Crop Sensors

The continuously decreasing agricultural land, ever increasing population and unpredictable climate changes have created the food shortage crisis and now food security has become a grave threat to mankind. Such serious challenges require drastic measures and need to adopt technological advancement in agriculture sector.

Advanced technologies like artificial intelligence (AI), the Internet of Things (IoT) and the mobile internet can provide realistic solutions to the above challenges. Precision or Smart agriculture is an agriculture technique which uses the above mentioned technological tools to increase efficiency and productivity, reduce wastage, erosion and pollution and conserve biodiversity.

The most important tool in modern agriculture management are sensors. Sensors provide data that helps farmers to improve crop conditions by measuring various crucial components and resources like plant water potential, yield quality, state of development, nutrient levels. pest and disease infections and area and distribution of plants and organs. A number of sensing technologies are used in precision agriculture, providing data that helps farmers monitor and optimise crops, as well as adopt to the changing environmental factors are as follows:

A. Location Sensors

These sensors use GPS satellite to determine latitude, longitude and altitude to within feet. Three satellites are minimum required to triangular a position. Precise positioning is the cornerstone of precision agriculture.

B. Electromagnetic Sensors

These sensors use electric circuits to measure the capability for soil particles to conduct or accumulate electrical charges. When using these sensors, the soil becomes part of an electromagnetic circuit and changing local conditions immediately affect the signal recorded by a data logger. These sensors help in recording the salinity, organic mater and moisture content along with residual nitrate and soil pH.

C. Optical Sensors

These sensors use light reflectance to characterise the soil. These sensors simulate the human eye when looking at soil as well as measure near-infrared, mid-infrared or polarised light reflectance. Close-range, subsurface vehicle based optical sensors have the potential to be used on the go, in a way similar to electromagnetic sensors, and can provide more information about single data points since reflectance can be easily measured in more that one portion of the spectrum at the time. Several researches have developed optical sensors to predict clay, organic matter and moisture content.

D. Mechanical Sensors

These sensors can be used to estimate soil mechanical resistance (often related to compaction). These sensors use a mechanism that penetrates or cuts through the soil and records the force measured by strain gauges of load cells. When a sensors cuts through the soil, it records the holding forces resulting from cutting, breaking and displacing soil. Soil represents the rates of the force required to enter the soil medium to the frontal area of the tool engaged with the soil.

E. Electrochemical Sensors

This type lf sensors can provide the information related to soil nutrient levels and pH. Sensor electrodes work by detecting specific ions in the soil. These sensors can be used in both outdoor farms and green house-based farming establishments to measure teh levels of ions like Potassium, Phosphorous, Calcium, Sodium, Nitrogen, Copper and Iron. With the advent of ISE(Ion Selection Electrode) and ISFET (Ion Selective Field Effect Transistor)sensors, the development of ion-specific nutrient supply system of crops/ plants is now possible.

F. Air Flow Sensors

These type of sensors measure soil permeability and record the number of gaseous substances present in the soil at a particular landscape other irrigation or to get an overview of the land that is to be cultivated before seeding process. It determines the optimum pressure required to pump air to aerate the soil and make it more fertile. It is also used tod ermine the properties of the soil, its composition, moisture holding capability and more.

G. Acoustic Crop Sensors

These sensors detect the plant geometric structure by emitting an ultrasonic wave signal to the plant. The sensor emits the signal as a reputed sweep under a certain frequency (usually 50-100 kHz). As a result of the application of ultrasound energy to the plant, several echos occur. Each echo contains information about the geometric structure of the plant, i.e. the structure of teh foliage. This information can be captured by extracting features from the echo signal into geometric features related to foliage structure (size, shape, orientation and overall orientation of the leaves). Additionally, the leaf structure greatly influences the amount of reflected ultrasonic sound. In general, the more leaves on the plant and the larger the leaves, the greater the percentage of ultrasonic sound that will be reflected by the sensor.

Acoustic sensors have a miscellaneous application in farm management. Some of the target applications are:

Soil cultivation with the object to detect the rows of plants from the soil
Weeding, for the detection of the plants from the weeds.
Herbicide application for the detection of the plants from the weeds.
Fruit harvesting to find the fruit inside the tree canopy.
Grain and forage harvesting in order to detect the harvested area, thus avoiding overlapping.

Application of the Sensor Output

Sensing technologies provide actionable data to be processed and implemented as need be to optimise the crop yield while minimising environmental effects. Below are a few of the ways that precision farming takes advantage of this data:

Yield monitoring systems are placed on crop harvesting vehicles such as combines and corn harvesters. They provide a crop weight yield by time, distance or GPS location measured and recorded to within 30cm.
Yield mapping uses spatial coordinate data from GPS sensors mounted on harvested equipment. Yield monitoring data is combined with the coordinates to create yield maps.
Variable rate fertiliser application tools use yield maps and perhaps optical surveys of plant health determined by colouration oto control granular, liquid and gaseous fertiliser materials. Variable rate controllers can either be manually controlled or automatically controlled using an on-board computer guided by realtime GPS location.
Weed mapping currently uses operator interpretation and input to generate maps by quickly marking the location with a GPS receiver and data logger. The weed occurrence can then be overlapped with yield maps, fertilisers maps and spray maps. As visual recognition systems improve, the manual entry will soon be replaced by automated, visual systems mounted to working equipment.
Variable spraying controllers turn herbicide spray booms on and off, and customise the amount (and blend) of the spray applied. Once weed location ae identified and tapped, the volume and mix of teh spray can be determined.
Topography and boundaries can be recorded using high-precision GPS, which allows for a very precise topographic representation to be made of any field. there precision maps are useful when interpreting yield maps and weed maps. Field boundaries existing road, and wetlands can be accurately located to aid in farm planning.
Salinity mapping is done with a salinity meter on a sled towed across fields affected by salinity. Salinity mapping interprets emergent issues as well as change in salinity over time.
Guidance systems can accurately position a moving vehicle within 30 cm or less using GPS. Guidance systems replace conventional equipment for spraying or seeding. Autonomous vehicles are currently under development and will likely to be out into use int the very near future.

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