**3.7 Variable-rate technology (VRT)**

VRT is a technology which adjusts and applies precise and measured the quantities of the inputs (e.g., fertilizers, pesticides, herbicides, seeds, and water) in agricultural production by considering data that is collected by sensors, maps, and GPS in a given landscape. VRT consists of the machines and systems for applying a desired rate of crop production materials at a specific time and a specific location; a system of sensors, controllers, and agricultural machinery used to perform variable-rate applications of crop production inputs.

## **3.8 Crop and soil sensors/remote sensing**

Crop and soil sensors/remote sensing technologies are mainly used to measure essential soil and crop properties on-the-go in the field. The measurement results obtained from these sensors are used either to control variable rate application equipment in real-time or in conjunction with a global positioning system (GPS) to generate field maps. There are numerous sensor types (e.g., location sensors, optical sensors, multispectral sensors, thermal infrared sensors, laser sensors, electro-chemical sensors, mechanical sensors, dielectric soil moisture sensors, and air flow sensors) on the market to measure crop, soil, and microclimate parameters. Kim and Lee [46], who reviewed the sensors widely used in the market in order to monitor the plant, the soil, and the environmental conditions that directly affect plant growth, presented the electrochemical sensors for monitoring plant health in PA as in **Figure 7**.

## **3.9 Image-based sensing and unmanned aerial vehicles**

Unmanned aerial vehicles (UAV), and unmanned ground vehicles (UGV) are utilized for plant health monitoring, pest control, livestock management, aerial survey, and soil analysis in PA. High-resolution cameras (RGB, multispectral, etc.) are used for the purpose of capturing the images for further investigation [47].

**Figure 7.** *The electrochemical sensors for monitoring plant health in precision agriculture [46].*
