Part 2: The neutron soil moisture meter and tensiometer

by Frikkie Koegelenberg, Pr Eng and Hendrik Jordaan

Since the neutron soil water sensor measures the absolute amount of water in volumetric units in the soil, this technique will also directly give an indication of the amount of water, in millimetres, to be irrigated. The method of soil water measuring by means of neutron dispersion dates back to the 1950’s. It has since been accepted as an effective and reliable technique used increasingly in Southern Africa.

We thank the ARC Agricultural Engineering in South Africa for making their manual on soil water sensors available to the readers of ProAgri Zambia.

The neutron water meter consists of two main components, namely, (i) a probe containing a source of high energy and fast-moving neutrons, as well as a sensor which is sensitive to slow-moving neutrons, and (ii) a microprocessor that can register the flow of slow moving neutrons in the soil. A schematic representation of the most important components that the instrument consists of is given in Figure 1.

The soil water content at a specific depth of soil is measured by lowering the probe in an uPVC pipe, which was installed in the soil beforehand to a desired depth and of which the bottom is closed. A neutron count is then taken for a minimum of 16 seconds, but preferably 32 seconds. It is however important to note that the shorter count time reduces the accuracy and dependability of the answer. The effective volume of soil of which the soil water content is measured, is determined by the radioactive strength of the neutron source and the degree of wetness of the soil.

The effective volume sampled by the neutron water sensor is therefore greater in dry than in wet soil. For the generally used radioactive sources such as Ra-Be and Am-Be, the radius of the spherical volume of soil in which the soil water content is measured varies typically between 0,1 m for a wet and 0,25 m for a dry soil. The practical implications of this are, strictly speaking, that a neutron water meter cannot be used to determine soil water near the soil surface (approximately 0,15 to 0,20 m).

A neutron water meter can be calibrated in the laboratory or in the field to determine the relation between the neutron counts and the volumetric soil water content. For calibration of the neutron water meter and other soil water meters, a specialist in this field must be consulted.

Benefits

1. Measuring is very fast. A minute per reading, or even less, is normally sufficient.

2. Measurements can be taken repeatedly on the same spot without disturbing the soil.

3. As many measuring points as required can be used without a significant cost increase, that is one instrument can be used on many different spots.

4. Measurements can be done at any depth, except the uppermost 150 mm, to obtain a continued profile of soil water with depth.

5. Soil water can be determined over the entire soil water spectrum.

6. Soil water is measured over a large soil volume and the soil water values are given directly in volume units, which simplifies irrigation calculations.

Disadvantages

1. Health and safety risk. Prolonged use can hold a radiation danger with accompanying problems. Because of the radioactive source it contains, the instrument is subject to certain regulations.

2. A large volume of soil is sampled, and this can cause problems when non-uniform soil water profiles are determined, for example where there is a sharp transition between a wet and a dry soil layer or where there are sharp texture differences in depth, such as a duplex soil.

3. It cannot be used summarily near the soil surface. A special shield is required for this, but is not very effective.

4. Neutron water measurements are influenced by soil density and soil type; therefore, many calibrations are necessary.

5. The cost of the instrument is high.

6. Readings can also not be taken automatically on a continuous basis.

Tensiometry

Tensiometry is an indirect way of determining the water content of a specified volume of soil. A tensiometer indicates the matrix potential (soil water tension), which is then converted to absolute soil water content by means of a soil water characteristic. A tensiometer consists of a porous point usually made from ceramic. The porous head is connected to a mercury manometer or a vacuum meter by means of a long water-filled tube. When the tensiometer is placed in the soil (in such a way that there is close contact between the soil and the porous head), water will move through the pores of the ceramic head. The water movement is caused by a difference in the soil water tension inside and outside the porous head. If the suction tension of the soil is higher than in the pores of the tensiometer, water will move from the tensiometer to the soil. Since the tensiometer and the manometer, or vacuum meter, form an airtight closed system, a vacuum will be created in the tensiometer.

Alternatively, the air pressure in the tensiometer will in time come into equilibrium with the air pressure in the soil, which is lower than that of the atmospheric pressure (that is, it is negative in relation to atmospheric pressure). These pressure differences (or vacuum amounts) are then registered on the manometer or vacuum meter.

Electronic tensiometers that can register automatically have an electronic vacuum sensor and are then connected to data loggers to store the data. If the soil is wetted again, as during a rain shower or by irrigation, and the soil water tension in the soil reduces to below that of the tensiometer, the tensiometer will indicate a zero reading when the soil is fully saturated. The fl uctuations in the tensiometer reading therefore indicates how the soil water tension reduces (during irrigation) or rises (during drying). A graph showing such fluctuations during an irrigation season is shown in Figure 2.

The installation and maintenance of tensiometers require considerable knowledge and attention. New tensiometers must first be examined for possible leakages, all the air must be removed, and they must then be set at a zero reading. Directives for this are supplied by Piaget (1991). The mercury manometer type tensiometer is more sensitive and accurate than the vacuum meter type, especially in the low water tension area. It is however more breakable than the vacuum meter type. Mercury manometertensiometers are therefore mostly used as research instruments, while the vacuum meter type is better suited for practical application by producers.

The effective and rational use of tensiometers in irrigation for agriculture requires that they should be placed correctly. At least two tensiometers are required for deep-rooted crops. The shallower tensiometer will indicate when irrigation should be started, while the deeper placed tensiometer is used to indicate whether irrigation is done correctly (that is that the matrix potential at that depth will not drop substantially lower than approximately -10 kPa, fi eld capacity). For shallowrooted crops, only one tensiometer is necessary. Note that, with this method, only the matrix potential is monitored.

To calculate the soil water content and consequentially the irrigation amount, the soil water characteristics curve of the specific soil must be known.

Benefits

1. The tensiometer readings are a direct indication of the amount of energy that a plant must apply to take up water from the soil.

2. After installation, the soil water content is determined in the same spot every time.

3. Changes in the soil matrix potential can continuously be captured with a data logger.

4. The soil water characteristics can be determined in situ in the soil with the aid of the apparatus.

Disadvantages

1. The apparatus is quite fragile – a single crack in the porous head is enough to render the tensiometer useless.

2. The tensiometer functions only in a relatively wet area of the soil water spectrum – between 0 and 80 kPa. If the tension rises above 80 kPa, the meniscus in the pores of the ceramic head breaks and air freely invades the tensiometer. The fact that the tensiometer will only indicate soil water contents that are bound with tensions lower than 80 kPa, is not a substantial problem on coarsely structured soils. At such soils, most of the plant’s available water is in any case bound in the tension area 0 to 80 kPa. In finer textured soils, however, a significant amount of the plant’s available soil water can fall outside the mentioned tension area.

3. The apparatus must be serviced regularly. It must be filled with de-aerated water weekly and a vacuum must be created again. This is done with a hand vacuum pump.

4. A time delay of up to ten minutes occurs in registering of the change in matrix potential readings. It is detrimental for continuous data capturing.

Next month we shall discuss pulse and frequency delay measurements.

Published with acknowledgement to the ARC Agricultural Engineering for the use of their soil water sensors manual. Visit www.arc.agric.za for more information.