Site-Specific Nutrient Management

Site-Specific Nutrient Management

Site-specific management can be thought of as a series of layers of information about each field, as depicted in Figure 1. Each time a measurement is made (soil tests, scouting reports, yield data, etc.), another layer of information is added. Over time, multiple layers of information are added and become part of the database that can guide future crop management decisions. By geo-referencing each data point to its precise geographic location, these data layers can be "stacked" for analysis to determine the relationship between layers for any point in the field. For example, the relationship between nitrogen rate applied and yield obtained might be determined, and then its variability mapped as an additional "calculated" layer of information.


A. Grid sampling, guided by GPS, provides more accurate soil test data.

B. Variable rate fertilizer application can improve efficiency. 

C. Variable rate seeding, variety changes and starter fertilizer can adjust for soil properties and productivity. 

D. Crop scouting with new digital technologies improves field records. 

E. On-the-go yield monitors can quickly track variability in the field. 

Precision Agriculture

The systematic implementation of best management practices into a site-specific system provides the best opportunity to develop a truly sustainable agriculture system. Managing the right source at the right rate, right time and in the right place is best accomplished with the right tools. Various technologies are available to help make decisions related to nutrient management, from soil sampling to fertilizer application to yield measurement. These tools enhance the ability to fine-tune nutrient management decisions and develop the site-specific nutrient management plan for each field.

Identify and Quantify Variability within Fields

Variability within fields is measured by soil sampling, field scouting, physical measurements, soil survey, and yield monitoring. Documenting this information in the GIS database for a field provides the basis for site-specific management decisions. Variability within fields comes from a variety of natural and man-made factors. Natural variability is largely due to physical properties of the soil including topography, texture and structure. Man-made influences on soil variability include:

  • Crop rotation, livestock pasture, fences, tile drainage, fertilizer and manure application;

  • Cropping systems and tillage operations affect soil tilth; and

  • Compaction (a result of a combination of natural and man-made factors).

Site-Specific Equipment and Technology


Special equipment is not required for site-specific management. Identifying areas requiring specific management can be done with conventional soil testing and scouting techniques. Different fertilizer rates can be applied to different areas by staking or flagging them, and then spreading the different areas separately. Estimates of within-field distances to identify these areas can be documented by measuring, counting rows, pacing or other relative means. But there are technology tools available that expand the capabilities for using site-specific management more effectively.

Technology Tools

GPS, GIS-based records and data analysis, sensors and variable-rate controllers are revolutionizing nutrient management to best meet crop needs and efficiently utilize available resources. Site-specific sampling, variable-rate fertilizer application and yield monitors are among the most common tools guiding today’s modern nutrient management systems.

Global Positioning System (GPS)

Most of the tools for precision agriculture involve use of data collection or controller systems that utilize the global positioning system (GPS). Each set of data collected is associated with its specific geographic coordinates (latitude, longitude, and elevation). This allows the understanding of precise relationships among the different layers of data, the resulting yield data, and other measurements. These layers can then be analyzed to make recommendations for future decisions.

GPS systems are used on planting equipment for collecting geo-referenced planting data, starter fertilizer application, and other inputs. With proper controllers, variable-rate application of inputs can be added to the management plan. Each of these steps can be added over time, increasing the value of the initial investment.

As more advanced military-technology becomes available for public use and new technologies develop to support GPS, this tool will continue to become more valuable to farmers in implementing site-specific management.

Real-Time Kinematic System (RTK)

GPS is used at different levels of precision, depending on the application and availability of information. The most precise system currently used in crop production applications is the Real-Time Kinematic (RTK) system.

The high-accuracy RTK guidance systems help avoid costly skips and overlaps, saving on input costs for seed, fertilizer and pesticides. Reduced operator stress and fatigue are major added benefits. RTK systems use a base station that transmits its location to the rover GPS receiver (on the implement), which is used to correct the position of the roving unit to the position of the known fixed base station. Such systems typically provide accuracy within 1 or 2 cm in position and from year to year. This enables accurate row-to-row positioning, eliminates skips and improves accuracy of harvest monitoring data. RTK is also used to provide similar accuracy for multiple passes, such as banding starter fertilizer in the fall matched with seeding in the spring.

Geographic Information System (GIS)

Geographic Information Systems (GIS) consist of data and software designed for spatial analysis of GPS-referenced data. Various databases in an agricultural GIS system might include soil survey data, soil test information, pest infestations, yield data, remote sensing imagery and other types of observations and records that can be collected and referenced with their geographic position (by GPS). These data sets can then be converted to maps to illustrate their spatial variability within the field and become additional layers in the field database.

The capability of GIS is more than mapping. The real power of GIS software lies in calculations and analysis of the georeferenced data sets to correlate their effects on yields and interactions with other production factors. By using models integrating the different spatially-variable data sets, responses to inputs can be predicted, or interactions affecting yield can be identified. Accumulated over time, the GIS data sets become increasingly useful as record-keeping and prediction tools.

Soil Surveys

For site-specific management, it is important to understand the variability in soil characteristics, which can be done through soil surveys. The Web Soil Survey provides over 2300 geographically-referenced digital soil surveys for free download from the USDA Natural Resources Conservation Service (NRCS) website. This information helps relate soil characteristics to site-specific variability observed in crop yield.


USDA-NRCS Web Soil Survey provides free access to digital soil survey maps and information for all of the United States.

Intensive Soil Sampling

Site-specific production systems will usually require more intensive soil sampling. The most common is a 2 to 3 acre grid, preferably taken on a systematic, unaligned grid basis. (See soil sampling chapter.) Research has shown an advantage to shifting to a 1-acre grid, or even smaller where the field is known to be highly variable. GPS-equipped sampling systems document the precise location of the samples, so the test results can be used to guide nutrient decisions and to facilitate correlation with yield maps, soil survey, and other datasets. Now subsequent samples, fertilizer and manure applications, and crop removal can all be analyzed as additional layers in the GIS database for the field, and used for calculating such details as fertilizer recommendations, nutrient use efficiency, and selected environmental parameters.

Remote Sensing

Remote sensing is becoming a useful tool for precision farming, using scanners on aircraft or satellites to monitor changes in wavelengths of light from fields and growing crops. Satellite imagery is also useful in more precise mapping of field boundaries and location of tile drainage lines, for example, and is often most effective when used in conjunction with field scouting ("ground truth observations") to help identify the reasons for variability. The data collected can be mapped and analyzed with the help of GIS tools, to provide additional data layers for GIS analysis and management decisions.

Remote sensing helps to define the extent of problems identified in field scouting by recognizing similar patterns. It is used to document such issues as pest problems, weather factors, nutrient management issues, and more. While it has taken several years to develop remote sensing technology to the point of providing dependable, cost effective products and services in a timely fashion, there are now such services available to add to the toolbox to aid farmers and their advisors in making crop management decisions.

Nutrient Management Plan Development

The value of GPS, GIS and remote-sensing technologies comes from incorporating the data into the management decision process. These tools can help to develop a comprehensive crop and soil nutrient management plan that can help improve production efficiency, increase yields and reduce potential environmental problems associated with crop production. The GIS system provides a means to monitor and evaluate nutrient needs, crop removal, and losses to the environment.

Taking Advantage of Information

Sites-specific management systems are more effective over field-average systems only if management is intensified to take advantage of the increased information available. Collecting data is only the first step. Too often, the data are collected and stored, and not really utilized. One common complaint from farmers who have collected precision farming data for several years is that they have not taken the time to analyze and interpret the data. Consequently, they are not gaining the benefits of the technology.

Information Integration

Site-specific management and the technology tools available require integration of many sources of information. Without the use of computers and GIS software, it is impractical to try to analyze all of the information available. Site-specific systems, including yield monitor data, generate large amounts of data that should be integrated into GIS and used to interpret the variability to move to a higher level of production, input efficiency and profitability.

Step-wise Implementation

For many farmers, the switch to site-specific management and investment in the new technology tools is a formidable challenge for both the learning process and the financial commitment. In which case, a stepwise implementation plan may be better than a complete transition. Starting with a good soil testing program and a yield monitor, a farmer can begin developing the databases and the experience needed to fully implement a site-specific management system. It takes four or five years of yield data to begin to identify true variability within the field and some of the cause/effect relationships involved. Stepwise implementation may make the whole process more acceptable as confidence in the new system grows.

Building a Digital Nutrient Management Plan for Each Field

A long-term goal might be to build a detailed geo-referenced database with layers of information on fertilizer use, crop yields, and nutrient removal for every field on every farm. Data from different layers (years, crops, yields, soil characteristics, nutrient additions, pest problems, etc.) could be analyzed for each part of a field, and used to interpret the cause/effect relationships among the various variables for which data are available. It becomes a very powerful management tool that gets better with each year of data that is added.

Documentation of Needs, Rates of Application and Yield Responses

Soil testing, either on a uniform grid basis, or based upon management zones, is the best way to determine and document variability in nutrient supplying power of the soil in a field. Along with documented variability in crop nutrient removal (such as by use of a yield monitor), soil test data used to determine nutrients needed from fertilizer and manure can then guide the development of site-specific nutrient application maps for more efficient nutrient use and protection from over-application, as well as prevention of under-application.

Moving Forward with Site-Specific Precision Agriculture Systems

As software and communication systems continue to improve, new outside databases, such as digitized soil surveys and weather information, can be linked with the farmer’s data for use in decision-support tools. Managing and interpreting those data often require outside help. Farmers can gain much more benefit by sharing the data with their advisor partners. Sharing data with other farmers makes a broader base of information upon which to make decisions. Each can still benefit from his unique experience and resources to make decisions on his own farm. Programs being implemented by seed, fertilizer, and chemical companies, or by technology data service providers, may be the answer to the growing information management needs of today’s farmers helping them to put the right nutrient source on at the right rate at the right time in the right place with 4R nutrient management.

In the near future unmanned remote-control drones and helicopters, and on-the-ground robotics will become a part of site-specific precision agriculture. They have the potential for dramatic changes in how we grow our crops.

Adapted from "The Efficient Fertilizer Use Manual",
Site Specific Farming chapter by Dr. Harold Reetz