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Know Your Soil Moisture

Irrigation management means soil moisture monitoring is imperative.

Irrigation management is about deciding when to irrigate and how much water to apply. When irrigations are managed properly, sufficient water is available for plant uptake for photosynthesis and transpiration during the growing season and excess water loss – through deep percolation and surface runoff – is minimized.

Numerous studies have quantified the magnitude of yield reduction when irrigation is delayed and available soil water within the root zone of the crop is appreciably reduced. Similarly, irrigation efficiency is reduced if water is applied in excess of what the soil can infiltrate or retain.

Knowing the soil water status within irrigated fields is critical information if irrigation efficiency is to be maximized, and timely irrigation to obtain the highest yields and the best quality is to occur.

Gravity and wheel move irrigation systems continue to be replaced by centre pivot irrigation systems. Labour savings and better control of water application amounts with centre pivot irrigation are some of the reasons for this conversion. Additionally, some processors of higher value crops dictate that fields under contract must be irrigated with centre pivot irrigation systems.

Soil water management under a centre pivot differs from that for wheel move or gravity irrigation. Frequent applications and applying less water at a time is the operating design for centre pivot management, and information regarding soil water status is important for timely irrigation scheduling.

Irrigation management has evolved, from early assessment of sufficient soil water by taking a shovelful of surface soil and performing a physical “hand-feel” assessment of soil water, to complex soil water sensor technologies that infer soil water status by measuring different properties of the soil. However, in recent producer surveys, done both in Canada and the United States, the majority of irrigators have not adopted the more complex soil water sensing instruments. Rather, they still rely on the hand-feel method for determining the need for irrigation applications. While there is nothing wrong with that approach, it is a qualitative measure. It obviously works well for many irrigators, but when it stops working, that is when other production priorities take precedence over timely soil water determinations, or the farming operation gets too large and soil water is allowed to decline to the point when crop yield is being compromised.

Most of the instruments available for soil water monitoring are indirect methods, in that they measure a feature of the soil that indicates the quantity of soil volume that contains water. The amount of soil water is then converted to a value (typically volumetric water content per cent, [VWC %]) with a calibrated equation. Other instruments measure the soil suction/tension/negative potential/matric potential: different words all referring to the same property.

Volumetric Methods

Volumetric water content (ratio of volume of water to total soil volume expressed as a per cent) is a convenient value when using these types of probes for irrigation scheduling. The value can be converted to a depth of water (millimetres or inches) and that value can be used to determine how much soil water the crop has used, and how much irrigation water needs to be applied. The majority of the probes that output the soil water status as VWC % are using a property of the soil components termed the dielectric constant.

The soil medium is comprised of mineral components (sand, silt, clay), air-filled porosity and water-filled porosity. The dielectric constant of air is minimal, less than 1; the dielectric constant of the mineral components is around 4 or so; and the dielectric constant of water is 81. The value of a soil water reading may be higher or lower with these instruments between sampling dates: air contributes minimally to the reading and the mineral component is constant, so the change has to be a result of more or less soil water.

The most common soil water instruments that use the dielectric constant to determine volumetric soil water content are grouped based on the method or technique employed for the measurement: time domain reflectrometry, time domain transmission, frequency domain reflectrometry, amplitude domain reflectrometry (impedance) and capacitance.

Tensiometric Methods

Water is held in soil pores against the force of gravity. The reason it is held in soil pores is a function of the polar nature of the water molecule, attraction of water molecules to the soil particles and polar orientation among the water molecules themselves. When all the soil pores are filled with water, the soil is saturated. As long as a soil volume has no restriction to drain (bedrock, high groundwater, underlying layer of slowly permeable clays), a soil volume will never be at saturation for long. If the water potential at saturation is zero, any soil water status less than saturation would be negative. Tensiometric methods measure this degree of negative potential termed soil tension/suction/matric forces. The more water that is in the soil, the closer to zero the reading will be; the less water in the soil volume, the more negative the reading.

Soil Water Probes

Soil water probes vary in the installation methods employed (see illustration at left). There are probes that have the sensors located along the instrument (a), prongs that are inserted in the soil (b), prongs buried in the soil (c), and those that have the measuring sensor at the end of the instrument that can be raised or lowered to varying depths (d).

All soil water probes work within certain limitations. Some are more sensitive to soil salinity than others, some sample a larger soil volume than others, some operate reliably only within a certain range of soil water values and some can be biased based on soil water heterogeneity.

There are soil instruments that measure soil tension, probes that measure the dielectric constant, probes that connect to a data logger for continuous monitoring, etc. Many countries have distributors for many of these soil water probes. In Canada, Hoskin Scientific Ltd. is a distributor of many soil water instruments.

It is important to remember that to use these probes for irrigation scheduling, interpretation of the data needs to occur to be able to determine the plant-available soil water status in the field. A display of a certain VWC % would not inform the user of volume or depth of plant-available water in the soil volume. The amount of water a soil can retain is dependent on the texture of the soil.

In the table above, a VWX % reading from the soil water sensor yielded a value of 19 per cent VWC. A 19 per cent VWC would mean the soil that is sandy loam in texture would be at field capacity; thus, the profile cannot retain any more water. If more water is applied, it will pass through the profile, termed deep percolation. However, the same 19 per cent in a clay loam-textured soil would mean the soil water status is very near wilting point; the plants are experiencing a limitation to transpiration and irrigation should have been started many days ago.

A reading of 19 per cent VWC does not inform irrigation scheduling unless additional calculations are done, or information referenced, that will identify the water holding capacity of varying textures of soil. (See graph at left.)

There are some soil probes that, depending on the input for the soil texture, do the calculations based on set field capacity and wilting point parameters. Output is then converted to available water content per cent within the algorithms of the program, and the display to the irrigator is in plant-available water. Additionally, the output, so far, informs the irrigator how much additional irrigation water is needed to “bring up” soil water content to field capacity.

The calculations are performed within the program, based on soil texture, so the display on the smartphone app informs the irrigator the current soil water status and the amount of water needed to refill the soil profile. Additionally, it is colour coded to alert the user the soil water status of the profile.

Some of these probes and apps include the Sensetion soil moisture station (dacom.farm/products/sensetion), the CropX wireless soil water probe (cropx.com/product) and the Sensoterra wireless probe (sensoterra.com/technology).

Knowing the soil water status in an irrigated field is very important for efficient and effective irrigation management. However, up until a few years ago, the adoption and incorporation of soil water sensors in a crop production system was mostly the domain of crop consultants, university extension or government institutions.

The newer probes appear to have the consideration of the irrigation producer in mind during development (easy installation, quick activation, reasonably accurate, smartphone enabled, meaningful display with easy interpretation). With more of these probes coming available in the marketplace, the incorporation of soil water monitoring into an irrigated production system becomes more realistic.

(Ted Harms is a soil and water specialist with the irrigation management section of Alberta Agriculture and Forestry)

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