Fertilizers for Corn
The guidelines of fertilizer rates given in the table "Fertilizers for Corn" are general guidelines for optimum economic corn production.
These general fertilizer guidelines should only be used in limited circumstances when a complete soil test has not been taken as the tables in this section are condensed for simplicity. More information is incorporated into the computerized guideline system than is considered in the general tables here. For example, in the Table the recommended rates of fertilizer application are broken down into five general soil groupings. Within each group, the years since a sod was plowed and the legume content of the sod are used to generalize the nitrogen guidelines at two rates of dairy manure additions. When these guidelines are formulated from a complete soil (and manure) test, they are tailored to the grower’s soil resources. Equations are used to calculate the guidelines based on yield potential (soil type specific), previous cropping practices, and the type, and rate of previous and present manure applications. The possible combinations range in the thousands.
For CAFO planning, see the relevant documents on the Cornell guidelines for field crops page accessible via the Cornell Nutrient Management Spear Program website.
When fertilizer and manure rates are applied as suggested for agronomic response, nutrient losses into the environment are relatively small and optimum economic production can be achieved.
As mentioned, for nitrogen, guidelines are based on crop history and manure use. Work on the Illinois Soil N Test (ISNT) as a tool for assessment of soil N supply from organic matter for corn over the past six years has shown the test to be 84% accurate in identifying sites that do not need extra fertilizer N due to the soil’s N supply capacity. These are sites where fertilizer savings can be made beyond what is recommended in the tables or soil test report based recommendation (i.e. where no N is needed due to high soil N supply even though the Cornell guidelines in the tables in this section state additional N could be needed). This test is most relevant in combination with the late season Corn Stalk Nitrate Test (CSNT) and for 2nd or higher year corn fields with a manure history where sidedressing of fertilizer N might not be needed. For more detailed nitrogen guidelines (Cornell guidelines for CAFO planning) and the Nitrogen for Corn website of the Cornell Nutrient Management Spear Program. See also Agronomy Fact Sheets #63, #77 and #78 for more information on the use of the ISNT and CSNT for fine-tuning of nitrogen management for corn in New York State.
Note: if soil test results are not available and previous crops do not have a history of adequate fertilization, use the fertilizer rates for medium soil test results. If the site does not have a history of fertilizer or manure, use the fertilizer rates given for the low soil test results.
The fertilizer used as a starter should contain a small amount of nitrogen; most, if not all, of the recommended phosphorus; and possibly some potassium. Thus, a good starter fertilizer might range from a ratio of 1-4-0, 1-3-1, 1-3-3, to 1-1-1, depending on the rate of fertilizer required. Do not apply more than 80 to 100 pounds per acre of N + K2O in the starter band. For example, 350 pounds per acre more of 10-20-20 can result in seedling injury. Urea and diammonium phosphate (DAP) can also cause seedling injury and should not be used in the starter band to eliminate this risk (see “Fertilizer Injury”).
Efficient Nitrogen Use
It is important to determine as accurately as possible the quantity of nitrogen necessary for optimum economical corn production and to apply only this quantity of nitrogen to prevent over-fertilization.
To determine the quantity of nitrogen that should be furnished by commercial fertilizer, all the nitrogen sources must be considered.
Soil organic matter supplies 40 to 80 pounds of nitrogen per acre per year. A good legume or legume-grass sod will supply 100 to 150 pounds per acre, or more, and a good grass or grass-legume sod will supply 75 to 100 pounds per acre in the first year after application. Thus, a legume sod and the soil itself will supply approximately 200 pounds per acre of nitrogen; hence, only a starter is needed to satisfy nitrogen requirements. Recent research in New York confirmed that for both optimum yield and quality a small starter N application (30 lbs N or less per acre) is needed for first year corn following sods in the rotation, independent of timing of sod kill (late fall or spring) or percentage legume in the sod (see Agronomy Fact Sheet #21). Trials were conducted in 2010 and 2011 to determine if manure can replace the need for starter N fertilizer too. The findings were: (1) fields with low or marginal ISNT needed starter N unless manure was applied at the full N rate; and (2) if no manure was applied to the field the current crop year, starter N use increased yield UNLESS the field had an optimal ISNT result (fields with optimal ISNT did not respond to starter N fertilizer). In all scenarios, the CSNT could be used to check on appropriateness of the manure and N fertilizer rates at the end of the season: optimally fertilized fields will have a CSNT between 750 and 2000 ppm. In order to identify sites where a starter N application can be omitted, we recommend producers sample the soil (0-8 inches depth) after harvest of 1st year corn and analyze for ISNT-N (and other soil fertility indicators), and follow up with a CSNT sample for the 2nd+ years. Note that ISNT and soil fertility data are valid for 2-3 growing seasons.
- For more information, see Agronomy Fact Sheet #67.
Since first year corn does not need additional N beyond a small starter application, no soil (e.g. PSNT or ISNT) or plant analysis (CSNT) tools are needed to determine N rates.Crediting manuire nutrients should be based on a representative manure sample. For further information on deriving N credits from manure, see Agronomy Fact Sheet #4 and #61.
The optimum economic N rate for corn after soybean can be reduced by 20-30 lbs N/acre for the first year of corn following soybean. See Agronomy Fact Sheet #30 for more information.
The optimum economic N rate for first year corn after clover, interseeded into a small grain, can be reduced by 70-120 lbs N/acre. See Agronomy Fact Sheet #60 for more details.
When needed, rates of nitrogen up to about 40 to 50 pounds per acre can be applied in the fertilizer band provided the N + K2O application does not exceed 80 to 100 pounds per acre. When rates above 40 pounds per acre of nitrogen are recommended, greatest nitrogen use efficiency can be obtained when a small starter rate (10 to 30 pounds per acre) is used in the fertilizer band, and the remaining nitrogen is applied at sidedress time just prior to the most rapid growth phase of the crop.
Enhanced efficiency fertilizers have been developed to minimize the potential for N loss to the environment. This includes technologies that delay nitrification (nitrification inhibitors), delay conversion of urea to ammonium (urease inhibitors), and/or use of sulfur or polymer coatings to allow release of N over a longer time period (slow or controlled release).
Nitrification inhibitors are substances that inhibit conversion of ammonium to nitrate. Nitrification inhibitors work by keeping N in ammonium form, which is done by inhibiting Nitrosomonas bacteria, commonly delaying conversion for four to ten weeks depending on the product, soil temperature and pH. These inhibitors can reduce N loss from leaching and denitrification but are only effective on fertilizers that either contain or are converted to ammonium, including anhydrous ammonia, urea, and ammonium sulfate. Nitrification inhibitors are effective for inhibition of the urea and ammonium in the fertilizer but not for nitrogen already in nitrate form (25% of the N in UAN).
In the humid conditions of New York State, nitrification inhibitors have the highest likelihood for a yield response when used on N applied at planting, in poorly drained soils (where denitrification losses might occur) or in sandy soils (where the leaching potential is high). They are less likely to be needed when N is sidedressed as conversion and rapid uptake is expected to be very rapid at that point. Proven chemistries include the use of dicyandiamide (DCD) and nitrapyrin.
Urease inhibitors are substances that inhibit conversion of urea to ammonia and carbon dioxide, reducing ammonia volatilization losses. Urease inhibitors can reduce or delay formation of ammonia for up to ten to fourteen days. Urease inhibitors are especially useful with N sources that have a high volatilization potential (e.g. urea) in situations in which tillage incorporation is not possible (e.g., no-till, pasture, and grass hay production). Treating with a urease inhibitor allows more time for rain to incorporate the N fertilizer. Ammonium sulfate is much less prone to volatilization and is hence a good alternative to the use of urease inhibitors. Documented chemistries that inhibit urease include N-(n-butyl) thiophosphoric triamide NBPT), phenylphosphorodiamidate, thiophosphoryl triamide, and ammonium thiosulfate.
Slow-release fertilizers minimize the potential of nutrient losses to the environment by slowly converting to ammonium and/or nitrate over time. These N sources can reduce N losses, especially in sandy soils more prone to N loss, and help extend N availability over a full growing season. Slow-release fertilizers release more slowly than soluble N sources. Their release is limited mostly by temperature and/or moisture.
Controlled-release fertilizers are usually common fertilizers such as urea coated with a polymer or with sulfur. The coating delays the availability of the nutrients for plant uptake after application and controls nutrient release over time. These products are not desirable when a quick release of available N is needed, for example, when sidedressing corn at the 6-leaf stage. In addition, some controlled release products, if applied on bare soil, should be incorporated to prevent runoff (particles may float) of the polymer coated fertilizers with heavy rains.
Nitrogen Status of the Corn Crop
One can determine whether the proper rate of nitrogen was used by examining the crop. This hindsight evaluation helps refine nitrogen management. Nitrogen sufficiency in one year does not necessarily imply that N rates should be decreased in the next year, and vice versa. Examine the lower leaves of the cornstalks. If three to five of the lower leaves are dead (or nearly so) by the early dent stage and the upper leaves on the plant remain medium to dark green, the proper rate of nitrogen fertilizer was used. If fewer than three leaves die by early dent and the top leaves remain moderately dark to dark green, too much nitrogen was used and the rate could have been decreased by 20 to 40 pounds per acre. If the leaves die up to or above the ear leaf or the entire plant has a light to very light green color and the leaves near the ear leaf are yellow, too little nitrogen fertilizer was used and the optimal rate was 20 to 40 pounds per acre higher than what was applied (in this evaluation, consider only leaves lost because of nitrogen deficiency and not losses caused by leaf blight or drought). Nitrogen deficiency starts as a V-shaped yellowing of the leaf tip, which proceeds toward the stalk, followed by gradual leaf death. Drought symptoms are almost the same as those of nitrogen deficiency, and drought will make nitrogen deficiency appear to be worse.
If a severe drought occurs late in the season (tasseling to early dent), the above descriptions are not valid. Instead, nitrogen rates even below those recommended would have been adequate because over fertilization does not compensate for lack of water.
The large quantities of N needed for optimal corn production can be supplied by soil organic matter, crop residues, manure, and/or commercial fertilizer. The pre-sidedress soil nitrogen test (PSNT) provides a way to determine if there will be sufficient nitrogen in the soil for maximum economic yields corn. PSNT determines the nitrate content of the top 12 inches of soil when the corn is 6 to 12 inches tall. The soil nitrate content at that time is an indication of the total nitrogen available to the plants for the remainder of the growing season.
If PSNT nitrate results are 25 ppm or above, there is sufficient N in the soil for maximum economic corn yields. If there is less than 21 ppm of soil nitrate, additional sidedress N is needed. When the results are between 21 and 25 ppm N, there is about a 10 percent probability that a yield response would be obtained from additional N. The quantity of N that is needed when the nitrate results are below 25 ppm is determined by computing the N requirements considering the soil, crop, rotation, and manure histories as described for corn, but subtracting any fertilizer N applied preplant or at planting.
For additional information on PSNT, instructions on sampling, soil testing kits, or interpretation of results, see the associated fact sheet or contact your local Cornell Cooperative Extension office.
A new tool released for use in New York in 2007 is the Late Season Stalk Nitrate Test (Agronomy Fact Sheet #31). This end-of-season test can be used to evaluate the N supply during the growing season. It is useful as a management tool as it helps identify if adjustments in N management are needed in future years.
For corn silage, samples should be collected from one week prior to harvest until one day after harvest (if stubble height is ≥ 14 inches).
The portion of the stalk used for the test is important as the test is calibrated for the nitrates that accumulate in this part of the stalk. First measure up 6 inches from the soil surface and cut the plant. Then measure 8 inches up from this first cut, and make a 2nd cut. These cuts result in an 8 inch sample of stalk taken from between 6 & 14 inches above the ground. Make sure not to touch the soil with the corn stalk segment; contamination with soil will greatly impact test results.
- Sample an 8-inch segment of the corn stalk between 6 and 14 inches above the ground.
In a uniform field (≤15 acres in size) fifteen 8-inch segments should be randomly cut and combined to make one sample to be submitted for analysis. Areas differing in management or soil type should be sampled separately. Similarly, fields that are more than 15 acres large should be subdivided into smaller sampling units. Split each stalk into four parts by cutting it lengthwise using a clean kitchen knife and toss out three of the four quarters. This will quicken the drying process and reduce the volume to be submitted to the laboratory.
Samples can be taken within 5 days after corn silage harvest as long as there is no major rainfall between harvest and sampling. If the silage cutting height in the field exceeds 14 inches, it is recommended to sample the standard 8-inch section of stalk from the 6-14 inch height as described above. When stubble height is less than 14 inches but greater than 8 inches, stalk samples can be taken between 2 and 8 inches off the ground. This alternative cutting height should be reported to the laboratory when submitting CSNT samples so the laboratory staff can properly adjust the results. More information on this alternative sampling protocol can be found in Agronomy Fact Sheet #72.
Samples should be submitted as soon after collection as possible but can be stored in a refrigerator for up to 6 days. Samples should be placed in a paper bag (not plastic). This allows for some drying to occur and minimizes growth of mold. Samples can be submitted to several different laboratories. Contact Quirine Ketterings at 607-225-3061 or qmk2 [at] cornell.edu () for further information.
Based on research conducted in New York, current interpretations are:
- Low = less than 250 ppm N
- Marginal = 250-750 ppm N
- Optimal = 750 to 2000 ppm N
- Excess = greater than 2000 ppm N
Low (deficient)- Plants had difficulty accessing enough nitrogen in these fields. Nitrogen access was hindered by inadequate supply, root restrictions, lack of moisture, or nutrient deficiency interactions. At harvest time, leaves are dead to or above the ear leaf and/or the entire plant has a light to very light green color.
Marginal- In some years, yields could have been increased with some additional N. In those years, plants look like described as above. In other years, the N supply was sufficient. Since it is difficult to predict what kind of growing conditions a season will bring, farmers are advised to target CSNTs in the optimal range.
Optimal (sufficient)- Nitrogen availability was within the range needed for optimum economic production of corn. In this range, three of the five lower leaves will be dead by harvest time while the top leaves remain medium to dark green.
Excess - If the sample has more than 2000 ppm N, the corn had access to more N than it needed for optimum yield. Most likely, fewer than three leaves from the bottom will have died; the top leaves remain medium to dark green. If manure and/or N fertilizer was applied, the application(s) supplied more N than the crop needed that growing season.
This test is not meant as a one time measurement; it is most effective when used for multiple years on the same field (or fields with similar histories) in order to determine how the fields respond to the way N is being managed. Crop history, manure history, other N inputs, soil type, and growing conditions all impact whether the stalk nitrate test will show that the crop is deficient, sufficient or excessive in N.
The greatest benefit of this test is that it allows evaluation and fine-tuning of N management for each specific field. It does, however, require multiple years of testing to gain experience with on-farm interpretation. Stalk nitrate tests of 2000 ppm or greater indicate excessive levels of available N during the growing season and if corn stalks test above 2000 ppm for two years or more, consider analyzing a soil sample for the Illinois Soil Nitrogen Test to determine soil N supply potential and evaluate the risk of a yield decline if fertilizer and/or manure application rates are lowered. See Agronomy Fact Sheet #63 for more information on use of the CSNT and ISNT for fine-tuning nitrogen management for corn.
Adapt-N – Incorporating Weather Information to Adjust Corn N Rates in Season
Appropriate nitrogen rates for corn vary greatly among locations and growing seasons. In some years corn is nitrogen deficient, while in others the same amount of fertilizer appears to be adequate. This happens because the appropriate N fertilizer rate is highly influenced by weather, soil, and management factors. Notably, early season precipitation has been shown to significantly affect corn fertilizer response. Much of the plant-available N is lost in years with wet springs, which then require more supplemental nitrogen fertilizer at sidedress time; lower. fertilizer rates are needed in dryer years. Soil type differences and management practices interact with weather and also affect the optimum N rate.
Adapt-N is a computational tool that incorporates the complex interactions among these factors to provide a field-specific, weather-adjusted N rate recommendation. It can be accessed from any computer or mobile device with Web access. Adapt-N simulates important processes day by day, incorporating high-resolution (3 x 3 mile) information on precipitation and temperatures, as well as soil type, organic matter content, previous crops, organic inputs (manure, etc.), tillage, planting date and population, cultivars, and yield potential. It can therefore provide a locally and seasonally adaptive N recommendation. At the same time, N losses - and therefore pollution of water and greenhouse gas emissions - are minimized. For more information on Adapt-N and sign-up instructions, visit adapt-n.cals. cornell.edu.