Site-Specific Farming – Number 1
What is Site-Specific Farming?
SF-1176 (1), June 1999
Dr. Dave Franzen, NDSU Extension Soil Specialist
Site-specific farming is doing the right thing at the right place at the right time.
Site-specific farming is a different way of thinking about the land. A field boundary
is defined by a surveyor and a legal description. Fields are square or rectangular in
shape because of the initial surveying conducted many years ago. However, the lands inside
these fields are variable- which means they are not the same. Some differences between
soils are small, but often the differences are very large. Site-specific farming is used
to measure what is different, record the differences at distinct and specific locations,
and then direct differences in management or input based on site-specific information. So
site-specific farming is managing areas within fields, rather than using the same
management on the field based only on legal description boundaries.
What does it take to conduct site-specific farming?
Many producers believe that site-specific farming is prohibitively expensive, and that
it is not relevant or useful in dryland environments. Recent studies suggest that this
area may in fact benefit most from site-specific farming for a variety of reasons, and
that site-specific management techniques can be used at relatively low cost.
To conduct site-specific farming, the producer must be able to do three things —
- Know where you are.
- Gather information at that location.
- Be able to do something about it.
Knowing where you are
Knowing where you are means knowing your geographic position. It is important to know
position to give a geographic address to information gathered, and because variable-rate
equipment needs to know where to change the rate of an input. Latitude and longitude have
been used for centuries to guide sailors, explorers and surveyors. Nearly every map or
globe contains rough latitude and longitude information.
Latitude and longitude can also be converted to UTM coordinates, which are latitude and
longitude in feet or meters from a specific point on the globe. Each method performs the
same end; it gives a specific address to each spot on earth. No two geographic locations
have the same latitude and longitude.
We can now determine our latitude and longitude very easily thanks to a system of
United States Department of Defense (DOD) satellites called the GPS (Global Positioning
Satellite) system (Figures 1 and 2). The GPS system can be accessed by purchasing a GPS
receiver. Due to inherent errors in the system, some of which originate with the DOD and
others simply due to time delay differences between satellites and atmospheric layer
differences, a differential signal is required for agricultural accuracy of 1 to 10 feet.
The differential signal may come from a separate satellite, an FM radio piggyback signal,
or a free Coast Guard signal if you are close to a navigable waterway (North Dakota has
little access to Coast Guard signals). GPS and differential GPS equipment and signals have
been reduced greatly in price over the past 10 years. Check with a local supplier (there
are several in North Dakota) for up-to-date pricing.
Figure 1. A GPS Satellite (24KB color photo)
Figure 2. The GPS Orbital Array (15KB color
illustration)
Gathering information
Important information can be gathered through several methods, including the use of
sensors and sampling. Sensors may be enhanced GPS systems that provide accurate elevation
data, or electrical conductivity detectors for management zone detection, or yield monitor
data. Sensors are desirable because many thousands of data points can be collected with no
laboratory analysis. Although there is work on soil testing sensors, the work is
frustrating because soils contain particles that easily wear away sensitive detection
equipment and plug up plumbing used to transfer solutions from one part of the sensor to
another, or plug up filters and separation columns on other equipment. Sensors that work
very well in chemical processing and water work very poorly in soils.
For soil fertility information, sampling is the best method to gather data. There are
two types of sampling methods — grid sampling and zone sampling.
Grid sampling uses a systematic method to direct where samples are taken. The samples
are taken densely enough so that when the sample results are mapped, they represent the
fertility patterns in the field. Grid sample density found to be consistently reliable is
about one sample per acre. Although many growers use the one sample per acre approach in
the Corn Belt states, many more do not and instead rely on less dense sampling, often one
sample per 2� acres, to represent the fields. In the Red River Valley of North Dakota and
Minnesota, where site-specific N application has been conducted since 1994, a 4-5 acre
grid was initially used. These less dense sampling grids seldom represent actual field
fertility levels. They are useful to demonstrate the variability of nutrients within
fields, but relying on the maps from these grids to accurately direct fertilizer
applications give only small improvements over a composite test if any improvements at
all. However, a one sample per acre approach for N sampling in North Dakota is not
reasonable and too expensive for anyone to use.
Zone sampling is sampling which is based on the idea that soil fertility patterns are
related to some logical reason. North Dakota research has shown that N levels are often
related to landscape patterns. Sampling based on landscape, using only 4 to 6 samples in a
40 acre field is similar or superior to patterns represented by a one sample per acre grid
(36 samples/40 acres). This finding means that sampling can be conducted relatively
cheaply and that sampling can be economical in wheat country and not confined only to
high-value crops like sugarbeets or potatoes.
Each sample in the landscape zone is a composite of 8 to 12 cores, not the 20 to 30
cores required for a composite sample. So the time taken to sample a field in a zone basis
will be comparable to that required if the field was sampled by composite, with the main
difference being that a few more lab analysis costs will be incurred. Zone sampling should
be directed by topography, but other information, such as yield monitor data, electrical
conductivity data, aerial photographs of bare soil/growing crops or satellite imagery,
should be considered along with landscape data to help define areas where hilltops end and
slopes begin and where slopes end and depressions begin. Zone sampling is especially
useful for N but has also been useful for P, soil pH, zinc, copper, sulfur and chloride.
Doing something about it
Variable rate controllers are available for whatever input needs to be varied on a farm.
Liquid materials, dry materials, anhydrous ammonia, seed, ag chemicals and starter
fertilizers can all be varied with any number of pieces of equipment. There are floater
rigs that can vary as many as five items simultaneously (Figure 3), or systems that cost
less that can fit on an existing floater to vary one input. Air-seeders (Figure 4) and
anhydrous ammonia rigs (Figure 5) have variable-rate equipment options. Seeders and
planters have variable-rate attachments. Controllers can be used to vary chemical rates
for herbicides, insecticides and fungicides. Some controllers can stay in a tractor cab
and can be used to control several different input rigs. Others are specific for a piece
of equipment. Good advice would be to have a unit that would be flexible for use in future
needs and serviced by a company that is likely to be around for a long time. A local
supplier available for servicing would also be helpful.
Figure 3. Pneumatic applicator
(27KB color photo)
Figure 4. Air-seeder
(26KB color photo)
Figure 5. Anhydrous ammonia applicator (34KB color
photo)
Managing the data
Data management and computer operation will be challenging for many producers who
already manage many parts of their farming operation. Teaming up with a local supplier
with these skills or a consultant that provides archiving and data management services
would be very helpful for many producers. There should be a clear understanding between
client and provider regarding who owns the data and how the data could be used. Many
growers would not want geographically addressed data going to neighbors, competitors, or
members of governing institutions without their permission. These issues should be agreed
to up-front.
The benefits
Site-specific farming allows producers to take charge of many aspects of production
that have been assumed to be acts of nature in the past. It makes producers curious about
problems in the field and gives them tools to correct them effectively. It provides a
means to test management and measure differences using yield monitors in a way not
possible before. Some will benefit from variable-rate application. Some will benefit from
changes in management. Others will benefit by archiving management decisions and outcomes
and being able to pass the information down to heirs, buyers or renters. The public will
benefit from improved efficiency of inputs and a reduction of nutrients and chemicals in
the environment.
Site-specific farming need not be prohibitively expensive. Sampling strategies have
been found to be reasonable in cost and high in information. Equipment and computer costs
have been greatly reduced. The system is for all North Dakota farmers, not just those with
high per-acre returns.
SF-1176 (1), June 1999
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