Soil Moisture Sensors for Irrigation Scheduling

 

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.

 

 

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 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:

 

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.

 

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