Measuring soil moisture for growing purposes is typically done in one of two ways:
IRROMETER soil moisture measurement is based on the tensiometric method, because of the fact that the amount of water is not as important as how difficult it is for the plant to extract it from the soil.
Soil water tension (or matric potential) has to be overcome for the plant to move water in to its root system. Different soil types will have different tensions even at the same volumetric measurement, making volumetric information relative to local conditions and often requiring site calibration for reading equipment. Because we use soil water tension, there is no site calibration required when using our sensors.
Due to the fact that tensiometers have been used in research since the 1920's and have been commercially available (from the IRROMETER Company) since 1951, years of published research by numerous universities and extension agencies have produced a wide field of reference for recommended tension levels to use with common crops and landscapes. Soil moisture information provided by IRROMETER equipment is inexpensive, accurate, and relevant.
There are two methods we offer for reading soil water tension:
The Tensiometer is the only direct measurement system available, which means that it actually reads the physical forces at work in the soil. Tensiometers act like a dummy root, allowing the soil moisture to interact with the instrument through the ceramic tip. Soil water tension outside of the instrument tries to remove the water from it, which creates a measurable tension inside the column. This tension is read with either a mechanical gauge or a transducer attached to the instrument. While this is the most accurate and proven method available, there is some maintenance required periodically to keep them full of water, and they must be removed from the field during the winter months to avoid freezing.
Our other option is the WATERMARK Sensor, which is a calibrated, indirect method of measuring soil water tension. These "Granular Matrix Sensors" electronically read the amount of moisture absorbed through a special "granular matrix", or mix of precisely composed materials. This special mix buffers the sensor against the effects of different salinities and ensures a much longer life than traditional "gypsum blocks". The readings are calibrated to reflect the same values that would be generated by a Tensiometer. These sensors are maintenance free and can be left in the ground permanently, with an expected life of 5+ years. WATERMARK sensors require very little power to read and integrate perfectly with electronic systems for data logging or telemetry.
Management - The key element in proper soil moisture measurement is the operator. Taking the time to interpret your sensor readings will give you a vivid picture of what is happening with the soil moisture in the root system of your crop. Usually 2 - 3 readings between irrigations are sufficient. A graphical display of your readings shows exactly how quickly (or slowly) your soil moisture is being depleted.
Use the following readings as a general guideline:
Perhaps the most important soil moisture reading is the difference between today's reading and that of 3 – 5 days ago. That is to say, how quickly is the reading going up? A slow increase means the soil is drying out slowly. But a big jump means the soil is losing water very rapidly. By analyzing such trends in the readings, you will determine WHEN to irrigate. A graph of readings over time makes it easier to see the trends, thereby making interpretation simpler. Your own situation may be unique because of differences in crop, soils and climate.
By using sensors at two or more depths in the root system, you can determine HOW MUCH water to apply. If the shallow sensor shows a rapidly increasing reading, but the deep sensor shows adequate moisture, you can run a short irrigation cycle as you only need to replenish the shallow root profile. If the deep sensor also shows a dry condition, then a longer irrigation cycle is needed to fully re-wet the entire root zone. The readings you take after an irrigation or rainfall event will show you exactly how effective that water application was.
Your own experience and management will soon point you in the proper direction. You will be practicing irrigation to achieve the positive results that come from any good management program.
Thresholds – Thresholds are reference lines that you specify for your own site and application. These identify the boundaries within which you want to manage moisture availability for your crop. How wet and how dry the soil should be depends on soil type, crop, the plants stage of development and cultural practices for managing the field. The following chart is offered as a reference guide to assist you in selecting appropriate threshold levels.
First, select the soil type(s) that most closely resembles that in your field.
Then, draw a vertical line from 10% available water depletion (represented by the blue/green boundary) down to the curve for your soil type and then horizontally over to the left axis labeled soil suction to obtain the reference WET value. This will determine the lower (wetter) threshold line.
For example, for a loam soil, this value would be 23 (as indicated by the blue arrow).
Next, draw a vertical line from 50% available water depletion (represented by the green/brown boundary) down to the curve for your soil type and then horizontally over to the left axis labeled soil suction to obtain the reference DRY value. This will determine the higher (drier) threshold line.
For example, for a loam soil, this value would be 84 (as indicated by the brown arrow).
There is no substitute for experience and agronomic knowledge to provide the best recommendations. Please consult a crop consultant, farm adviser, NRCS agent or extension agent for more specific advice on proper soil moisture management. A list of crop consultants that specialize in irrigation management can be found here.
Suggested placement depths for IRROMETER and WATERMARK Sensors - The following are suggested placement depths for various crops based on deep, well drained soils. In lighter or shallow soils, place instrument accordingly or set them at an angle. With drip or trickle irrigation 12" and 24" depths are recommended, with an added 36" instrument for deeply rooted crops.
Links to reports containing insight into specific applications and explanation of this instrumentation, as well as some scientific background references:University of Nebraska "Nebraska Agricultural Water Management Demonstration Network (NWAMDN): Integrating Research and Extension/Outreach"