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.
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.
With the volatility in energy costs, fertilizer N cost has risen dramatically. This increase does affect the economic optimum N rate. To account for this change, the ratio of the price of N per lb. 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.
Table 1. Guidelines for use of nitrogen fertilizer for corn grown on soils considered to be highly productive.
|Value Ratio||MRTN||Acceptable Range||MRTN||Acceptable Range|
|— lb. N/acre —|
|0.05||155||130 to 180||120||100 to 140|
|0.10||140||120 to 165||110||90 to 125|
|0.15||130||110 to 150||100||80 to 115|
|0.20||120||100 to 140||85||70 to 100|
MRTN = maximum return to nitrogen.
Table 2. Guidelines for use of nitrogen fertilizer for corn grown on soils considered to have medium productivity potential.
|N Price/Crop Value Ratio||Corn/Corn||Corn/Soybeans|
|— lb. N/acre —|
Table 3. Suggested nitrogen guidelines for corn grown on non-irrigated loamy fine sands with less than 3 % organic matter.
|N Price/Crop Value Ratio||Corn/Corn||Corn/Soybeans|
|— lb. N/acre —|
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 5. 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 4. Nitrogen credits for different previous crops for the first year of corn.
|Previous Crop||1st Year Nitrogen Credit|
|4 or more plants/ft2||150|
|2 to 3 plants/ft2||100|
|1 or fewer plants/ft2||40|
|Group 1 Crops||75|
|Group 2 Crops||0|
*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.
Crops in Group 1:
Alsike clover, birdsfoot trefoil, grass/legume hay, grass legume pasture, fallow, and red clover.
Crops in Group 2:
Barley, buckwheat, canola, corn, grass hay, grass pasture, oat, potato, rye, sorghum-sudan, sugar beet, sunflower, sweet corn, triticale, and wheat.
Figure 1. Flow chart decision-aid for determining the probability of having significant residual NO3--N in the soil.
Table 5. Nitrogen credits for some forage legumes if corn is planted two years after the legume.
|Legume Crop||2nd Year Nitrogen Credit|
|4 or more plants/ft2||75|
|1 or fewer plants/ft2||0|
Use of the Soil NO3--N test encouraged.
The use of the soil NO3--N management tool for corn producers in western Minnesota. The use of this test is appropriate for the shaded counties shown in Figure 2. The NO3--N soil test is particularly useful for conditions where elevated residual nitrate-N is suspected. Figure 1 is a decision tree that indicates situations where the nitrate-N soil test would be especially useful.
Figure 2. Reference Map for lime
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 NO3 --N.
When using the soil NO3 --N test, the amount of fertilizer N required is determined from the following equation:
NG = (Table 1 value for corn/corn) – 0.60 * STN(0-24 in.))
NG = Amount of fertilizer N needed, lb./acre
Table 1 value = the amount of fertilizer needed, adjusted for soil potential, value ratio, and risk.
STN (0-24) = Amount of NO3 --N measured by using the soil NO3 --N test, lb./acre.
South-Central, Southeastern, East-Central Minnesota
Research has led to the inclusion of a soil NO3 --N test to adjust fertilizer N guidelines in south central, southeastern, and east-central Minnesota (non-shaded areas of Figure 2). This test, in which soil NO3 --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 NO3 --N test on coarse textured soils derived from glacial outwash is generally not worthwhile because these soils consistently have low amounts of residual NO3 --N.
The amount of residual NO3 --N 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 NO3 --N. However, following relatively wet years, such as the early l990s, little residual NO3 --N can be expected.
This soil NO3 --N testing option, which estimates residual NO3 --N, will not be appropriate for all conditions. Figure 1 can be used to help decide which fields may need to be sampled. This flowchart uses such factors as previous crop, manure history, and knowledge of previous rainfall.
Nitrogen fertilizer guidelines for corn can be made with or without the soil NO3 --N test. The University of Minnesota's N guidelines (Table 18) are still the starting point. A five step process is suggested when the soil NO3 --N test is considered.
- 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 will be followed for the specific conditions.
- 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 NO3 --N is small and soil testing for NO3 --N is not recommended, use the N guideline derived in Step 1.
- If conditions suggest that a soil NO3 --N test is warranted, collect a preplant, 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 NO3 --N.
- Determine residual N credit based on the measured soil NO3 --N concentrations. Use Table 6 to determine this credit.
- 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 preplant and/or as a sidedress application.
This soil NO3 --N test SHOULD NOT be used when commercial fertilizer was applied in the previous fall. The variability in the degree of N conversion to NO3 --N before spring makes this test meaningless in these situations.
For more information on this soil N test option, see A Soil Nitrogen Test Option for N Recommendation with Corn.
Table 6. Residual N credit values based on the concentration of NO3 --N measured before planting in the spring from the top two feet of soil.
|Soil NO3 --N||Residual N credit|
|— ppm —||— lb. N/acre —|
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.
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 NO3--N 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.
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 7. The suggestions for potash fertilizer use are listed inTable 8.
Rate Changes with Placement
The phosphate suggestions provided in Table 7 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.
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.
As with phosphate, the suggested rates of potash vary with the soil test for potassium (K), expected yield, and placement (Table 8). 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.
Table 7. Phosphate suggestions for corn production in Minnesota.*
|Soil test P (ppm)|
|v. low||low||medium||high||v. high|
|Expected Yield||Bray:||0-5||6-10||11-15||16-20||21 +|
|bu./acre||— P2O5/acre to apply (lb./acre) —|
* Use one of the following equations if a P2O5 guideline for a specific soil test value and a specific expected yield is desired.
P2O5rec = [0.700 — (.035 (Bray P ppm)] (expected yield)
P2O5rec = [0.700 — (.044 (Olsen P ppm)] (expected yield)
No phosphate fertilizer is suggested if the soil test for P is higher than 25 ppm (Bray) or 20 ppm (Olsen).
Table 8. Potash suggestions for corn production in Minnesota.*
|Soil test K (ppm)|
|v. low||low||medium||high||v. high|
|Expected Yield||0-40||41-80||81-120||121-160||160 +|
|bu./acre||— K2O/acre to apply (lb./acre) —|
* Use the following equation if a K2O guideline for a specific soil test value and a specific expected yield is desired.
K2Orec = [1.166 — 0.0073 (Soil Test K, ppm)] (expected yield)
No potash fertilizer is suggested if the soil test for K is 175 ppm or higher.
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 7 and 8 cannot be rigid across the entire state. There are some special situations where rates might be changed. See Fertilizing Corn in Minnesota, for a description of this situation. 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 decline, 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.
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 for more information.
CAUTION! Do not apply urea, ammonium thiosulfate (12-0-0-26) or fertilizer containing boron in contact with the seed.
The addition of sulfur (S) to a fertilizer program should be a major consideration when corn is grown on sandy soils, reduced tillage systems, or in a long term continuous corn rotation.
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. No sulfur is suggested on fine textured soils unless organic matter content in the top 6-8 inches is less than 3.0% or fields with long term continuous corn with high amounts of residue. In this case 10-15 lb. S per acre should be broadcast before planting.
There are several materials that can be used to supply S. Any fertilizer that supplies S in the sulfate (SO42-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.
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 9.
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 10.
The use of iron (Fe), copper (Cu), manganese (Mn), and boron (B) is not suggested for corn fertilizer programs in Minnesota.
Table 9. Suggestions for magnesium use for corn production.
|Magnesium soil test||Relative level||Mg to apply|
|— ppm —||— lb./acre —|
* 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.
Table 10. Zinc suggestions for corn production in Minnesota.
|Zinc soil test*||Sinc to apply|
|— ppm —||— lb./acre —|
* Zinc extracted by the DTPA procedure.