Predicting Organic Matter Zones

Southern Minn Co-op Develops Model to Assist Nitrogen Management & Boost Profitability

By Chris Dunsmore, Jody Steffel & Mark Bredehoeft*

    Can the level of organic matter (OM) influence sugar percent and purity in one’s sugarbeet crop?  And, assuming it can, how might organic matter zones be mapped in order to allow growers to appropriately modify nitrogen application rates to take advantage of this relationship?

    Those were core questions raised by Southern Minnesota Beet Sugar Cooperative (SMBSC) ag research staff several years ago.  As in many other areas, organic matter levels vary significantly in the Southern Minn growing district. 

    Research conducted in cooperation with University of Minnesota-St. Paul soil scientist John Lamb during 2003-05 documented the effect that OM had on both sugar percent and purity.

    Another study was initiated in 2006 to determine the influence that organic matter has on nitrogen mineralization — and to learn whether OM could be successfully predicted across the Southern Minn growing area.  That research, done in cooperation with John Lamb and Albert Sims of the UM Northwest Research & Outreach Center at Crookston, showed that nitrogen mineralized throughout the growing season does indeed have a good correlation with organic matter level.

    In that study, Landsat 5 satellite imagery of bare soil was used to aid in the development of a model for prediction of variation in organic matter within a given field.  The premise was that the color of the soil on a greyscale image would correlate to OM.  Wavelength bands with the highest correlations to actual OM were used for the model; and, since OM tends to follow elevation, that element also was added to the model.**

    So the completed model utilizes elevation data along with three different wavelength bands and correction factors.  Mapping software was used to come up with a predicted OM map using the model.  It includes a maximum of five zones, numbered “2” through “6.”  (The predicted zone number, it should be emphasized, identifies similar OM zones within the field; it does not predict the actual organic matter level.)  Soil samples were compared as a means of testing the model’s accuracy for prediction of OM zones.

    A pilot program was then designed to test whether the OM model would influence sugarbeet yield and quality on a whole-field basis, with the program put into action during the 2010 Southern Minn growing season.  Seven fields were selected for the test, with each organic matter zone’s soil sampled to a 48-inch depth, and nitrogen then adjusted to a given level to compensate for predicted OM mineralization.  For example, if the OM ranged from zero to 3%, N was adjusted to 120 pounds/acre; if OM was 3-4%, N was adjusted to 110 pounds; 4-5% OM, 100 pounds of N; 5-7% OM, 90 pounds; and finally, for organic matter soils above 7%, the nitrogen was adjusted down to 70 pounds.

    Within each field, one test strip using grid sampling technology and a second strip using conventional sampling were added in order to compare the zone program to these different soil sampling methods.  Total N to the four-foot depth averaged 51 pounds in zones 3-5 and 321 pounds in zone 6.

    A total of 406 samples were collected from six fields in the 2010 study year and analyzed at the SMBSC quality lab.  In the first analysis, beet samples from OM zones adjacent to the test strips were used to compare zone, grid and conventional fertility management.  In four of the six fields, sugar content was higher with the zones.  Five of the six fields had higher purity with the zones, while three had higher tonnage.  Five of the six fields had higher net revenue with the OM zone measurement compared to grid or conventional.  (Zone 6 was not included in the analysis since its nitrogen level was very high, so the N could not be managed based on organic matter.)

    It was concluded that zone data needed to be “weighted” in order to equalize the data across each zone.  For instance, if one zone covered 30 acres and another just five acres, the larger zone would have a greater influence on the mean — and thus would bias the data toward that zone’s reading. 

    The bottom line on the 2010 data was as follows:

• Sugar in the OM zones increased 0.1% over the grid method and 0.7% over conventional sampling.
• Purity in the OM zones increased 0.3% versus the grid method and 0.9% compared to conventional.
• Tonnage per acre was 0.8 ton higher in OM zones over grid and 1.2 tons better than the conventional.
• Per-acre net revenue in the OM zones was $69.81 more than with the grid method and $78.55 better than the conventional sampling method.

    The tests were repeated in 2011, but results were inconclusive due to the type of growing season experienced in the Southern Minn area.  Delayed planting, heavy spring rains and an abnormally hot and dry summer and autumn combined to produce a greater impact on the final results than did the N treatments, making it impossible to sort out what impact the N management program had on final crop yield and quality.

    The plan for 2012 calls for seven sugarbeet fields to be managed using this approach.  Testing also will be conducted in fields planted to corn whose nitrogen will be managed using this program.  Another related study will look at whether plant populations by OM zone can enhance whole-field production.

    Data to date suggest that the organic matter zone program should be successful under most growing conditions. 
    Since the objective is to enhance the grower’s profitability by optimizing production of all crops in the rotation, it’s important to engage other parties who play a role in this effort: crop consultants, retailers and advisors for the other crops, not just for sugarbeets.

    An organic matter mapping system is now available to soil sample contractors (consultants) in the Southern Minn region via the SMBSC website.  This system has been patented by SMBSC.  Consultants who have been approved for access to the site can select a targeted field.  The consultant will then be able to observe the number of zones within that field — and the acreage per zone — to determine whether they wish to purchase the selected field.  (For 2012, fields being used for sugarbeet production this year can be purchased at no charge.)

    The above image provides an example of a purchased map.  The download options for the OM zone maps include a shape-file, geo-referenced bmp or tiff, or pdf.  Layers of information (such as yield and Veris maps) can be overlaid to enhance the organic matter map.

    At present, the geographic area available for organic matter maps is determined by the Landsat image used during the project’s development.  It covers the bulk of the SMBSC growing area.  Area expansion will be considered as the program is refined and grower needs are further assessed.

* Chris Dunsmore, Jody Steffel and Mark Bredehoeft are research assistant, statistical assistant and research agronomist, respectively, with Southern Minnesota Beet Sugar Cooperative, Renville.

** Dan Humberg, South Dakota State University, assisted with the employment of the Landsat 5 satellite imagery.  Richard Horsley, North Dakota State University, conducted analysis of the data and produced an algorithm used to define the organic matter zones.