common plants is 0 to – 15 bar.

Determination method for soil water potential

We can use the rapid soil moisture meter to determine soil moisture. In contrast, for measuring soil water potential, we can use the portable soil water potential meter. Of course, there are many methods to measure soil water potential, such as the tensiometer, centrifuge, and so on.

Tensiometer:

Under certain conditions, the transfer efficiency of soil water between the tensiometer and the soil cannot be fast enough to allow the correct measurement of the change level of soil water potential. So, this is the reason why the accuracy of soil water potential measured by tensiometer is low.

Although the measurement results of the tensiometer are not accurate enough, it is still a practical device for agricultural production and measurement of soil water potential in the lab.

The principle:

Place the tensiometer filled with water in the soil. Under the action of soil matrix potential, water will pass the soil through the head. If the soil water potential is equal to the pressure potential in the tensiometer, it will stop the water from moving. So, that is as soil water potential value.

Measurement range: 0-85kpa (about 800 cm water column).

Measurement steps:

degassing the instrument / correcting the zero position/field installation/balancing for 1-2 days / avoiding the influence of temperature when reading (generally in the morning)

Pros:

• Not affected by soil solute potential;
• Easy installation and observation;
• Low cost and easy application in a large area.

Cons:

• Stability is affected by bubbles;
• Frequent exhaust required;
• The measurement results are affected by temperature;
• Regular irrigation is needed.

However, with the development of technology, portable soil water potential meter is more and more popular in determining soil water potential.

Portable soil water potential meter

It can measure and observe soil water potential in the field to further get soil water conservancy property parameters such as soil moisture, temperature, and hydraulic conductivity.

Pros:

• simple operation, convenient carrying, and flexible use,
• automatically get the peak value of soil water potential.

Centrifuge:

Suppose we use the centrifuge to measure the soil moisture characteristic curve. It has the advantages of a short measurement cycle and relative accuracy. In the measurement process, if we can consider the influence of bulk density, it can significantly improve the accuracy of the measurement. In addition, if we use a centrifuge to find the soil water potential, it will be affected by the change in soil temperature.

Pressure film instrument

The shape of the soil-water characteristic curve measured by the pressure film instrument is roughly consistent with the inherent characteristic curve of the soil, which can be used to simulate the dynamic change process of soil water quantitatively.

However, it takes up to half a year to thoroughly measure the water characteristic curve of soil (such as soil). Of course, if we only want to get the relative shape of the characteristic curve, we can reduce the measurement cycle. Still, we need to find out the proportional relationship between the curve of the relative shape and the standard characteristic curve.

Pros:

High accuracy

Cons:

Take time

Comparison of soil water potential measurement methods

Among the common test methods for measuring soil water potential, the Portable soil water potential meter is the simplest method with high accuracy.

The soil moisture characteristic curve measured by centrifuge is higher than that measured by pressure film instrument. Still, the measurement cycle is short, and the relative shape of the curve is more consistent with the inherent characteristic curve of soil.

If we can consider the influence of bulk density, it will increase the accuracy of measurement. As long as the suction of bulk density change in the measurement process is converted into the suction of a specified bulk density, It can mutually correct the measurement results with the measurement results of pressure film.

Since the tensiometer’s accuracy is low, the shape of the measured water characteristic curve deviates from the shape of the inherent characteristic curve of the soil.

Because of the conductivity (k) of the porous ceramic cup and the tensiometer’s vacuum meter device’s sensitivity (s), it can not accurately reflect the actual situation of the change of soil water potential in time, resulting in having a certain deviation compared with other measurement methods.

However, a tensiometer can be used to monitor the dynamic change trend of soil moisture in the field, which can not do by other measurement methods. Tensiometer is widely used to determine the period of farmland irrigation.

In practice, the tensiometer is commonly buried in two depths. The upper branch should be placed in the area with the strongest root activity, and the lower branch should be placed near the bottom of the root activity area. For most crops, when the upper tensiometer value at 300 ~ – 500kPa and the lower tensiometer starts to show dry, this time we should make irrigation.

Although the tensiometer has disadvantages, it is still a practical instrument that can be used to measure soil water potential. It is also one of the most simple and cheap instruments for accurately measuring soil water potential.