EFFECT OF GRID SOIL TESTING AND VARIABLE RATE FERTILIZER APPLICATION ON SUGARBEET YIELD AND QUALITY

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.

OBJECTIVE:

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.

PROCEDURE:

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.

SUMMARY:

  1. Grid soil testing better represented the soil nutrient status of the field.
  2. Production of a 20 Ton/A sugarbeet crop of reasonably good quality was achieved with conventional soil sampling and testing. Results of this study confirm the overall reliability and validity of the current North Dakota State University and University of Minnesota soil testing recommendations for sugarbeets.
  3. Conventional soil sampling over fertilized 50% of the six conventionally fertilized grids and under fertilized 50% of them.
  4. The 1995 crop very efficiently used deep soil nitrate-nitrogen (2-4').
  5. The topography at the Bradow site has much more variation in elevation than most of the Red River Valley. However, crop responses still very closely paralleled the 1994 and 1995 research results of Dr. Larry Smith at the Northwest Experiment Station at the University of Minnesota, Crookston, Minnesota.
  6. The limited phosphorus and potassium data obtained from this study indicates less correlation between P & K test levels and sugarbeet crop yields than are observed with nitrogen. Growers considering grid soil sampling and testing might best consider only testing and fertilizing with nitrogen at this time, especially if P & K test levels are reasonably high. However, a full soil nutrient content analysis, as well as pH, organic matter, soluble salts, and other measurements will establish baseline data for a field and may indicate field areas of concern.

ACKNOWLEDGEMENTS:

  1. Thanks is expressed to the Sugarbeet Research and Education Board of Minnesota and North Dakota for partial financial support of this project.
  2. A special thanks to Larry Sax, Centrol, Morris, Minnesota for his grid sampling assistance.
  3. Jeff Loeks, and Tom Knudsen, Minn-Dak Coop. agronomists, Wahpeton, North Dakota, provided much worthwhile assistance.
  4. Great appreciation is extended to Don and Gary Bradow and their harvest crews for patience with project researchers under difficult harvesting conditions.

1995 Sugarbeet Research and Extension Reports. Volume 26, pages 105-112.


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