Fertilizing corn in Minnesota

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 compared to previous approaches. This process has been in place since 2005 and is the product of a multi-state effort to use a similar philosophy/approach for determining N rate guidelines for corn.

Because of technology improvements in corn production practices such as weed and pest control, expected yield is not as important of a factor in determining N rate as it has been in the past.

Soil productivity has become a better indicator of N need. 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 yield potential due to erosion, reduced water holding capacity caused by lower organic matter content, sandy soil texture, poor drainage and restricted root growth. The fluctuation in fertilizer price affects 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 $0.40 per lb N (or $656 per ton of anhydrous ammonia), and corn is valued at $4.00 per bushel, the ratio would be 0.40/4.00 = 0.10.

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 Minnesota experiments supporting these guidelines. 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-averse and cannot tolerate risk associated with less-than-maximum yields in some years, even though economic return to N may not always be highly profitable, may want to use the N rates near the high end of the acceptable range shown in Table 1.

On the other hand, if corn is grown on medium or fine-textured soils considered to be of low or medium productivity 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.

The acceptable range gives the producer flexibility in arriving at an acceptable and profitable N rate that is calculated as the rate +/- $1 from the MRTN rate.

The N rate guidelines in Table 1 are used if corn is grown in rotation with soybean or following corn when NOT irrigated. Corn grown on sandy soils deserves special consideration.

If irrigated, the guidelines listed in Table 2 are appropriate when corn is grown in rotation with corn. If corn is grown following soybean on irrigated sandy soils, a credit of 30 lbs of N per acre should be taken from the suggestions 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.

Soils considered medium productivity in the past were given special consideration. More recent data has not shown strong support for a separate suggested application rate of N for medium-productivity soils.

The rate of N can be adjusted based on the acceptable range if a soil is considered to be medium productivity and has shown to be more or less responsive to fertilizer N.

Alfalfa, which includes pure stands of alfalfa and alfalfa-grass mixtures with at least 50% alfalfa in the stand, can eliminate or greatly reduce the need for N from fertilizer or manure during the two subsequent years if corn is grown.

Past guidelines assigned N credits to corn based on alfalfa stand density, but analyses of field trials from across Minnesota and the Midwest indicate that the frequency and level of yield response to N in first and second-year corn following alfalfa are more closely associated with soil texture, age of alfalfa at termination, alfalfa termination timing and weather conditions.

It is well established that first-year corn following alfalfa rarely responds to N except on sandy soils, on fine-textured soils when there are prolonged wet early-season conditions and on medium-textured soils when following very young alfalfa stands or in some cases when following spring-terminated alfalfa.

In past field trials from across Minnesota and the Midwest, yield of second-year corn following alfalfa did not respond to N in half of the fields studied.

Suggested rates of N for first and second-year corn following alfalfa are in Table 4. In some cases, the optimal rate of N can vary greatly due to weather-related variability in soil N mineralization. In such cases, limit the amount of N from fertilizer and manure applied before and near corn planting and apply additional N to corn during the growing season if necessary based on weather and crop conditions.

^a Includes pure stands of alfalfa and alfalfa-grass mixtures with at least 50% alfalfa in the stand.
^b Coarse = sands and sandy loams; medium = loams and silt loams; fine = clays, clay loams and silty clay loams.
^c Alfalfa age at termination, including the establishment year if alfalfa was direct-seeded without a small grain companion crop.
^d An additional 30 to 40 lbs N/acre can be applied to corn during the growing season if necessary based on the Corn calculator for supplemental nitrogen.

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 5.

In Table 5, several crops are divided into Group 1 and Group 2. The crops for each group are listed in Table 6.

The N rates listed in Tables 1 and 2 define the total amount of fertilizer N that should be applied. All N applied should be accounted for in the calculation, including N in starter fertilizer, weed and feed program, DAP (di-ammonium phosphate) or MAP (mono-ammonium phosphate) applied late fall (after 4” average soil temperatures stabilize at 50^oF) on non-sandy soils or for all soil types in spring, and with sulfur.

It is 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.

Red clover is the only crop other than alfalfa that may provide a second-year N credit. If red clover was grown two years before the current crop, 35 lbs of N per acre should be subtracted from the N rate when corn follows the crops listed in Group 2, Table 5.

The use of manure as a fertilizer source can raise questions about adequate nitrogen rates. The economics of manure application are not straightforward when on-farm sources are used in corn production.

Manure presents challenges as not all of the nutrients are 100% available to crops in the first year of application. Plant available N (PAN) is a term used when applying manure to identify the amount of N applied that is plant available in any given year and may be less than the total N applied.

Suggestions for N application when manure is the primary nutrient source are given in Table 7. If commercial fertilizer is used along with manure, the suggested rates in Table 7 should not be exceeded. Lower application rates similar to the 0.10 price ratio may be considered based on the productivity of the soils in your fields, economics or environmental concerns.

The pre-plant soil nitrate test (PPNT) can be a useful tool for assessing situations where residual soil nitrate can be credited to the corn crop. The PPNT should not be used when commercial fertilizer or manure was applied in the previous fall or in the spring prior to the sample being taken.

Western Minnesota

The use of the fall or spring PPNT is a key management tool for corn producers in western Minnesota. The suggestion that residual N in the fall can impact the need for nitrogen is contingent on the fact that the evapotranspiration of water historically has exceeded precipitation in this area of the state.

Use of the fall PPNT is appropriate in the maroon counties shown in Figure 1. The PPNT 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 the PPNT, soil should be collected from a depth of 6 to 24 inches in addition to the 0 to 6-inch sample that is used to test for pH, phosphorus and potassium.

Corn growers in western Minnesota also have the option of collecting soil from 0 to 24 inches and analyzing the sample for nitrate-nitrogen (NO₃-N). This 0 to 24-inch sample should not be analyzed for pH, phosphorus and potassium because the results cannot be used to predict lime needs or rates of phosphate and potash fertilizer needed.

When using the spring or fall PPNT, the amount of fertilizer N required is determined from the following equation:

NG = (Table 1 value for corn/corn) - (0.60 x STN(0-24in.))

• NG = Amount of fertilizer N needed (lbs N/acre)
• Table 1 value = the amount of fertilizer needed to be adjusted for soil potential, value ratio and risk
• STN(0-24 inch) = Amount of nitrate-N measured by using the fall PPNT (lbs N/acre)

South-central, southeastern, east-central Minnesota

Research has led to the inclusion of a spring PPNT to adjust fertilizer N guidelines in south-central, southeastern and east-central Minnesota (gray counties in Figure 1). Soil nitrate-N, 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 the 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. The PPNT should not be used following alfalfa.

The spring PPNT 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 (2012 for example) a majority of fields will have significant residual nitrate. However, following relatively wet years, little residual nitrate can be expected.

Nitrogen fertilizer guidelines for corn can be made with or without the 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.

1. Determine N rate guideline using Table 1 using soil productivity, price/value ratio, and previous crop for the specific field. The prescribed (rate assumes that best management practices (BMPs) will be followed for the specific conditions).
2. Determine whether conditions are such that residual nitrate-nitrogen may be appreciable. Figure 2, which includes factors such as previous crop, manure history and previous fall rainfall can provide insight as to the applicability of testing for nitrate-nitrogen. If conditions are such that the probability of residual nitrate is small and soil testing for nitrate is not recommended, use the N guideline derived in Step 1.
3. If conditions suggest that a soil nitrate test is warranted, collect a pre-plant, 0-2 ft. soil sample taking enough soil cores from a field so that the sample is representative of the entire field. The sample should be sent to a laboratory and analyzed for nitrate-nitrogen.
4. Determine residual N credit based on the measured soil nitrate-nitrogen concentrations. Use Table 8 to determine this credit.
5. Calculate the final N rate by subtracting the residual N credit (Step 4) from the previously determined N guideline (Step 1). The resulting fertilizer N rate can then be applied either pre-plant and/or as a side-dress application.

Because of the diversity of soils, climate and crops in Minnesota, there are no uniform statewide guidelines for the 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 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, but they are available at the Minnesota Department of Agriculture.

Currently, the use of these BMPs is voluntary. Corn growers should implement BMPs to optimize N use efficiency, profit and protect against increased losses of nitrate-nitrogen to the environment.

When needed, phosphate and/or potassium (potash) fertilizer can produce profitable increases in corn yields. Soil test categories represent the probability the soil will supply all the needed crop nutrients.

Table 9 shows field research data summarizing the expected percent of time when a measurable response to P fertilizer will occur and the percentage of maximum yield produced when no fertilizer is applied.

The chance of a yield response to P and the increase in yield is greatest when soil P tests very low and decreases as soil test P increases. Corn yield may still be increased by P at high and very high soil tests but the net return to P may not be profitable.

* Use one of the following equations if a P₂O₅ guideline for a specific soil test value and a specific expected yield is desired. 
P₂O_5suggestions = [0.700 - .035 (Bray P ppm)] (expected yield)
P₂O_5suggestions = [0.700 - (.044 (Olsen P ppm)] (expected yield) 
No phosphate fertilizer is suggested if the soil test for P is greater than 25 ppm (Bray) or 20 ppm (Olsen).

* Use one of the following equations if a K₂O guideline for a specific soil test value and a specific expected yield is desired.
K₂O _suggested = [1.12 - 0.0056 (Soil Test K, ppm)] (expected yield)<
No potash fertilizer is suggested if the soil test for K is 200 ppm or greater.

In general, the results of the Olsen test should be used if the soil pH is 7.4 or greater. There are some situations where the results of the Bray test are greater than the results of the Olsen test when soil pH values are greater than 7.4. For these cases, the amount of phosphate suggested should be based on the soil test value that is the higher of the two.

Measurement of P by the Mehlich III procedure is not suggested for use in Minnesota. However, some soil testing laboratories analyze P with this analytical test. For these situations, use the guidelines appropriate for the results of the Bray procedure.

The definition of categories is the same for both the Bray and Mehlich III analytical procedures when P is determined colorimetrically. If the soil pH is greater than 7.4, use of the Mehlich III test is not suggested as the results may not correlate to the Olsen P test.

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 very low P soil test values, plan on using the suggested band rate at planting then subtract the amount of P banded from the suggested broadcast P rate, then broadcast and incorporate the remainder of P 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. 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 greater 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 11. 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. The grower can choose either broadcast or band placement if the soil test for K is in the low (41-80 ppm) or medium (81-120 ppm) range. Applying potash in a band is emphasized if the soil test for K is in the high range (121-160 ppm).

Many growers would prefer to maintain soil test values for P and K in the medium to high range to reduce the risk of yield loss due to insufficient P or K. 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 and soils are not tested frequently enough to make adjustments for decreasing soil test values. In these circumstances, the application of P and K based on crop removal may be warranted.

Median rates of P and K removed per bushel of corn grain are listed in Table 12. High rates of P or K applied for maintenance will typically result in a less return in crop value per pound of nutrient applied.

The most economical use of P and K fertilizer is to only apply what is needed year to year. Recent research suggests that crop removal based on a proven yield (5-10 year yield average) can be economical for soils testing in the medium to high soil test categories.

For very high testing soils, a small starter rate of P or K is typically sufficient to achieve maximum crop yield in situations where corn will respond to applied fertilizer. Application of 50% or less of crop removal is suggested for soils testing very high in available nutrients to allow soil tests to gradually draw down to the high STP classification.

Strict crop removal of P and K may not provide sufficient nutrients for soils that test very low or low for either nutrient. Extra P can be applied to build some soils to the medium or high soil test category.

A general rule is that 16-18 lbs P₂O₅ and 7-10 lbs K₂O are required to increase the Bray-P1 or ammonium acetate K tests by 1 ppm, respectively. Rapid buildup of soil test P or K is not suggested. Research has shown that the rates of P suggested for the very low and low STP categories will slowly build STP to the medium classification for neutral to acid soils.

The amount of P or K needed to build the soil test greatly depends on soil chemical properties. For soils in western Minnesota where the Olsen P test is used, aggressively building soil test P values will not be cost-effective due to the reaction of ortho-phosphate with calcium. Under these circumstances, applying only what the crop needs to maximize yield potential is suggested.

Excessive building of P can lead to increased risk for P loss to the environment. The strategy outlined in Table 13 shows the STP range which is optimal for maintenance within the medium to high STP categories and suggests drawing STP down using P application based on partial crop removal in order to maintain STP in a more profitable zone. Soil test P ranges are not given for the Olsen P test as it may not be possible to build and maintain some high pH soils.

The example in Table 13 could be used for K. However, research has demonstrated increased seasonal variability in the soil K test. Collecting samples at the same time is critical to best evaluate maintenance-based strategies for K.

Potassium fertilizer guidelines for corn were revised based on recent research on medium and fine-textured soils in Minnesota. Currently, these guideline rates are not adjusted based on a soils’ ability to hold potassium on cation exchange sites of clays.

Coarse-textured soils, such as sands and loamy sands, have very little clay and low cation exchange capacity (CEC). Potassium can leach on low CEC soils, potentially wasting K fertilizer and reducing economic return to fertilizer.

Research in Minnesota is on-going to determine if K guidelines need to vary based on soil CEC. Recent research on sandy soils with a CEC around five meq per 100 grams showed sandy soils needed less potassium fertilizer than medium and fine-textured soils with the same soil test K level and had a lower critical soil test level.

Due to K leaching potential and a lower critical level of low CEC soils, it is not recommended to build soil test K greater than 120 ppm. Until more research data is available, K fertilizer could be applied on low CEC soils using the equation below.

However, use of this equation will reduce K fertilizer application rates on low CEC soils and should be done on a trial basis to ensure K is not limiting yield on irrigated corn grown on low CEC sandy soils.

K₂O_suggested = [1.08 - 0.0084 (Soil Test K, ppm)] (expected yield)

Because of the diversity of Minnesota’s soils and climate, rental and lease arrangements for land and goals of individual growers, the phosphate and potash suggestions listed in Tables 10 and 11 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

This region of the state usually has high native levels of soil test P and strict interpretation of the guidelines suggests that no phosphate is needed in a fertilizer program. However, many corn growers 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 lbs phosphate per acre is suggested for use in a banded 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 suggested rates for phosphate and potash listed in Tables 10 and 11 are small and fertilizer spreaders cannot be adjusted to apply these low rates. 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.

Research in Minnesota has shown that soil test levels for P and K should not change rapidly with time. Yearly decreases have been small for situations where no phosphate or potash fertilizer has been applied but can vary from year to year depending on environmental conditions. Always use long-term soil test trends over five years when assessing changes in soil test values using maintenance-based strategies.

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 in place, 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.

Animal manure is an excellent source of plant nutrients. When using manure, first determine the amount of plant-available nutrients it will provide in the first year (80% of P and 90% of K).

Adjust the fertilizer program to make up the difference between what was applied and what is needed according to the fertilizer guidelines. Extension resources that describe in detail the use of manure are listed at the end of this publication.

The use of a banded fertilizer at planting is an excellent management tool for corn production in Minnesota, especially when soils 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 or when recommended rates of broadcast P or K are applied.

The rate of fertilizer that can be applied in a band directly on the seed at planting varies by fertilizer source and soil texture. See a summary of appropriate rates for Banding fertilizer on the corn seed.

Application of fertilizer two inches beside and below the seed row presents a very low risk for reduced germination and higher rates of nutrients can be applied when there is one or more inches between fertilizer and seed placement. All nutrients applied in starter fertilizer should be accounted for in the total fertilizer program.

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

The addition of sulfur (S) to a fertilizer program for corn should be a major consideration based on soil texture, crop rotation, soil drainage and soil organic matter concentration in the top six inches. Suggested sulfur application rates as sulfate-sulfur forms are given in Table 14.

Banding sulfur fertilizer can increase effectiveness and reduce the required application rate by as much as one-half. Keep in mind that ammonium or potassium thiosulfate should not be placed in contact with the seed. Thiosulfate will not harm germination or emergence if there is at least one inch of soil between seed and fertilizer.

Visual sulfur deficiency symptoms early in the growing season are common in Minnesota due to limited mineralization and uptake of S early in the growing season. Some of these symptoms may be temporary and will go away as the soil warms.

If a deficiency of S is suspected, recent data have shown that fertilizer S can be applied when corn is 12 inches tall or less without a reduction in yield potential. Plant-available sulfur in the sulfate form is an anion and is susceptible to leaching loss.

Recent data suggests that sulfate can carry over from one year to the next. Total corn uptake of S can range from 20-25 lbs of S per year. Application in excess of this amount will not result in increased grain yield but S not used by the current corn crop may be carried over to the next year, reducing the need for S application for the following crop.

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.

Fertilizer sources containing elemental sulfur are commonly sold in Minnesota. Elemental sulfur must be oxidized to sulfate sulfur before it can be taken up by the plant. If elemental S is used, sulfate S may be required to ensure adequate availability of S early in the growing season and the application rate of elemental S should be 20 lbs S per acre or greater or double the amount of S suggested in Table 14.

Liquid S is commonly sold as thiosulfate products. The thiosulfate ion contains half of the S in sulfate and half in the elemental S form. Research has shown a greater availability of S from the elemental S fraction in thiosulfate fertilizers.

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 acidic 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 or when soils are irrigated and the water source used contains a high concentration of Mg.

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 15.

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

Plants take up less than one pound of micronutrients per acre, with only a few ounces required for optimal plant production. While micronutrients are needed for optimal plant growth, they may not need to be applied.

Boron

Research trials conducted throughout Minnesota indicate that zinc (Zn) is the only micronutrient that may be needed in a fertilizer program for corn. This nutrient, however, is not needed in all fields.

The DTPA 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 16.

* Zinc extracted by the DTPA procedure.

Because corn is the only agronomic crop that will consistently respond to Zn fertilization, the use of Zn in a banded fertilizer is suggested. 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.

Chelated zinc is commonly used when liquid starter fertilizer is applied directly on the corn seed. Chelated Zn can increase the availability of Zn by preventing the precipitation of low-solubility Zn compounds, but these products typically cost more per lb of Zn applied.

Zinc fully chelated with EDTA has been shown to provide the greatest stability of zinc across a range in soil pH values. Using chelated Zn does not increase the potential for a yield response for soils testing 0.75 ppm or greater in soil test Zn and should be targeted to soils where a response to Zn is expected.

Boron

Recent research has shown that boron (B) is not likely to increase yield of corn across a variety of soils across Minnesota. Boron availability in soils is affected by soil organic matter concentration and soil moisture.

There is a small chance of a response to B on sandy soils with SOM less than 1.0% and boron soil tests of 0.08 ppm or less. If a deficiency to boron is suspected, the application of boron should not exceed 1-2 lbs per acre of B broadcast applied on a trial basis. There are no guidelines for foliar-applied boron as low rates of B applied to foliage do present a risk for boron toxicity.

Other micronutrients

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