15.1: Introduction

Chemigation is the application of chemicals with irrigation systems by injecting chemical solution into the irrigation water stream. The advantages and disadvantages of chemigation are important factors to be considered. All components of the chemigation system must be made of non-corrosive materials. The prevention of any chemicals from entering the water supply is crucial and the practice of chemigation is regulated by federal, state, and local agencies. An important component of a chemigation system is the injection device to assure accurate chemical application. As precision agriculture increases in popularity, the injection devices will become more sophisticated to account for spatial differences of chemical needs within a field.

In chemigation the term “chemicals” usually refers to fertilizers and pesticides with pesticides being inclusive of herbicides, insecticides, fungicides, nematicides, rodenticides, etc. According to data from USDA (2019), of the more than 25 million acres of irrigated field corn, vegetables, cotton, and orchards irrigated in the U.S., fertigation (application of fertilizers) was practiced on 32% of the irrigated area and pesticides were applied with chemigation on over 10% of the area. In some locations the term chemigation also includes the application of chemicals that are necessary for irrigation system maintenance, such as chlorination and acid treatment of microirrigation systems for preventing plugging of emitters by algae, slimes, and chemical precipitates (Chapter 14).

Advantages of Chemigation

The advantages of using chemigation can include better uniformity of chemical application, more timely application of chemicals, effective chemical incorporation, reduction in the number of field operations and the associated soil compaction and crop damage, improved efficacy of pesticides, and reduced environmental contamination (Threadgill et al.,1990 and van der Gulik et al., 2007). Timely application of soluble fertilizers using chemigation can reduce leaching losses on sandy soils, especially during years with greater than normal precipitation (Watts and Martin, 1981). Chemigation allows for the application of nitrogen at times that better match the time of crop uptake especially in taller crops such as field corn. Also, under the tall crop conditions, chemigation is often a viable alternative to aerial application of pesticides. During pest outbreaks timeliness of pesticide application can be critical, and rather than waiting for a commercial aerial sprayer irrigators can take advantage of using their irrigation system for the application. Even when irrigators could use their own field equipment to apply fertilizers or pesticides, using the irrigation system helps minimize field operations.

Precision agriculture depends on variable application of chemicals within a field to better match spatial differences in crop or pest control needs. Variable rate irrigation, discussed in Chapters 6 and 13, makes chemigation a viable application method for precision agriculture. Lo et al. (2019) documented how variable-rate chemigation is compatible with the needs in precision agriculture on a field scale.

Disadvantages of Chemigation

Water resource contamination, application onto non-target areas, increased risk of human exposure to chemicals, and limitations of the chemical products to be applied are some of the potential disadvantages of chemigation. Often with chemigation the chemical is mixed with the irrigation water through injection into the water stream. This poses an environmental risk when the irrigation system shuts off creating the potential of the chemical in the irrigation pipelines and chemical supply tank to backflow into the water source. In Section 15.3 we discuss methods to reduce the risk of this contamination.

In irrigated areas the public and field workers often become accustomed to working with and around irrigation systems and may regard the irrigation water as fresh water that they might consider safe for drinking or other uses and not know that hazardous chemicals might be mixed in the water. Also, with chemigation there is the potential risk of water contamination due to drift, runoff, or application of chemical onto non-target areas. An example of the latter case is a sprinkler system that applies water onto a stream or irrigation ditch that traverses the field.

As discussed by Threadgill et al. (1990) the chemical compatibility to chemigation must be considered. For example, by federal law in the U.S., pesticide labels must specifically state that it is legal to apply the chemical with an irrigation system. Also, there are fertilizers or pesticides that may not be good choices for chemigation if there is potential for precipitation of solids in the water when mixed with the chemical.