WHY GUESS - SOIL TEST

Ron Torkelson

Department of Soils

North Dakota State University

Profitable sugarbeet production requires a high tonnage crop containing a high percentage of sucrose. To accomplish this, growth limiting factors, including soil fertility, must be handled effectively.

Nitrogen

It has long been recognized that sugarbeets will respond favorably to commercial fertilizer. Before we know how much nitrogen to apply to a field that is going to be seeded to beets, it is essential that we know how much is needed to satisfy the requirements of a beet crop. A general rule of thumb that is often suggested states that 10 pounds of nitrogen is needed for each ton of beets produced per acre. On this basis, a 20 ton beet crop in the Red River Valley would require 200 pounds of nitrogen per acre two feet. (Most of the soil nitrogen is located and consequently utilized from the top two feet of soil). The
beet utilizes almost exclusively the nitrate form of nitrogen (NO3-) and relies on three sources for this nitrogen; (1) nitrogen mineralized or released during the growing season through the decomposition of soil organic matter, (2) nitrate-nitrogen present in the soil at or near planting time, and (3) inorganic nitrogen applied as a commercial fertilizer.

Sugarbeets are unique in their requirement for nitrogen. Too little nitrogen results in premature yellowing and reduced yields while too much nitrogen leads to reduced sucrose percent accompanied by a high concentration of impurities. To assure that the soil has adequate nitrogen (200 lbs/acre) to meet the requirements of a 20 ton beet crop, we assume that through organic matter decomposition approximately 50 pounds of nitrate-nitrogen will become available during an average growing season in the Valley. The remaining 150 pounds per acre two feet must be supplied by soil nitrates, as determined by a soil test from a soil testing laboratory, and fertilizer nitrogen. Therefore, to answer the question, "how much fertilizer nitrogen do I apply", a reliable soil test and recommendation is essential.

It is accepted that summer fallowing in a crop rotation accumulates nitrate-nitrogen. It is not well understood how much will accumulate in any particular year, and therefore, predictability of soil nitrate levels based on management practices alone leave something to be desired. NDSU soil test data (see Table 1) indicate that 250 fields being planted to sugarbeets in 1972 have been soil tested for nitrate nitrogen. Of the 250 fields, 84 or 35 percent were cropped in 1971. The number of sugarbeets going on old cropland is increasing and a continued increase is anticipated. The data in Table 1 further points out that:

1. 81% of the non-fallow fields tested less than the required 150 lbs/acre 2 feet and received a
nitrogen recommendation.

2. 44% of the fields summer fallowed in 1971 tested less than 150 lbs and received a nitrogen
recommendation.
To further emphasize the importance of a reliable soil test, selected soil test data and fertilizer recommendations compiled from grower submitted samples are found in Table 3 on the following page. The purpose of these recommendations is to illustrate that there are fields with high levels of soil nitrogen after being cropped that did not receive a nitrogen recommendation for sugarbeets and there are also summerfallow fields with low levels of soil nitrate that received a recommendation.
Nitrogen recommendations are based only on a 24 inch sampling depth. For sampling depths less than 24 inches, a general recommendation in Table 2 can be followed by the grower. This recommendation will be high for some fields, low for some, and "in the ball park" for others.
Prior to the 1970 growing season it was recommended that no more than 10 pounds of N+K20 be applied with the seed. Recent research conducted in Alberta, Canada, as well as similar work conducted in other parts of the United States, indicates that even small amounts of nitrogen fertilizer can result in germination damage. In 1968, G. M. Guccione of Manitoba Sugar Co. Ltd. found that 9 1/4. pounds of nitrogen per acre placed with the seed reduced the number of seedlings from 140 to 103. Therefore, at the present time it is recommended that all nitrogen fertilizer be broadcast.

Phosphorus

Red River Valley soils are inherently low in phosphorus but through continuous heavy applications of phosphatic fertilizers to sugarbeet ground, the P levels of many fields have increased. Table 4 shows that approximately 35 percent of the sugarbeet fields for 1972 tested very high and did not receive are commendation. Phosphorus fertilizer is essential for early growth and is not as detrimental to seed germination as nitrogen. It is therefore suggested that even though most recommendations (drill row) are doubled and broadcast, some of the phosphorus should be placed with the seed. If broadcasting phosphorus, Canadian work indicates that a yield response may result with a fall application and incorporation compared with a similar spring application. Other work conducted in Canada shows that 10 pounds of phosphorus applied with the seed may produce a considerable increase in the vigor of seedlings, particularly those planted early. This could prove significant in years when a killing frost occurs after emergence.

Potassium

Some of the coarse and moderately coarse textured soils in North Dakota and western Minnesota may show potassium deficiencies that would require supplemental potassium as fertilizer but in general the levels are quite adequate. Soil test data in Table 5 however, indicates that approximately 40% of the fields being planted to sugarbeets in 1972 received a potassium recommendation. Potassium, like nitrogen can have adverse effects on germination and emergence of seedlings. Therefore it is suggested that all potassium be broadcast.

Micronutrients

For the most part, soils on which sugarbeets are produced in the Red River Valley are adequately supplied with the essential micronutrients. However, zinc (Zn) responses to application of zinc fertilizer shave been observed on some fields. It is suggested that zinc fertilizer be applied only on a trial basis to evaluate the need for zinc as determined by yield responses.

Suggested rates of zinc fertilizer on areas where zinc deficiency is known to exist are 10 to 20 pounds per acre zinc (Zn) from- zinc sulfate or 2 to 4 pounds per acre zinc (Zn) from a zinc chelate. Both broadcasting and banding of zinc can be expected to be equally effective. In most cases zinc sulfate has been as effective as zinc chelates in correcting deficiency problems.

Fertility Research
The fertility studies conducted in 1968, '69, '70, '71 and the work underway for 1972 are all designed to strengthen the reliability of the present soil test-fertilizer recommendation program.

Work in 1971 was conducted at three sites - Casselton, Auburn, and at St. Thomas, North Dakota. Site characteristics, treatments and yield data for the respective plots are found in Tables 6, 7 and 8 on the following pages. Nitrogen was spring applied as ammonium nitrate (34-0-0), phosphorus as superphosphate (0-46-0), potassium as muriate of potash (0-0-62), and all rates were incorporated.

It is no secret that 1971 was an unusual growing season. Many growers and researchers alike watched timely precipitation in many areas prevent severe moisture stress throughout the growing period. Probably most important was the late season moisture that increased some yields considerably. Sugarbeets at the Auburn and particularly at the Casselton plots showed an early response to nitrogen that was eliminated by such late season moisture. Some of the low testing nitrogen plots as well as some grower fields that contained less than 150 pounds of fertilizer plus soil-nitrogen yielded exceptionally well. Under optimum growing conditions, it is quite possible that a substantial amount of nitrogen mineralized from organic matter in addition to the estimated 50 pounds is responsible for some of the yield. Late season yield increases also result from a release of immobilized nitrogen from dry surface soils that receive significant amounts of precipitation. A good distribution of nitrogen at greater soil depths could therefore become very important during lengthy, precipitation-free periods in the latter part of the growing season. In most years however, the probability of receiving sufficient late season precipitation for substantial beet yield increases is low. It is therefore very important that soil nitrogen levels are adequate during early and mid-season growth when the soil moisture status is much more favorable.
The effect of over fertilization on sugarbeet quality is always a question. At St. Thomas, such a study(Table 8) shows that over fertilization with nitrogen can be much more degrading to beet quality than over fertilization with potassium. At a soil test nitrate level of 190 pounds per acre 24 inches, the amino N (ppm) at the 200 pound rate lowers beet quality much greater than the 800 pound potassium rate. The impurity index which considers both N (amine) and K as impurities and is a measure of beet quality, is 1119 for the 200 pound N rate and 857 for the 800 pound K20 rate. (The lower the index, the more white sugar that is extractable, and the higher the beet quality.) It is very desirable to produce a high tonnage of high sucrose beets with an impurity index less than 1000. Tonnage and impurity index increases and sucrose percent decreases when the soil nitrogen levels are more than adequate.
Factors Affecting Fertilizer Recommendation
Fertilizer recommendations at the present time are based strictly on a soil test. Personal observations made in research plots and grower fields during the 1971 growing season indicated that:
1. Nitrogen rates may have to be adjusted for beet crops that growers consider early or late planted.
2. Grower beet populations are increasing and 85-90 beets/100 ft. of row may require more nitrogen
than 70 beets/100 ft. of row.
3. Nitrogen distribution to depths of 24 inches under cropped land is usually less uniform than
under fallow and may require an additional "shot" of nitrogen or deeper incorporation.
Work conducted by Dr. W. C. Dahnke, NDSU, indicates that:
4. A substantial amount of nitrate-nitrogen may be mineralized and will accumulate over the winter period suggesting a need for two recommendations--one based on fall sampling and one on spring sampling just prior to seeding.

The above observations may all be important factors to consider when arriving at the proper rate of fertilizer application. Work is being done in some of these areas and other areas need a much closer look. Never-the-less, these are practices that growers, fieldmen, county agents, researchers, and others associated with sugarbeet production in the Red River Valley should be giving some thought to, particularly when making their observations.


1971 Sugarbeet Research and Extension Reports. Volume 2, pages 32 - 39.


Red River Trade Corridor
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