Fertilizing Corn in Minnesota

George Rehm, Nutrient Management Specialist
Gyles Randall, Soil scientist, Southern Research and Outreach Center
John Lamb, Nutrient Management Specialist
Roger Eliason, Director, Soil Testing Laboratory

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In Minnesota, corn is grown on more acres than any other crop. Nationally, Minnesota ranks among the top five in production. Average corn yields have improved steadily over the past several decades. While general fertilizer use contributed substantially to yield increases in the past, total fertilizer management which optimizes nutrient efficiency will be needed to increase future production and profitability.

Nitrogen Suggestions

Minnesota corn growers receive substantial return for money invested in nitrogen (N) fertilizers. For many situations, the most profitable yield cannot be achieved unless N fertilizers are used.

There are many management decisions involved in the use of N fertilizers. The most important, however, is the selection of a N rate that will produce maximum profit while limiting the potential for environmental degradation. The choice of an appropriate rate of fertilizer N is not easy because of the transient nature of N in soils.

The productivity potential of the soil, the previous crop, the ratio of the price of N to the value of the crop, and risk assessment provide the basis for N rate guidelines. In western Minnesota, these guidelines should be adjusted for any residual nitrate-nitrogen measured.

Standard N Guidelines

The consideration of soil productivity, price/value ratio, and previous crop are used to arrive at the fertilizer N guidelines for corn. This represents a significant change in the process of determining the rate of N to use. This process is a product of a seven-state effort (Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio, and Wisconsin) to use a similar philosophy/approach for determining N rate guidelines for corn. The data used in the project was from research conducted since 1989 and involved over 700 locations in the corn belt.

Because of technology improvements in corn production practices such as weed and pest control, expected yield is not as important a factor in determining N rate as it has been in the past. Soil productivity has become a better indicator of N needs. A majority of Minnesota soils are highly productive and have generally produced maximum economic corn yield with similar N rates over the last 15 years. Some soils have a reduced potential attributed to erosion, reduced water holding capacity caused by lower organic matter content, sandy soil texture, poor drainage, and any restriction to root growth.

With the current volatility in energy costs, fertilizer N cost has risen dramatically in the past three years. This increase does affect the economic optimum N rate. To account for this change, the ratio of the price of N per pound to the value of a bushel of corn has been added to the N rate decision. An example calculation of the price/value ratio is if N fertilizer costs $0.30 per lb N or $492 per ton of anhydrous ammonia, and corn is valued at $2.00 per bushel, the ratio would be 0.30/2.00 = 0.15.

Once the soil productivity and price/value ratio have been determined, a producer’s attitude towards risk must be factored into the process. A producer who is risk adverse and cannot tolerate risk associated with less-than-maximum yields in some years even though economic return to N may not always be the greatest may want to use the N rates near the high end of the acceptable range shown in Table 1. On the other hand, if water quality concerns are an issue and/or localized N response data support lower N rates, producers may choose N rates near the low end of the acceptable range in Table 1 if they are willing to accept the possibility of less-than- maximum yield in some years without sacrificing profit. This acceptable range gives each producer flexibility in arriving at an acceptable and profitable N rate. The maximum return to N value (MRTN) shown in Table 1 is the N rate that maximizes profit to the producer based on the large number of experiments supporting these guidelines.

The N rate guidelines in Table 1 are used if corn is grown in rotation with soybean or following corn under high productivity conditions and guidelines in Table 2 are used under medium productivity conditions. Corn grown on sandy soils deserves special consideration. If irrigated, the guidelines listed in Table 1 are appropriate. For non-irrigated corn grown on sandy soils (loamy fine sands, sandy loams, loams) with more than 3% organic matter, use the guidelines given in Table 2. For non-irrigated corn grown on soils with a loamy fine sand texture and less than 3 % organic matter, use the guidelines provided in Table 3.

To arrive at a guideline following other crops, an adjustment (credit) is made to the corn following corn guidelines. The adjustments can be found in Table 4. In Table 4, several crops are divided into Group 1 and Group 2. The crops for each group are listed in Table 5.

Table 1. Guidelines for use of nitrogen fertilizer for corn grown on soils considered to be highly productive.

Table 2. Guidelines for use of nitrogen fertilizer for corn grown on soils considered to have medium productivity potential.

The N rates listed in Tables 1, 2, and 3 define the total amount of fertilizer N that should be applied. Any N applied in a starter fertilizer, weed and feed program, DAP (diammonium phosphate) or MAP (monoammonium phosphate) should be included in the calculation of the total amount of N applied during the growing season.

It’s generally accepted that legume crops provide N to the next crop in the rotation. Some forage legumes provide some N in the second year after the legume was grown. These second year N credits are listed in Table 6. If corn is grown in the second year following alfalfa and red clover, these N credits should be subtracted from the N rates that would be used when corn follows the crops listed in Group 2, Table 5.

Table 3. Suggested nitrogen guidelines for corn grown on non-irrigated loamy fine sands with less than 3 % organic matter.

Table 4. Nitrogen credits for different previous crops for the first year of corn.

*Use this credit if any small grain stubble in southeastern Minnesota counties was tilled after harvest; if there was no tillage, use guidelines for crops in group 2. Use guidelines for crops in group 2 if corn follows small grain in the remainder of the state.

Table 6. Nitrogen credits for some forage legumes if corn is planted two years after the legume.

Use of the Soil Nitrate Test Encouraged

Western Minnesota

The use of the soil nitrate test is a key management tool for corn producers in western Minnesota. The use of this test is appropriate for the shaded counties shown in Figure 1. The nitrate-N soil test is particularly useful for conditions where elevated residual nitrate-N is suspected. Figure 2 is a decision tree that indicates situations where the nitrate-N soil test would be especially useful.

For this test, soil should be collected from a depth of 6-24 inches in addition to the 0-6 inch sample. The corn grower in western Minnesota also has the option of collecting soil from 0-24 inches and analyzing the sample for nitrate-nitrogen (NO₃^--N). This 0-24 inch sample should not be analyzed for pH, phosphorus, and potassium because the results cannot be used to predict lime needs and rates of phosphate and/or potash fertilizer needed.

When using the soil nitrate test, the amount of fertilizer N required is determined from the following equation:

South-central, southeastern, east-central Minnesota

Research has led to the inclusion of a soil N test to adjust fertilizer N guidelines in south-central, southeastern, and east-central Minnesota (non-shaded areas of Figure 1). This test, in which soil nitrate-N is measured in the spring before planting from a two-foot sampling depth, is an option that can be used to estimate residual N. In implementing this test, the

user should first evaluate whether conditions exist for residual N to accumulate. Factors such as previous crop, soil texture, manure history, and preceding rainfall can have a significant effect on accumulation of residual N.

A crop rotation that has corn following corn generally provides the greatest potential for significant residual N accumulation. In contrast, when soybean is the previous crop, much less residual N has been measured. This test should not be used following alfalfa.

The soil N test works best on medium- and fine-textured soils derived from loess or glacial till. The use of the soil N test on coarse-textured soils derived from glacial outwash is generally not worthwhile because these soils consistently have low amounts of residual nitrate-nitrogen.

The amount of residual nitrate-nitrogen in the soil is also dependent on the rainfall received the previous year. In a year following a widespread drought, 1989 for example, a majority of fields will have significant residual nitrate. However, following relatively wet years, such as the early l990s, little residual nitrate can be expected.

This soil N testing option, which estimates residual nitrate-nitrogen, will not be appropriate for all conditions. Figure 2 can be used to help decide which fields may need to be sampled. This flowchart uses such factors as previous crop, manure history, and a knowledge of previous rainfall.

Nitrogen fertilizer guidelines for corn can be made with or without the new soil N test. The University of Minnesota’s N guidelines (Table 1) are still the starting point. A five-step process is suggested when the soil nitrate-nitrogen test is considered.

This soil nitrate-nitrogen test should not be used when commercial fertilizer was applied in the previous fall. The variability in the degree of N conversion to nitrate-N before spring makes this test meaningless in these situations.

For more information on this soil N test option, see Extension Bulletin FO-6514, A Soil Nitrogen Test Option for N Recommendation with Corn.

Table 7. Residual N credit values based on the concentration of nitrate-N measured before planting in the spring from the top two feet of soil.

Best Management Practices for Nitrogen

Because of the diversity of soils, climate, and crops in Minnesota, there are no uniform statewide guidelines for selection of a source of fertilizer N, placement of the N fertilizer, and use of a nitrification inhibitor. In order to accurately address this diversity, Minnesota has been divided into five regions and best management practices (BMPs) for N use in each region have been identified and described. The listing of these management practices for all regions is not appropriate for this publication. This information is available at local County Extension Offices.

Currently, the use of these best management practices is voluntary. Corn growers should implement BMPs to optimize N use efficiency, profit, and protect against increased losses of nitrate-nitrogen to groundwater aquifers and surface waters. Time of application, selection of a N source, placement of fertilizer N, and decisions regarding the use of a nitrification inhibitor are topics that are discussed in detail in other Extension publications listed at the end of this folder.

Phosphate and Potash Suggestions

When needed, the use of phosphate and/or potash fertilizer can produce profitable increases in corn yields. The suggestions for phosphate fertilizer use are summarized in Table 8. The suggestions for potash fertilizer use are listed in Table 9.

Rate Changes with Placement

The phosphate suggestions provided in Table 8 change with soil test level for phosphorus (P), expected yield, and placement. In general, the results of the Olsen test should be used if the soil pH is 7.4 or higher. There are some situations where the results of the Bray test are higher than the results of the Olsen test when soil pH values are higher than 7.4. For these cases, the amount of phosphate recommended should be based on the soil test value that is the higher of the two.

P₂O_5Rec = [0.700 - (.035 (Bray P ppm)] (expected yield)
P₂O_5Rec = [0.700 - (.044 (Olsen P ppm)] (expected yield)

Measurement of P by the Mehlich III procedure is not recommended in Minnesota. However, some soil testing laboratories analyze P with this analytical test. For these situations, use the recommendations appropriate for the results of the Bray procedure. The definition of categories is the same for both the Bray and Mehlich III analytical procedures.

A combination of band and broadcast applications is suggested if the soil test for P is very low (0-5 ppm for Bray; 0-3 ppm for Olsen). For fields with these very low values, plan on using the suggested band rate in a band at planting, subtract this amount from the suggested broadcast rate, then broadcast and incorporate the remainder before planting. Phosphate fertilizer can be applied as either a broadcast application or in a band fertilizer if the soil test value for P is in the low (6-10 ppm for Bray; 4-7 ppm for Olsen) or medium (11-15 ppm for Bray; 8-11 ppm for Olsen) ranges. For any banded application, use the rates suggested for band use.

Broadcast applications of phosphate fertilizer have a low probability of increasing corn yields when the soil test for P is in the high range (16-20 ppm for Bray; 12-15 ppm for Olsen). The use of phosphate in a banded fertilizer is suggested for these situations. No phosphate fertilizer is suggested for either broadcast or banded application if the soil test is higher than 25 ppm (Bray), or 20 ppm (Olsen), and conventional tillage systems are used.

As with phosphate, the suggested rates of potash vary with the soil test for potassium (K), expected yield, and placement (Table 9). A combination of broadcast and band applications is suggested when the soil test for K is in the range of 0-40 ppm. For fields with these values, plan on using the suggested rate in the band at planting, subtract this amount from the suggested broadcast rate, then broadcast and incorporate the remainder needed before planting.

No potash fertilizer is recommended if the soil test for K is 175 ppm or higher.

The grower has the choice of either broadcast or band placement if the soil test for K is in the low (41-80 ppm) or medium (81-120 ppm) range. The application of potash in a band is emphasized if the soil test for K is in the high range (121-160 ppm).

There is a low probability of response to broadcast applications of potash if the soil test for K is higher than 160 ppm. No potash will be needed in either broadcast or a band application if the soil test for K is 175 ppm or higher, and conventional tillage systems are used.

Special Considerations

Because of the diversity in Minnesota’s soils and climate, rental and lease arrangements for land, and goals of individual growers, the phosphate and potash suggestions listed in Tables 8 and 9 cannot be rigid across the entire state. There are some special situations where rates might be changed. Some, but not all, of these situations are described in the following paragraphs.

East Central Minnesota Soils in this region of the state usually have high native levels of soil test P and strict interpretation of the guidelines suggests that no phosphate is needed in a fertilizer program. Yet, many have observed responses to phosphate when applied in a band at planting. Soils in this region are frequently cool and wet in the spring and these conditions can lead to a requirement for phosphate fertilizer early in the growing season. Therefore, a rate of 15-20 lb. phosphate per acre is suggested for use in a bamded fertilizer placed close to the seed at planting for corn production in these situations, regardless of soil test level for P.

Broadcasting Low Rates Some of the suggestions for phosphate and potash use listed in Tables 8 and 9 are small and fertilizer spreaders cannot be adjusted to apply these low rates. In some situations, the suggested broadcast rate of phosphate can be blended with the suggested broadcast rate of potash and the mixture could then be applied with available equipment.

In other situations, broadcast applications of low rates of only phosphate or potash may be suggested. For these fields, it may be more practical to double the suggested broadcast rate and apply on alternate years.

Changes in Soil Test Values Many growers would prefer to maintain soil test values for P and K in the medium to high range. This is especially true if they own, rather than rent, the land. There is justified concern that soil test levels for either P or K will drop substantially if low rates of phosphate or potash fertilizers are applied year after year.

Research in Minnesota has shown that soil test levels for these two nutrients do not change rapidly with time. Yearly decreases have been small for situations where no phosphate or potash fertilizer has been applied.

A small decrease in soil test levels for P and K can be expected when phosphate and potash are used repeatedly in a banded fertilizer. Likewise, some reduction can be expected when low rates of phosphate and potash are used year after year. When soil test values drop, broadcast applications of higher rates of phosphate and/or potash fertilizers are justified if profitability and cash flow is favorable and the grower wants to maintain soil test values in the medium or high range.

Unless long-term leases or rental arrangements are used, the use of a banded placement for phosphate and/or potash may be the most profitable management system for rented land. It is difficult to economically justify the use of high rates of phosphate and/or potash to build soil test levels on rented acres.

Adjusting for Manure Use

The plant nutrients used in a fertilizer program for corn should be reduced if manure is used. The nutrient value of manure, however, varies with type of livestock, handling system, and method of application. Old rules of thumb are no longer appropriate when calculating the nutrient value of manure. Manure nutrient credits should be subtracted from the fertilizer guideline. There are several extension publications that describe in detail the use of manure. These publications are listed at the end of this folder.

Using a Banded Fertilizer

The use of a banded fertilizer at planting is an excellent management tool for corn production in Minnesota especially when soil conditions are cold and wet at planting. Yield increases are not always guaranteed with the use of a starter when soil test values are in the very high range. More recent research shows frequent responses to banded fertilizer when soil test values for P and/or K are in the high range and yield potential is high. The use of this management practice, however, can be considered to be a good insurance policy.

The rate of fertilizer that can be applied in a band below and to the side of the seed at planting varies with the nutrient used, the distance between seed and fertilizer, and soil texture. See Use of Banded Fertilizer for Corn Production (FO-74250) for more information.

CAUTION! Do not apply urea, ammonium thiosulfate (12-0-0-26) or fertilizer containing boron in contact with the seed.

Sulfur Use

The addition of sulfur (S) to a fertilizer program should be a major consideration when corn is grown on sandy soils.

The use of a soil test for sulfur is not a reliable predictor of the need for sulfur in a fertilizer program. Soil texture is a reliable predictor. If the soil texture is a loamy sand or sandy loam, either apply 12 to 15 lb S per acre in a banded fertilizer or broadcast and incorporate 25 lb S per acre before planting. Keep in mind that ammonium thiosulfate should not be placed in contact with the seed. This material will not harm germination or emergence if there is 1 inch of soil between seed and fertilizer.

There are several materials that can be used to supply S. Any fertilizer that supplies S in the sulfate (SO₄^2--S) form is preferred. Because the greatest need for S occurs early in the growing season, application of any needed S in a starter fertilizer is preferred.

Magnesium Needs

Most Minnesota soils are well supplied with magnesium (Mg) and this nutrient is not usually needed in a fertilizer program. There are some exceptions. The very acid soils of east-central Minnesota might need Mg. There should be no need for the addition of Mg if dolomitic limestone has been applied for legume crops in the rotation. There is a soil test that can be used to predict the need for this nutrient. The suggestions for using Mg in a fertilizer program are summarized in Table 10.

* Apply 10 -20 lb. Mg per acre in a band only if a Mg deficiency is suspected or if a deficiency has been confirmed by plant analysis.

Micronutrient Needs

Research trials conducted throughout Minnesota indicate that zinc (Zn) is the only micronutrient that may be needed in a fertilizer program for the corn crop. This nutrient, however, is not needed on all fields. The soil test for Zn is very reliable and will accurately predict the needs for this essential nutrient. The suggestions for Zn are summarized in Table 11.

Because corn is the only agronomic crop that will consistently respond to Zn fertilization, the use of Zn in a banded fertilizer is highly recommended. However, carryover to succeeding years will be better with broadcast applications. There are several fertilizer products that can be used to supply Zn. Except for large particles of zinc oxide, all are equally effective. Cost should be a major consideration in product selection.

The use of iron (Fe), copper (Cu), manganese (Mn), and boron (B) is not suggested for corn fertilizer programs in Minnesota.

Related Publications

Check your local University of Minnesota County Extension Office or www.soils.umn.edu/extension/extension_publications.php for these publications that may provide more details on management practices related to fertilizer use for corn production.

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