Allan Cattanach, Dave Franzen, Harold Stanislowski, and Larry Sax
Extension Sugarbeet Specialist,
North Dakota State University, University of Minnesota, Fargo, North Dakota
Extension Soil Specialist, North Dakota State University, Fargo, North Dakota
West Otter Tail County Extension Agent, Fergus Falls, Minnesota and Centrol, Inc., Morris, Minnesota
Sugarbeet grower adoption of the practices of grid soil sampling and testing and variable rate fertilizer application have increased dramatically since 1993. A few growers used these practices on a few hundred acres of sugarbeets in 1993. The sugarbeet production practice survey data presented in another paper in this publication indicates 25% of all sugarbeet acreage to be planted in 1996 was grid sampled in the fall of 1995. Nearly 40% of the acreage in some counties was grid sampled. Grower acceptance of these practices is progressing more rapidly than the research data base required to guide use of the technology.
Objectives of the study were to:
1) determine field variability in level of nitrate-nitrogen, phosphorus, and potassium;
2) compare variable rate application versus conventional fertilizer application for effects on sugarbeet yield and quality and;
3) determine soil nutrient status following sugarbeet harvest to ascertain if, how, and why variability occurs and changes.
A sugarbeet field on the Don Bradow farm near Foxhome, Minnesota was selected for grid soil sampling in the fall of 1994. The field had been planted to small grains in 1994. Nitrate-nitrogen content was determined at the 0-6", 6-24", and 24-48" depths. Other nutrient levels were determined on the 0-6" depth samples. Yield and quality comparisons were made on grids about 3.5 acres in size. Six soil cores were taken from each grid. Mapping was done by Centrol, Inc., Morris, Minnesota by Mr. Larry Sax. Soil samples were analyzed by Agvise, Inc., Northwood, North Dakota. Six grids, the length of the field were fertilized by conventional application techniques and six by variable rate application. The field was planted the last week in April to a Minn-Dak cooperative approved variety. A plant population of about 175 plants/100 foot of 30" row was achieved.
The experiment was harvested on October 13, 1995. A total of 80 quality samples were taken from the 13 truck loads of beets harvested on the conventional and variable fertilizer application grids. Two quality samples per grid on each harvest pass were taken as lifting proceeded to enable quality variability across grids to be determined. All loads were delivered to the same Minn-Dak piling ground for truck weight determinations. Yields were determined only for grids from area 3 and 4 of the field, Table 1.
RESULTS AND DISCUSSION:
Grid soil sampling gave a much more detailed determination of soil nitrogen status of the field. The part of the field conventionally soil sampled had 74 lbs. of nitrogen per acre in a 4' depth soil sample. Table 1 shows the fall 1994 soil test nitrate-nitrogen level for the field.
The levels of available nitrate-nitrogen by grid are presented in Table 2. Available nitrogen ranged from a low of 34 to a high of 146 lb/A. This compared to 74 lb/A available from area 3 that was conventionally fertilized. Table 3 shows the nitrogen recommended by grid.
All 6 grids of area three were treated as the conventional application check and they received 105 lb/A urea or 48 lb. of N. Thus three grids were over fertilized by 48, 23, and 48 lbs. of N and three grids were under fertilized by 29, 38, and 11 lbs.
The effect of type of fertilizer application technology used on sugarbeet yield and quality is shown in Table 4.
The grid sampled and variable rate application of nitrogen in this comparison resulted in a $74/acre increase in gross revenue. The cost of grid sampling, soil testing, and variable rate fertilizer application was assumed to be about $23/acre for this study with a $51/acre net return.
Figure 1 shows the variability in soil nitrate test level, nitrogen fertilizer application rate, and beet sucrose percent. Sucrose concentration tended to be higher and less variability was apparent where variable rate fertilizer application was used.
After harvest of the 1995 sugarbeet crop the field was again soil sampled on the same grid basis as used in the fall of 1994. The grid sampling more accurately reflected soil nutrient levels following sugarbeet harvest, Table 5. Seventy eight percent of all deep (2-4') nitrogen was used by the 1995 crop, Table 6. Seventy percent of the field contained 31 lb/A or less nitrate-nitrogen after harvest in the top 4' of the soil profile.
Phosphorus, and potassium soil test levels are shown in Tables 7 & 8. Values are given for grid tests completed in the fall of 1994 and after sugarbeet harvest in 1995 for each nutrient. Phosphorus soil test values changed little, although they increased in all grids but one. Potassium soil test values varied greatly from fall 1994 to fall 1995.
1995 Sugarbeet Research and Extension Reports. Volume 26, pages 105-112.