By Viacheslav I. Adamchuk, Precision Agriculture Engineer, and Paul J. Jasa, Extension Engineer University of Nebraska-Lincoln
Sensors that measure a variety of essential soil properties on the go are being developed. These sensors can be used either to control variable rate application equipment in real-time or in conjunction with a Global Positioning System (GPS) to generate field maps of particular soil properties. Depending on the spacing between passes, travel speed, and sampling and/or measurement frequency, the number of measurement points per acre varies; however, in most cases, it is much greater than the density of manual grid sampling. The cost of mapping usually is reduced as well.
Measuring Soil Properties
When thinking about an ideal precision agriculture system, producers visualize a sensor located in direct contact with, or close to, the ground and connected to a "black box" which analyzes sensor response, processes the data, and changes the application rate instantaneously. They also hope that the real-time information detected by the sensor and used to prescribe the application rate would optimize the overall economic or agronomic effect of the production input. This approach, however, does not take into account several difficulties met in the "real world":
1. Most sensors and applicator controllers need a certain time for measurement, integration, and/or adjustment, which decreases the allowable operation speed or measurement density.
2. Variable rate fertilizer and pesticide applicators may need additional information (like yield potential) to develop prescription algorithms (sets of equations).
3. Currently, there is no site-specific management prescription algorithm proven to be the most favorable for all variables involved in crop production.
Rather than using real-time, on-the-go sensors with controllers, a map-based approach may be more desirable because of the ability to collect and analyze data, make the prescription, and conduct the variable rate application in two or more steps. In this case, multiple layers of information including yield maps, a digital elevation model (DEM), and various types of imagery could be pooled together using a geographic information system (GIS) software package designed to manage and process spatial data. Prescription maps can be developed using algorithms that involve several data sources as well as personal experience.
Sensors for Automated Measurements
Scientists and equipment manufacturers are trying to modify existing laboratory methods or develop indirect measurement techniques that could allow on-the-go soil mapping. To date, only a few types of sensors have been investigated, including:
Electromagnetic sensors use electric circuits to measure the capability for soil particles to conduct or accumulate electrical charge. 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. Several such sensors are commercially available:
- Mapping electrical conductivity (Veris® 3100, Veris Technologies, Salina, Kansas)
- Mapping transient electromagnetic response (EM-38,Geonics Limited, Mississauga, Ontario, Canada)
- Using electrical response to adjust variable rate application in real-time (Soil Doctor® System, Crop Technology, Inc., Bandera, Texas)
Electromagnetic soil properties, for the most part, are influenced by soil texture, salinity, organic matter, and moisture content. In some cases, other soil properties such as residual nitrates or soil pH can be predicted using these sensors. Several approaches for applying electromagnetic sensors have been observed in recent years.
Optical sensors use light reflectance to characterize soil. These sensors can simulate the human eye when looking at soil as well as measure near-infrared, mid-infrared, or polarized light reflectance. Vehicle-based optical sensors use the same principle technique as remote sensing. To date, various commercial vendors provide remote sensing services that allow measurement of bare soil reflectance using a satellite or airplane platform. Cost, timing, clouds, and heavy plant residue cover are major issues limiting the use of bare soil imagery from these platforms.