Rapid growth in population, climate change, reduction in rainfall, and demand for more food to feed billions of people worldwide are placing a lot of pressure on arable farming with negative impacts on conventional farming practices.
The current situation requires agriculture to become “smart” by using modern smart technologies to find solutions for the effective utilization of scarce resources, thereby meeting the global population’s ever-increasing consumption needs.
With the advent of the Internet of Things (IoT) and the digital transformation, these technologies can be leveraged to remotely monitor soil moisture, crop growth, and preventive measures to detect crop damages and threats.
Remote management of agricultural activities, through automation using wireless sensors and the Internet of Things, is the area of focus today in agriculture to provide new insights and improved decision making.
A sensor is a simple and small device that measures or detects real-world conditions such as motion, heat, or light and converts the condition into analog or digital representation. Compared with other analytical techniques such as chromatography or spectroscopy, the advantages of sensors are their robustness, reduced size, versatility, and low mass-production costs.
Conventional soil sampling and analysis have mixed economic returns due to high labor-intensive sampling and analysis procedures. A limited number of soil samples also does not produce an accurate representation of soil properties. Sensors provide more accurate soil properties necessary to successfully implement site-specific management decisions.
1. Electromagnetic sensors
Electromagnetic sensors use electric circuits to measure the capability for soil particles to conduct or accumulate electrical charge. Soil becomes a part of the electromagnetic circuit, and the change in the electrical conductivity is immediately recorded in a logger.
There are two types of EC sensors: contact or non-contact:
- Contact EC sensors uses electrodes, usually in the shape of colters, that make contact with the soil to measure the electrical conductivity. This approach has 2-3 pairs of colters mounted on a toolbar, and one pair applies electrical current into the soil, while the other two pairs of colters measure the voltage drop. The data is recorded in a data logger along with location information. Contact EC sensors are very popular in precision agriculture because it is easier to cover large areas and is less susceptible to outside interference.
- Non-contact EC sensors work on the principle of Electromagnetic Induction (EMI) without contacting the soil surface directly. The instrument has a transmitter and a receiver coil, installed at opposite ends of a non-conductive bar located at opposite ends of the instrument. This is a digital and multi-frequency sensor that can operate in a frequency range of 300 Hz to 24 kHz.
2. Optoelectronic sensors
Optoelectronics deals with the interaction of electronic processes with light and optical processes. Devices where such interaction takes place, are called optoelectronic devices. Optoelectronic sensors, in general, use the characteristics of different materials to have a defined spectral signature. The system is composed of a light source (emitter) that emits light in a wavelength range and a sensor (detector) for specific wavelength ranges. Soil organic matter (SOM) sensor is one widely used under this category.
Although inexpensive, the single-wavelength sensor needs to be recalibrated for the soil and moisture conditions that prevail at the time of use. The multiple wavelength sensors can utilize a single calibration to predict SOM over a range of soil moistures and soil types within a geographical area of several hundreds of kilometers. Besides, it can be used to sense soil moisture and cation exchange capacity (CEC).
3. Electrochemical sensors
Electrochemical sensors provide the most important information on nutrient availability and pH in the soil. When soil samples are sent to the soil testing laboratory, a set of standardized laboratory procedures is performed. These procedures involve sample preparation and measurements. Some measurements are performed using an ion-selective electrode. These electrodes detect the activity of specific ions (nitrate, potassium, or hydrogen in case of pH). The value obtained may not be as accurate as a laboratory test, but the high sampling density may increase the overall accuracy of soil nutrients or pH.
4. Ion Selective Electrode sensors
Ion Selective Electrode sensors measure the potential of a specific ion in a solution. They are also used to determine the nitrogen content in the soil. The potential is measured against a stable reference electrode of constant potential. The potential difference between the two electrodes depends on the activity of the specific ion in a solution. This activity is due to the concentration of the specification, allowing the end-user to make an analytical measurement.
5. Mechanical sensors
Mechanical sensors measure the mechanical resistance of the soil. These sensors penetrate/cut through soil and record the force measured by the strain gauge or load cells.
Airflow sensors: These are used to measure the soil air permeability. The sensor measures the pressure to squeeze a given volume of air into the soil at a fixed depth.
Acoustic sensors: Soil texture is investigated by measuring the change in the noise level due to the tool’s interaction with the soil particles.
6. Other sensors
A range of sensors is used with the right combination of intelligent integration, logic, and customizable software in soil science. They include:
- Soil pH sensors: They measure soil acidity and alkalinity that can cause common fertility problems, stunted growth, and abnormally colored leaves or poor plant health.
- Ammonium sensors: They enable precise determinations of ammonia and nitrogen concentrations.
- Nitrate sensors: They measure the amount of nitrate in the soil/water.
- Potassium sensors: They monitor the potassium in the soil to provide an understanding of the relationship among free potassium in soil nutrients reserves, soil texture, and root growth, which could optimize fertilizer consumption.
- Soil-water sensor: They provide valuable information about the soil-water content, which is critical for determining local energy and water balance, transport of applied chemicals to plant and groundwater, irrigation management, and precision farming.