SUMMARY OF SITE-SPECIFIC SAMPLING RESEARCH 1994-1998
D.W. Franzen and V.L. Hofman
NDSU Extension Soil Specialist
and
Extension Ag and Biosystems Engineering
In 1994, commercial sampling for variable-rate fertilizer application began in the Red
River Valley for sugarbeet production. Variable-rate P, K and limestone application in the
central Corn Belt was directed by grid sampling, a systematic method designed to eliminate
sampling bias. Field nutrient patterns are formed from analysis of the grid samples. The
density of grid sampling influences how closely field nutrient maps reflect actual levels.
Research in Illinois and Wisconsin suggested that grids be about one sample per acre ( a
200-220 foot grid), which is reasonable in these areas considering sampling is conducted
only once every four years with a hand probe and an ATV or unmodified pickup truck.
However, sampling for NO3-N to 4 feet in depth for sugarbeet requires much more time and
much higher equipment costs than surface P, K and pH sampling. Sampling must also occur
each year for N responsive crops (sugarbeet, wheat, barley, canola, sunflower, corn,
potatoes).
Based on economics alone, samplers in the Red River Valley began to sample in 5 acre
grids. At that time, samplers were told that this method would reveal some of the nutrient
variability within fields, but the grids were not tight enough to consistently show actual
field patterns to direct variable-rate application long-term. Samplers were asked to be
flexible enough that following sufficient sampling research, adjustments may have to be
made in sampling strategies. Based on information from these studies, many samplers are
already modifying their earlier sampling methods.
METHODS
Sites were established near Gardner from 1994-96, Colfax from 1995-98, east of Hunter from
1997-98, and sister sites were located outside the Red River Valley near Valley City and
Mandan. All sites were densely gridded, with cores to 4 feet taken when possible in the
Valley locations and to 2 feet outside the Valley, or when the subsoil was so saturated
with water that deep sampling was impossible (Hunter site, 1998).
Grid samples were collected in a 110 foot grid (1/4 acre), which helped reveal underlying
fertility patterns within the fields for comparisons with less dense sampling methods.
Grain yields were collected at Colfax, to determine profitability in various crops such as
corn and wheat from variable-rate application of N. At many of the sites, additional
samples were collected in 10 foot and 2 foot grids to determine how many sample cores
might be needed to represent each grid or zone. The methods for comparison of grids and
zones have been given in previous Sugarbeet Research and Extension Reports from 1995-1997.
Topography was measured in each field using a laser-surveying device with recording taken
in a 110 foot grid.
RESULTS
Colfax
Soils
The Colfax site as described by the Richland county soil survey consists of
Tiffany/Embden and Glyndon/Wyndmere soils, with the Glyndon/Wyndmere soils in the east 2/3
and Tiffany/Embden in the west.
Figure 1. Soil series as described by soil survey
Figure 2. Colfax site as mapped by M.D. Sweeney, 1998
The Colfax site as described in a detailed soil survey conducted in 1998 consists of a
natural Tiffany drainage way bordering the east 1/3 of the field in a north-south
direction. This channel was widened to allow surface drainage of water just prior to the
1998 sampling. Some grading has been conducted in the past prior to 1995 in the same area.
To the east of the drainage way are Wyndmere and Hecla soils. To the west are Wyndmere and
soils, with some disturbed Maddock along the southwest corner.
Changes in NO3-N over time
The NO3-N patterns changed little between 1995 and 1998. The Tiffany soils shown in Figure
1 were consistently low each fall, probably as a result of denitrification from excessive
water during each summer. The Hecla soils and parts of the Wyndmere soils were higher in
N.
Figure 3. Colfax NO3-N levels, 0-2 feet, 1995-1998
In 1995, the field was in corn, in 1996 it was seeded to spring wheat, with soybeans
planted late into the Tiffany area because of early season wetness. In 1997, the field was
seeded to corn, and in 1998, the field was planted to soybeans. As each crop was
harvested, similar patterns of yield were revealed (Figure 4).
Figure 4. Wheat and corn yields, 1996 and 1997
The patterns of yield and nutrients corresponded to patterns of topography
(Figure 5.)
Figure 5. NO3-N, P and corn yield over topography
Grid and topography comparisons with 110 foot grid values
Area-based topography sampling represented 110 foot grid values similar to the 220 foot
grid in most years. Relatively low correlations of chloride would not cause concern,
considering that chloride ranged from 100 to 400 lb/acre in all study years. The
recommendation would be the same whether the value sampled was 100 or 400 lb/acre; no
additional chloride would be needed. Area-based topography correlation with P was probably
low in 1995 because the samples were obtained with an automatic probe under wet
conditions, which may have degraded the quality of the 0-6 inch sampling depth.
Table 1. Comparison of topography-based and grid sampling
strategies, Colfax, 1995-1998
The 220 foot grid tended to have a better correlation than area-based topography sampling,
however, it would take 36 samples to construct a map on the 40 acre field, compared to 5
samples for the area-based topography sampling. The tendency for N correlation to decrease
with time is probably a result of variable-rate N applications in 1996 and 1997. In 1996,
there was a significant increase in net return due to topography-based variable N and S
application, compared to both a uniform N application and grid sampling at a 440 ft. grid.
In 1997, there were no significant differences between treatments, partially due to
treatment overlap through a misapplication of the fertilizer on the west side of the farm,
and partially due to a generally low range in N values. Most sample N values were within
40 lb/acre of each other.
Gardner
The Gardner site was a 40 acre farm split into a north 15 acre field and a south 25 acre
field. The north 15 acres was in alfalfa from 1994-1996. During the winter of 1994-95,
some of the alfalfa suffered winter kill due to ice buildup. The stand continued to
decline until after the 1996, it was plowed under. The south field was in spring wheat in
1994, barley in 1995, and was seeded to alfalfa in 1996. Although the soil survey from
Cass county describes the field as one soil mapping unit, Fargo/Hegne, a detailed soil
survey conducted during 1998 shows more interesting features (Figure 6).
Figure 6. A detailed soil survey of the Gardner site. M.D. Sweeney,
1998
The Hegne soil tends to be a little higher in elevation than the Fargo, and the Enloe
areas are particularly poorly drained. The line of Enloe soils in the south field tend to
form a natural drain together from west to east, before reaching the man-made drain in the
east. A road borders the west boundary.
Change in nutrients over time
Patterns of NO3-N changed over time. N levels were spatially variable in the south field
in 1994, but not the north field where the vigorous alfalfa was growing. In 1995, both
fields were spatially variable, and N levels were more related to landscape than any other
year studied.
Figure 7. NO3-N levels, Gardner, 1994-1996
Figure 8. P levels, Gardner, 1994-1996
In 1996, following the new alfalfa seeding, and the continued decline of the alfalfa stand
in the north field, there was no spatial variability. This phenomena is very similar to
that following sugarbeet at the Hunter site from 1997-1998, and also has been noted
following sunflower at the Mandan site-specific farms from 1995-1998.
When N levels were spatially variable, high levels tended to be in the upland positions in
the south field, while high levels tended to be in the depressions in the alfalfa. In the
south fields, the depressions tended to be wet, but in the alfalfa field, even following
heavy rains, it was possible to drive in the field within 24 hours, indicating soil
moisture levels were relatively low. Denitrification was probably not proceeding at as
high a rate as in the south, annually cropped field.
The P levels varied between years. P levels were lower in 1995 than in 1994, then
recovered again in 1996. Patterns within fields were relatively similar, but the values
changed. Canadian researchers have also noted that during alfalfa degradation, P levels
decrease. It may be the case that our extracting solutions do not extract organic P very
well.
Table 2. Comparison of topography-based and grid sampling
strategies, Gardner, 1994-96
Area-based topography sampling was based on five sampling zones, compared to 36 samples
for the 220 ft. grid. Topography sampling, both area and point compared well to the 220
ft. grid.
The Gardner site was important not only because it supported topography-based zone
sampling to gather nutrient information meaningfully and cheaply, but also pointed out the
impact that certain crops may have in altering the spatial patterns of nutrients within
fields. A combination of soil sampling in the winter-killed areas, combined with
previously crop credits from the remaining areas of alfalfa would have been necessary to
describe the effects of the crop on subsequent crop N requirements.
HUNTER
The Hunter site is about 5 miles west of Gardner, and consists of Bearden and Perella
soils. Because of the wet weather in the fall of 1998, the field has not been subjected to
a detailed soil survey, but a survey is planned for spring or fall, whichever is most
practical. The wet weather also prevented the 2-4 foot depth sampling this fall. Only the
0-6 inch and 6-24 inch depths were taken. The field was in spring wheat in 1997 and
sugarbeet in 1998. The N level was spatially variable following spring wheat, but was not
spatially variable following sugarbeet.
The field slopes gently from west to east, with a few locally upland positions near the
field center. In 1997, N levels were highest along the east slopes of the central upland
positions and lowest near the center and in the southeast where water tends to stand
(Figure 8).
Figure 8. NO3-N levels to 4 feet draped over topography, 1997
The pattern of NO3-N completely changed following sugarbeet (Figure 9). Instead of high
areas of N in the central uplands, the high N remained on the south edge of the field, but
levels in other areas were low and uniform.
The difference in spatial variability between NO3-N levels following spring wheat and
sugarbeet is reflected in the correlation between sampling strategies and the 110 foot
grid. There was low correlation between the 110 foot N levels and sampling strategies in
1998, compared to higher correlation values in 1997 (Table 3). This is the same problem
observed at the Gardner site when the field was in a vigorous stand of alfalfa. Sugarbeet
and alfalfa are accumulating large amounts of soil N, and soil N levels alone may not be
sufficient to reveal the true N status of the field. Following alfalfa it has been common
to use a previous crop credit to reduce N recommendations below that directed solely by
soil test. Based on work by Moraghan and an ongoing study in Pembina county in a
sugarbeet/potato rotation, the level of N in sugarbeet tops is a much better predictor of
field N status than soil testing alone.
Figure 9. NO3-N levels draped over topography, 1998
Table 3. Correlation of grid and topography sampling strategies,
Hunter, 1997-98
One of the problems with topography sampling alone in determining management zones is
determining boundaries on slopes. Satellite images of the Hunter site will be made
available this winter to further define boundaries beyond estimates made from topography
data. When this is done, any adjustments in zone sampling compared to grids will be
reevaluated using both the 1997 and 1998 data.
SUMMARY
This study has shown that the relatively flat soils of the Red River Valley can be
extremely variable in nutrient levels. The variability may be associated with landscape
and/or the activities of man. If sampling by grid, the 220 foot grid established in the
central Corn Belt as the recommended grid strategy is valid here as well. However,
topography-based zone sampling was also shown to be as effective as the 220 foot grid in
representing field values and often superior to the 220 foot grid in fertility boundary
definition. A 4-5 acre grid is a generally poor method of directing variable-rate
fertilizer application, useful mostly in revealing some range of variability to a grower
new to site-specific farming. The topography-based zones require fewer samples and lower
sampling and analysis costs than grid, while providing a high level of quality information
to the producer. Sampling by zone should use several sources of information, including
aerial images of crops and bare soil, satellite imagery, topography, yield maps,
electrical conductivity detection, and detailed soil survey information (not published
survey information).
