When I was hired by NDSU in my current position as an Extension Soil Specialist, I joined the Soils Department in spite of my experience in precision nutrient management, corn and soybean production, and tile drainage. Now, most of my presentations deal with all of these topics.
My experience in the Illinois private fertilizer industry and in my PhD work consisted of grid sampling for P, K and soil pH. In Illinois, grid sampling often is the best approach for P, due to the high rates of P applied in fields since the late 1960’s. There is most often great variability in soil P levels, but it is mostly in the high range. For K and soil pH, grid sampling may be appropriate, but my work with Ted Peck’s (PhD) intensive grid sampling project showed that a zone approach may provide better information to direct a lime application or a K application (see link to Illinois grid technical bulletin). A zone approach for these two nutrient factors is also supported by Antonio Mallarino’s work in Iowa (Mallarino and Wittry, 2004, Agronomy Journal). To be effective when soil P, K levels are not all high, grid sampling should be conducted using at least 1 sample per acre grid density. Most growers are reluctant to use this intensity even using 0-6 inch depth sample cores.
In North Dakota, my first research projects were intensive grid sampling projects, using the same fields for more than one year. Since most soil sampling conducted in North Dakota is primarily for residual fall soil nitrate, I assumed that I would find that a grid sampling density of 1 sample per acre would be required (which it is), and that site-specific nutrient management in the region would be impractical. However, the second year sampling revealed that residual soil nitrate patterns were similar to the first year. That finding indicated that soil nitrate patterns were not random and that they were present due to some logical reason. Residual soil nitrate patterns could be defined not only with very intensive grid sampling, but using easily obtained information to define the pattern, and then sample modestly within the patterns (zones) to define the nitrate value. Zone sampling was therefore presented as the preferred method for soil sampling within the state.
Zone may be defined using at least two zone delineation tools. These tools could be:
Topography, not elevation, but the shape of the landscape (ridge top, shoulder slope, foot slope,depression).
Aerial imagery- color or some form of NDVI (normalized differential vegetative index)
Satellite imagery- not necessarily the finest resolution availableElectrical conductivity, such as the output of the Veris EC sensor
Electromagnetic sensor output, such as the EM-38 (Geonics, Ltd, Ontario, Canada)
Foliage NDVI sensing with active optical sensors (GreenSeeker/Holland Crop Circle)
Multi-year combine yield maps (See NDSU Ext. Circ. 1176-3 for more details)
See NDSU Ext. Circ. 1176-2 for more details.
Notice that soil survey maps are not on the zone delineation list. Although some areas near the Red River Valley are ‘fortunate’ to be mapped with ancient beach ridges defined, and the zone maps follow delineation of these well-defined features, most fields within the Red River Valley, the glacial till plain and fields west-river are not so well-defined. Soil survey maps only delineate features that are at least 2.5 acres or more in size, and soil type boundaries as depicted in these maps do not represent boundaries of nutrient management zones. A detailed discussion of this topic can be found in Franzen et al., 2002 (link to journal article).
Economic and environmental rewards are often possible using precision nutrient management. See NDSU Ext. Circ. 1176-4 for more details. In addition-
-An archive of spatial yield data that can be a valuable record, but can also be used to delineate production management zones.
-An understanding of the production of certain soil types under your management, which leads to an understanding of the productivity of neighboring farms under consideration for purchase or for rent.
-A record both in time and space of the application of fertilizers or pesticides, that might be useful in litigation or in environmental enforcement.
Zone sampling is now the primary precision nutrient management strategy in North Dakota. An exception to this is the corn and soybean growing area in SE North Dakota. I think that growers and farm fertilizer suppliers in the area think they are Iowa because they grow corn and soybeans. I can safely assure them that they are not Iowa. In this area, grid sampling in a 2.5 acre grid for soil P and K is routinely conducted, with soil nitrate not often included. This strategy is ill-conceived unless soil P and K levels are all in the high range. Most P levels, and in many fields, K levels are in the low to medium range, and their levels are defined by topography and natural factors and not by high P and K fertilization rates. Growers often call me to relate that their P levels are still low despite their ‘high’ P fertilizer rates. Rates they cite seldom exceed 150 pounds per acre of 11-52-0. When I was working with central Illinois growers, common P rates were 250-300 pounds per acre, and some growers who started farming a piece of land for the first time would apply up to 800 pounds per acre of 18-46-0 the first year. There are few North Dakota growers who have ever applied 250 pounds per acre of 11-52-0 to any crop, with the exception perhaps of potatoes. The 2.5 acres per grid strategy in Illinois/Iowa is acceptable because all the variability of P and K is in the high range. When it is not, it is necessary to go to a 1 sample per acre grid strategy, or better yet, zone the field. There is no one in the world with more grid sampling history than I have, so take this to the bank.
The new corn N recommendation strongly urge side-dress N application on N-loss prone soils in addition to enough preplant N to keep the corn healthy until side-dress time (V4-V8). To better direct a side-dress N rate, the use of active-optical sensor algorithms for on-the-go N application is recommended. At preplant, a small area with a high N application rate within soil category within intended variety is applied. At side-dress, the applicator enters the field with the proper side-dress algorithm programmed into the rate controller, and the sensor in front of the applicator. The applicator first senses the N non-limiting area, which then using the algorithm predicts the highest yield possible with any N rate. If areas of the field read within 5% of the reading of the non-limiting area, no N is applied. If the field areas read lower than the 5% difference, the controller calculates how much N is required to achieve the higher yield with consideration of an N application efficiency factor and directs the N application to the area. Using one sensor on the front of the applicator, these rates are applied to applicator width the distance from the sensor to the applicator boom/gang, in blocks as the applicator progresses through the field. More details on this side-dress method can be found in NDSU Ext. Circ. 1176-5.