Nitrogen management is a critical factor in the production of a high quality and profitable sugar beet crop. Applying more N than recommended reduces beet quality and net income. New N guidelines this past year have reduced the N guidelines to a total of 100 lbs N/acre (for a 24″ deep soil sample) and 130 lbs N/acre (for a 24″ plus a 42″ or 48″ depth soil sample). These N guidelines are recommended for sugar beet production in the Red River Valley (American Crystal Sugar Co. and Minn-Dak Farmers Coop) and southern Minnesota (Southern Minnesota Beet Sugar Coop).
The following information was collected from soil analysis in the Southern Minnesota Beet Sugar Coop area in the fall of 2002. Widespread and intensive soil sampling was highly encouraged and supported by SMBSC. Local crop consultants and agronomists collected soil samples for this project. About 75% of the fields sampled were tested by AGVISE Laboratories in Benson, MN with 90% of the samples being conventional field composites and 10% being sampled either on 4.4-acre grids or zones of 10-40 acres. 93% of the fields sampled were corn in 2002 and 95% of the fields sampled were deep sampled (either 36″, 42″ or 48″ in depth).
If the composite sample is high in N, what does that mean?
Since sugar beet quality is negatively correlated with excessive nitrogen fertility, it is very important to know the soil test nitrogen (STN) levels for each field and manage nitrogen inputs accordingly. Of the conventionally sampled fields in this project, 70% had STN values greater than 70 lbs N/acre, 50% had STN greater than 85 lbs N/acre and 40% had STN greater than 100 lbs N/acre. In general, as the STN increases, so does the in-field variability for STN. You can see this trend when you look at the table below. This table looks at the STN variability within fields that have a composite soil test level in different ranges for the 54 grid sampled fields. In the table, you can see that fields testing low in average STN have smaller N ranges, whereas fields that have higher average STN levels have a wider range and are more variable for STN. With this information, growers can make better decisions on their fertility management. Conventional composite soil test results can be used as a screening tool to identify which fields would be good candidates for grid sampling and variable rate N management. If a field has a high STN level (>70 lb/a), there is a much better chance that a variable rate N program will be profitable on that field. If a field has a composite STN level less than 70 lbs N/acre, a grower may not want to grid sample that field. If you look at the percentage of each field that would be over or under fertilized, based on the composite STN levels in the table, you can see that the higher the composite STN level, the more likely large parts of the field get the wrong N rate. For example, fields with a composite STN level of 40-50 lbs would have 20% of acres under fertilized, 66% adequately fertilized and 14% over fertilized, on average. Fields testing from 80-89 lbs would have 41% of acres under fertilized, 28% with adequate fertilizer and 30% of the acres over fertilized. As the composite STN level gets higher, a variable rate N application is much more likely to provide a higher quality beet crop and be much more profitable on that field. Soil sampling fields first with a composite sample right after corn harvest is a good screening tool to help identify these fields. These fields can then be grid or zone sampled so that variable rate N can be applied to increase crop quality and profitability of the growers. If you have any questions on the data collected in this project, please give our staff a call.
|Soil test nitrogen variablity in 54 grid sampled
fields for SMBSC, Fall 2002
|1. Composite field soil test N is the average N of al grid points added together.|
|2. Average oil test N range from lowest testing grid to highest testing grid for fields in this N range.|
|3. Percent of filed considered to be under fertilized.|
|4. Percent of field considered to be adequately fertilized (+/- 15 lbs N/acre).|
|5. Percent of field considered to be over fertilized.|