Soil moisture sensors for irrigation scheduling

Vasudha Sharma, Extension irrigation specialist and Anne Nelson, Extension educator

Measuring soil moisture and electrical conductivity
in a corn field.

Efficient irrigation management can improve yields, grain quality, conserve water and energy, and reduce nutrient leaching. One of the easiest and most effective ways to improve irrigation efficiency is to implement soil sensor technology in irrigation scheduling. This article provides basic knowledge and practical recommendations for using soil moisture sensors for irrigation scheduling.

Types of soil moisture sensors

Soil moisture sensors are divided into two categories depending on the technology they use: 1) Sensors that measure volumetric water content and 2) Sensors that measure soil tension when placed in the soil profile (Figure 1).

Figure 1. General categorization of soil moisture sensors.

Volumetric water content (VWC) soil moisture sensors

Volumetric water content is the volume of liquid water per volume of soil. It is usually expressed as a percentage. For example, 25% volumetric water content (VWC) means 0.25 cubic inch of water per cubic inch of soil. When this %VWC is multiplied by the desired depth (effective rooting depth), it results in the total soil water in that soil depth in inches.

For irrigation scheduling, the soil water content measured using soil moisture sensors (current soil water content) is compared to the soil water content at field capacity to calculate the total water depletion in the soil profile. Check https://z.umn.edu/soilwaterbasics for basics of soil water for irrigation scheduling.

Field capacity can be measured very easily in the field using soil moisture sensors. The VWC measurements provided by the soil moisture sensor after 12-24 hours of heavy irrigation or rain is the field capacity of the soil. Web Soil Survey can also be used to get the field capacity and available water holding capacity (AWC) values of your soil.

For irrigation scheduling, it is also important to understand the soil water content when the crop begins to experience stress. In general, most crops begin to experience stress when soil water depletion is 30-50% of AWC. This is called management allowable depletion (MAD) or irrigation trigger point. MAD can be varied depending upon crop, growth stage and irrigation system’s pumping capacity. Check https://z.umn.edu/strategiesbyseason for growth stage/season-based MAD recommendations. Irrigation should be started when the soil water depletion is equal to the management allowable depletion.

The most common soil water content or volumetric water content sensors are:

• Electromagnetic sensors: These sensors indirectly measure VWC based on the dielectric and electric properties of the soil medium (soil bulk permittivity or soil dielectric constant). The dielectric constant is a measure of the ability of the substance to store electrical energy. Since soil particles, water, and air, all have different dielectric constants, their ability to store or dissipate electrical energy is different. This is how it can be correlated to soil water content.
• Neutron Probe: This sensor measures volumetric water content at various depths with a radiation source. However, the operator must be licensed and receive special training to use this sensor. The unit is expensive and requires a lot of time to take field readings.

Example calculations: Using a volumetric water content (VWC) sensor to estimate soil water depletion

Example: Consider the soil moisture readings at three depths given in Table 1 for center pivot irrigated corn in a Hubbard soil series. The field capacity (FC) and available water holding capacity (AWC) at each depth is also given in Table 1. Assume the root depth is 36 inches and management allowable depletion (MAD) is 50%. Assume daily crop water use or evapotranspiration is 0.20 in/day.

Calculations for 0-12 inches soil layer:

• Depth of water at AWC i.e. Col.F=Col.D x Col.B = AWC x thickness = 0.08 x 12 in = 0.96 in.
• Depth of water at MAD i.e. Col.G=Col.F x 0.50 = Depth of water at AWC x 0.50 = 0.96 x 0.50 = 0.48 in.
• Soil water depletion (D) i.e.Col.H = (Col.C - Col.E = (FC - current soil moisture) x thickness = (0.16-0.11) x 12 = 0.60 in.
• Do the same for other depths

Table 1. Soil water depletion calculations at different depths using volumetric soil moisture sensors.

A	B	C	D	E	F	G	H
		Soil water content (in/in)			Depth of water (inches)
Soil layer	Thick- ness	At FC	At AWC	Current soil moisture	At AWC	At MAD	Soil water depletion (D)
0-12	12	0.16	0.08	0.11	0.96	0.48	0.60
12-24	12	0.15	0.08	0.13	0.96	0.48	0.24
24-36	12	0.11	0.08	0.10	0.96	0.48	0.12
				Profile total	2.88	1.44	0.96

In this example

• Total available water holding capacity in 36 inch soil profile = 2.88 inches.
• Crops can deplete 1.44” of AWC in 36-inch soil profile before irrigation (col. G).
• Currently, the soil water depletion is 0.96 inches (col. H). That means water available in the soil profile before stress occurs = 1.44 - 0.96 = 0.48 inches.
• Crop water use or evapotranspiration is 0.2 inches/day, assuming no rainfall will occur, when should the next irrigation be applied? 
• Days for next irrigation = water available before stress occurs divided by crop water use per day = 0.48 in /0.20 in ~ 2 days

Notes:

• Crop water use, irrigation system efficiency, and soil observations should also be used as feedback to make decisions. 
• Irrigation amount should be applied to replenish soil moisture to 85% of FC to leave some room for rain.
• Consider your pumping capacity and application efficiency for scheduling irrigation. Check https://z.umn.edu/irrigationpumpingcapacity

Soil water tension or matric potential sensors

Soil water tension indicates the energy required by plant roots to extract water from soil particles. As soil water is removed from soil, soil tension increases. Soil tension is expressed in centibars or bars of atmospheric pressure. When the soil is full of water, soil water tension is close to zero. For coarse textured soils, at 25-45 centibars, 50% of AWC is depleted (https://www.irrometer.com/basics.html), so in these soils crop should be irrigated before the sensor indicates 25-45 cb (Irmak et. al 2016). 

However, soil tension measurements are soil specific and can be inaccurate, so depending on your crop and soil observations, soil tension limits should be refined. For example, note the soil tension at the earliest indication of water stress and always make sure that you irrigate before it reaches that point. You can also track your water movement by taking measurements right after an irrigation event. If your bottom sensor after irrigation indicates zero reading, that means you might have irrigated more than required. On the other hand, if it shows no movement, that means you irrigated less. Laboratory-developed charts like Table 5 in  https://z.umn.edu/soil-water-deficit can also be used to convert the tension readings from sensors to soil water deficit values. 

More information about the soil water tension sensors and sensor installation can be found at  https://z.umn.edu/soil-water-deficit.

Other recommendations

1. Use data loggers to store and log the data. This will help in data interpretation and quick decision making.
2. Use soil moisture data with irrigation scheduling tools such as checkbook method or daily crop water use data from Central Ag weather network (http://agweathernetwork.com/).
3. Read the sensors every two to three days.
4. Flag the location of the sensor for easy accessibility.
5. Always irrigate to replenish the soil moisture to less than field capacity so that there is some room for potential rainfall.

This article was first published in April 2019.