Information about the Kentucky State University Cooperative Extension Program 2022

Mohammad Valipour, PhD, Assistant Professor, Kentucky State University

Although only 8.2% of farms in Kentucky have been equipped with soil moisture sensors (SMS), this amount is higher than in 27 other states in the US (Kukal et al., 2020). On most Kentucky farms (more than 90%), irrigation scheduling has been applied based on empirical methods including condition of crop, soil type, personal calendar schedule, and when neighbors begin (United States Department of Agriculture (USDA), 2018). Most SMS are easy to use and affordable for monitoring soil moisture and irrigation scheduling. SMS can help farmers to enhance water use efficiency (WUE) and increase profits while substantially reducing use of water and energy. In addition, using SMS decreases erosion and leaching of chemicals such as pesticides, fertilizers, and soil nutrients. The other advantage of SMS is helping farmers apply water at optimum rates and only where needed. Blonquist Jr. et al. (2006) evaluated the performance of SMS under Kentucky Turf Grass in a silt loam soil. Relative to evapotranspiration (ET) based irrigation recommendations, the SMS helped to save 16% less irrigation water. Also, relative to a fixed irrigation rate of 2 inches per week, the SMS helped to apply 53% less water. In a newer study, Viloria and Dunwell (2017) demonstrated that SMS can help reduce irrigation water applications by 40 - 70% of irrigation without using SMS. It is worth mentioning that an average 50% reduction in irrigation can save over 43 million gallons of water and $6,500 in pumping cost annually in Kentucky (Viloria and Dunwell, 2017). Using SMS in fields can increase average net income by almost 20% and, for some crops like soybean, by 64% (Payero, 2021). Now the question is which SMS is best for Kentucky conditions. The objective of this fact sheet is to present a guideline for Kentucky farmers to select the best SMS for their farms with respect to various factors, including cost, soil texture, soil salinity, ease of operation, calibration, and performance accuracy.

SMS usually measure soil volumetric water content. PAW is the water available to support plant growth. PAW as a function of soil volumetric water content. PAW capacity equals the difference between field capacity (FC) and the permanent wilting point (PWP). FC is the maximum amount of water that a soil can hold. The PWP is the point when there is no water available to the plant. Table 1 shows FC, PWP, and PAW capacity for different types of soil texture. For example, a silt loam soil (which is the predominant soil texture in Kentucky) has a PAW capacity of 0.21. For a plant with a root depth of 19 inches, the total amount of PAW capacity is 0.21 × 19 = 3.99, or about 4 inches. If more than 4 inches of water is applied to the soil, either by precipitation or by irrigation, some of it will be lost to runoff or leaching below the root zone. According to Table 1, loam and silt loam soils are the best for agriculture because they have the highest PAW capacity compared to the other types of soil texture. In fact, loam and silt loam soils drain excess water quickly and are not prone to drought conditions (Easton and Bock, 2016). Table 1. FC, PWP, and PAW based on soil texture (adapted from Easton and Bock, 2016).

The top 10 SMS that are highly used for irrigation purposes are tensiometers, granular matrix sensors (GMS), heat dissipating sensors (HDS), capacitance, time domain reflectometers (TDR), frequency domain reflector (FDR), amplitude domain reflector (ADR), time domain transmission (TDT), neutron probe, and wireless sensors. Tables 2 and 3 show a short description, photos, pros, cons, and an estimated price of each SMS. It should be taken into account that “the best” SMS is not the same for all farmers based on their different point of views and limitations. For example, a farmer with a budget limitation has to opt for