Monitoring Soil Moisture

The ongoing dry spell for some and spotty rains for others has had irrigation going full blast and rightly so! It is easy to get caught by drier than expected soil especially early in the spring/summer. Irrigation scheduling or monitoring soil moisture will give you a "heads up" before lack of soil moisture affects the crop.

Why monitor soil moisture: Although the rule of thumb is "an inch of water/week" for horticultural crops, the exact amount of water to apply to a crop depends on how much the crop needs and how much water the soil can store.

The crop water needs depend on the Evapotranspiration (ET) which is affected by:

• Temperature & Humidity
• Solar radiation
• Crop growth stage
• Presence of Mulch

The amount of water that the soil can store (available to the plant) depends on:

• Soil texture
• Rooting depth
• % Organic Matter

Use of water and nutrients is most efficient when water is applied in amounts needed by the crop and that the soil can store - not more, not less.

Understanding soil moisture levels and crop response:

Definitions
Field capacity: As much water as the soil can hold. (More accurately, it is the amount of water held in the soil two or three days after it has been saturated by rainfall. There is very little downward movement of soil water due to gravity and very little suction due to capillary action.)
Permanent Wilting Point: The amount of water remaining in the soil when the plant wilts in a humid atmosphere. The water remaining in the soil is held tightly by soil particles and plant roots cannot absorb it.
Available water: This is the amount of water in the soil between field capacity and permanent wilting point. Start irrigation before soil reaches 50% of available soil water.

Availability of soil water:

Equipment to help you get a handle on soil moisture levels:

Tensiometer ( eg. Irrometer)

The tensiometer reads the soil water tension or suction, in centibars. The higher the tension, the dryer the soil. The tensiometer is made of a closed plastic tube with a ceramic tip attached to one end, and a vacuum gauge with an air tight seal at the other end. The tube is filled with water and sealed. When the ceramic tip comes to equilibrium with the surrounding soil the gauge registers the soil water tension.

Tensiometers must be installed carefully to ensure that the ceramic tip is in contact with the soil. Use a soil sampling tube to drill a hole to the proper depth. Fill the soil with a slurry of soil and water. Push the tensiometer into the hole until it reaches the bottom.

Tensiometer

Reading the tensiometer gauge:

Routine maintenance is important. The liquid in the tube must be refilled and air bubbles removed with a hand pump. It can take a year or two of experience to gain confidence in the readings.

Electrical Resistance Blocks (trade names include Watermark):

These units measure soil water tension in centibars, similar to tensiometer readings.
Watermark is one trade name for electrical resistance blocks. The Watermarks measure the electrical resistance to current flow between electrodes embedded in a material resembling fine sand surrounded by a synthetic porous material.

Electrical Resistance Blocks

Watermarks and other electrical resistance blocks must be installed carefully to ensure that the blocks are in contact with the soil. Use a soil sampling tube to drill a hole to the proper depth. Fill the soil with a slurry of soil and water. Push the block into the hole until it reaches the bottom, leaving the attached wires above the soil surface. Replace the soil above the block, and firmly pat it into place. Flag each Watermark for easy access. A hand-held digital reader is attached to the wires to read the data.

The sensors can be installed in the soil, to any depth. They should be installed in groups of two. Because they require good contact with soil, they are not suited to gravelly, sandy or peat soils.

Time Domain Reflectometry ( TDR's):

Time domain reflectometry is a relatively new way to measure soil moisture. Probes inserted into the soil measure the velocity of electromagnetic waves in the soil. These waves are slowed by soil moisture. The measurements are very accurate, and the equipment comes factory-calibrated.

The disadvantage to TDR technology is that complex electronics and expensive equipment are required. This is still a research tool, but is becoming more affordable.

Figure 1

Figure 2

The Gro.Point™ Probe Figure 1: The Gro.Point™ probe is a TDR based probe manufactured by Environmental Sensors Inc in Canada.. Gro.Point™ probes are buried in the ground at the desired depths, and are easy to use and install. A display unit converts the sensor's output signal directly into a percentage volumetric moisture reading.

FieldScout TDR 300 Figure 2: This is a portable shaft mounted probe with a built in data logger that allows you to take multiple measurements anywhere in the field. The FieldScout will give almost instant readings.

Capacitance based instruments:

Until recently capacitance based technology was used primarily in research trials due to the expense. This technology provides repeatable and continuous readings. Per unit costs have come down in recent years and capacitance based probes are in use in a variety of large, intensive irrigation operations. Current capacitance probes are designed to be installed in a fixed location in the field for the entire season and are compatible with high levels of automation and/or telemetry. Siting of the probe is critical to ensure representative information for the field monitored.

C-Probe, EnviroSmart, EasyAg, Profile Probe, etc.: This probe is used by installing a waterproof access tube and inserting the probe into the tube. Numerous moisture sensing points may be fixed along the length of the probe to give readings at numerous depths specific to your crop roots.

For more information on monitoring soil moisture and using these types of instruments to make irrigation decisions see the Best Management Practices booklet Irrigation Management - available free in your local OMAFRA office or order a copy on-line through the OMAFRA website.

Related Links

• Best Management Practices Booklet Irrigation Management

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