Conventional to Modern Methods of Soil NPK Sensing: A Review

In this review, we have evaluated several methods and recent technologies to determine the soil nitrogen, phosphorus, and potassium (NPK) content to increase yield production. Sensing technology has been used for a long time to determine the exact amount of soil nutrients without going through a conventional time-consuming, and expensive laboratory testing process. Several types of sensors are developed to measure the quantity of Nitrogen (N), Phosphorous (P), and Potassium (K) present in the soil, basically known as NPK sensor that provides on-the-go detection. A significant need for compact, inexpensive, low-power, and rapid soil NPK sensors is anticipated due to the demand for enhanced agriculture production and the future widespread usage of the Internet of Things (IoT). This fact implies using advanced technologies for both sensing elements and rapid detection. In recent years, machine learning (ML) techniques have also been utilized to optimize soil NPK levels. These techniques involve training models on large datasets of soil samples and their corresponding nutrient levels, allowing for accurate prediction of nutrient concentrations based on sensor data. Only the sensing devices primarily used in agriculture applications will be discussed in this review. The prominent technologies and methods used in agriculture will be explored with regard to research and development ever held in agricultural sensing devices for measuring soil parameters. These elementary methods of determining the soil parameters may be grouped into two classes: conventional and emerging. We expect this review to guide the researchers and provide an approach to the study related to producing a convenient, affordable, and efficient soil NPK detection sensor.