Soil sampling gets on-the-go
By Larry Dreiling
In the last 20 years, as precision farming techniques have moved from gee-whiz stuff to everyday use, many different concepts for creating variable-rate prescriptions for crop inputs like seed and fertilizer have been developed.
Originally, the use of yield monitors coupled to Global Positioning System satellites was the big thing in creating yield maps for proper subsequent management decisions.
It led to higher use of soil sampling, and farmers and crop consultants patrolled fields in four-wheelers tied to GPS units to capture samples at just the right spot to determine everything from soil pH to nitrogen uptake. Since then, satellite-based auto-guidance and steering systems have been introduced.
Studies have shown that use of yield map and soil survey information has its limitations, since yield data are not always readily available to growers and soil properties are often not well defined near survey boundaries.
That's why 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 those using electromagnetic, optical, mechanical, electrochemical, airflow and acoustic.
Configured with GPS, soil sensors produce the rich mapping coverage needed to better define soil boundaries and improve a farmer's management capabilities.
Perhaps the biggest of the big new thing of the past decade has been the advent of apparent soil electrical conductivity mapping as a simple, inexpensive tool farmers can use to quickly and accurately characterize soil differences within their fields.
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 to produce soil EC in a measurement known as milliSiemens per meter, according to a University of Nebraska-Lincoln scientist. "We term this apparent electrical conductivity, which is different than what a lab sample is. Soil texture is predominant in determining structure of conductivity," according to Richard Ferguson, Ph.D., soil fertility specialist at UNL. "It gives producers an idea of how soils vary in fields that can influence water holding capacity and nutrient supply."
Soil texture, salinity, organic matter, and moisture content, for the most part, influence electromagnetic soil properties, Ferguson said. 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.
Soil EC measurements are correlated with soil properties that affect crop productivity, including soil texture, cation exchange capacity, drainage conditions, salinity, and subsoil characteristics.
There are two methods currently available for mapping soil EC in the field. The first, called the "contact" method, uses coulters placed in direct contact with the soil to measure soil EC; the second, referred to as the "non-contact" method, uses electromagnetic induction. The measurement of soil EC by either contact or non-contact methods is said to have comparable results.
Veris Technologies of Salina, Kan., introduced the first on-the-go soil EC mapping system in 1996 and has been selling it at the commercial level for the last decade, primarily to the crop consultancy and seed sales trade. It measures EC at two depths (0 to 12 inches as well as 0 to 36 inches) as the device is pulled across a field to develop management zones, guide soil sampling, and provide additional quantitative data for field analysis.
"We use these systems in our research to characterize special variations in fields, to use that information to go along with other layers of data we collect," Ferguson said. "We encourage producers to use it as they map their fields, to understand how variable they really are and to manage them better."
Conductivity can be used as a way to determine zones to sample for soil nutrient levels, Ferguson said, to sample in regions of high, medium and low conductivity to understand how fertility might change. Interestingly, higher organic matter content can increase conductivity, but the impact may be proportionately lower than texture.
"As we reach into excessive levels of conductivity is where we see salinity and poor drainage, usually in soil with high clay content. This is where high conductivity can be a negative factor, as well," Ferguson said.
When thinking about an ideal precision agriculture system, Ferguson admits most 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 doesn't take into account all the things you meet up with in the real world," Ferguson said.
A few of those problems:
--Most sensors and applicator controllers need a certain time for measurement, integration or adjustment, which decreases the allowable operation speed or measurement density.
--Variable rate fertilizer and pesticide applicators may need additional information, like yield potential, to develop precise prescription algorithms.
--There currently 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, Ferguson thinks 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.
"From this approach, multiple layers of information including yield maps, a digital elevation model, and various types of imagery could be pooled together using a GIS (geographic information system) software package designed to manage and process the data."
Ferguson said a team of researchers at UNL will continue work on vehicle-based soil sensors, which could be used for research and commercial applications.
"These can be very useful tools to help define variable input rates," Ferguson said.
Larry Dreiling can be reached by phone at 785-628-1117, or by email at firstname.lastname@example.org.