Fertilizing Irrigated Corn

Introduction

Adequate soil fertility is one of the requirements for profitable corn production. Nitrogen (N) is the most yield-limiting nutrient, unless previous manure applications or excessive N fertilizer rates leave high residual nitrate (NO₃-N) levels in the soil. Phosphorus (P) is the next most limiting nutrient, while zinc (Zn), iron (Fe), and potassium (K) also may be limiting in some Colorado soils. 

Basis of Fertilizer Suggestions

Base fertilizer rates on realistic expected yields (EY) minus credits for 1) residual soil nitrates, 2) estimated nitrogen mineralized from soil organic matter, 3) previous legume crop residues and manure or other organic wastes, and 4) nitrogen present in irrigation water. These rates also assume adequate irrigation and proper management practices, including weed and insect control. 

Expected corn yields for individual fields are best determined by adding 5% to the most recent five-year average yield of corn, excluding the years when yields are reduced by hail, early frost, etc. EY can be increased by using higher yielding varieties, higher plant populations, or improved irrigation, weed, and/or tillage management. However, EY should rarely change more than 20 bushels per acre from one year to the next. 

Soil Sampling

The value of a soil test in predicting nutrient availability during the growing season is directly related to how well the sample represents the field. Take surface samples from the 0-8 inches soil depth. Subsoil samples should also be taken from 8 to 24 inches deep to determine available NO₃-N. 

Sample 4 to 6 feet if a more accurate N rate is desired, especially with corn. A good sample is a composite of 15 to 20 soil cores taken from an area uniform in soil type. Areas with major differences in soil properties or management practices should be sampled and evaluated as separate samples. 

Thoroughly air dry all soil samples within 12 hours after sampling by spreading the soil on any clean surface in a well-ventilated area where the soil will not be contaminated. Do not oven-dry the soil because this can change the soil test results. Place the air-dried soil in a clean sample container or bag for shipment to the soil test laboratory. 

Submit a carefully completed information form with the soil sample. This form provides information so fertilizer application suggestions can be tailored to your specific situation. Take soil samples for fertility analysis every year for optimum fertilization recommendations. 

Soil tests should include the determination of NO₃-N, extractable P, K, Zn, and Fe, as well as soil pH, soil organic matter, and soluble salts. The laboratory that generates soil test results should be calibrated for Colorado soils. Fertilizer programs for corn are based on such studies. 

Nitrogen Suggestions

Base nitrogen rates for corn on the expected yield for each field. Nearly all corn crops will require some N fertilizer to optimize yield, unless there is a substantial N carryover. High N rates in excess of crop needs can result in groundwater contamination with NO₃-N under irrigated or wet conditions. 

Credit should be given for the level of NO₃-N in the soil. Other credits for N include the amounts expected to be available during the season from mineralization of soil organic matter, manure, and previous legume crop residues, as well as NO₃-N in irrigation water. These credits are subtracted from the total crop needs to determine the suggested N fertilizer rate for the expected yield.

Soil nitrate-N credit

Residual NO₃-N in the soil is immediately available to plants; therefore, decrease the fertilizer rate to give credit for the amount of NO₃-N in the rooting zone. The suggested N rate is reduced 8 lb/A for each ppm of NO₃-N (average concentration in the soil sample depth) in the soil from a 0-24 inch sampling depth. The method to calculate a depth-weighted NO₃-N concentration in the root zone where surface and subsoil samples have been taken is as follows (Table 1): 

Table 1: Example calculation for determining soil N from test results. 

Soil Layer Sampled (inches)	Thickness (inches)	Measured NO₃-N (ppm)	Calculations
0 – 8	8	20	8″ × 20 ppm = 160
8 – 24	16	8	16″ × 8 ppm = 128
Total:			(160 + 128) ÷ 24″ = 12 ppm
Soil N Concentration (0–24 inches):			12 ppm × 8 lb N/A = 96 lb N/A
N Fertilization Deduction:			96 lb N/A

Soil organic matter credit

Nitrogen in soil organic matter becomes available to plants through mineralization. Conditions that favor high yields also favor the activity of soil microorganisms that are responsible for mineralization. Therefore, estimated credits for N released from organic matter are related to expected yields. The suggested N rate is reduced by 14 lb/A for each percent organic matter for each 100 bu/A of corn. When a soil test for organic matter is not available, a level of 1.0 to 1.5 percent organic matter can be assumed for most Colorado croplands. Testing soils is highly recommended because of the large variability in soil organic matter concentration in a given field or across fields. 

Legume and manure credit

Previous legume crop residues will release N to the succeeding corn crop after incorporation and decomposition. Therefore, reduce fertilizer rates by a legume credit (see Table 2).  

Table 2: Nitrogen credits for previous crops and manure application. 

Crop	lb N/A Credit*
Alfalfa > 80% stand 60–80% stand 0–60% stand	100–140 60–100 0–60
Sweet clover and red clover	80% of credit for alfalfa
Dry beans	30 lb N/A
Sugar beets**	50 lb N/A

*For the second year after changing crops, use ½ of the first-year N credit.
**Sugar beets are included due to the incorporation of beet tops; they are not a legume crop.
***For the second and third years after manure application, use ½ and ¼ of the first-year N credits, respectively.
****DM = Dry Matter.

Manure Type	lb N/ton Credit*** (Dry Basis)	lb N/ton Credit (As Is)
Beef	10	5 (at 50% DM****)
Dairy	15	3 (at 20% DM)
Poultry	25	20 (at 75% DM)

Use values toward the lower end of the ranges for sandy soils and use the upper end of the ranges for medium and fine textured soils. 

Understanding manure as a fertilizer 

Manure is a common source of nutrients and is especially beneficial when applied to land that was intensively plowed or leveled for irrigation, where topsoil has been incorporated, removed or lost by erosion. Manure helps improve the soil physical condition and supply N, P, K, and micronutrients to the crop. Hazards from excessive manure applications include potential weed problems, soluble salt buildup, excessive nutrient levels, potential nitrate leaching to ground water, runoff into surface water bodies, and erosion of soils high in P. Application rates should be governed by nutrient needs of the crop and the nutrient composition of manure.  

The N content of manure varies considerably, depending on source, handling techniques and moisture content. Obtain a laboratory analysis for nutrient and moisture content to determine the N credit. In the absence of an analysis, the minimum N credit is 10 pounds per ton for beef feedlot manure and 15 pounds per ton for dairy manure (dry basis) for the first year after application and less for the next two years (see Table 2). For more information on how to apply manure for soil fertility purposes, please refer to CSU Extension Bulletin 568A – Best Management Practices for Manure and Wastewater Utilization https://www.extension.colostate.edu/docs/pubs/crops/568A.pdf. 

Irrigation N credit

Irrigation water may contain NO₃-N, which is available to plants. The amount of N contained in 1 acre-inch of irrigation water contains 0.223 pounds of N for each ppm NO₃-N (alternatively, 1 acre-foot of irrigation water is 2.7 pounds of N for each ppm of NO₃-N). To determine the amount of NO₃-N in your irrigation water, collect a water sample during the irrigation season directly from the source, such as a ditch, well, or pivot, after allowing the system to flush for a few minutes. Use a clean plastic bottle, fill it completely 3 times to rinse the bottle, then fill it to the top to minimize air space, and immediately place it on ice or refrigerate at 40°F to preserve the sample. Ship the sample to a certified lab as soon as possible, ideally within 24 hours, using cold packs to maintain temperature. Request analysis for nitrate-nitrogen (NO₃-N) in ppm to calculate its contribution to your nitrogen budget. For more information on water sampling and selecting an analytical laboratory, please reference CSU Extension Factsheet Selecting an Analytical Laboratory.  

Irrigated Corn for Grain Production

The basis for suggested N rates is an algorithm (equation), originally developed by the University of Nebraska. Nitrogen rate is determined as follows: 

where EY = expected yield and % OM = percent organic matter. 

Step	Formula	Description
1. Calculate the Corn N Need	35 + [1.2 × EY (bu/A)]	Estimate total N needed based on expected yield (EY).
2. Subtract the Soil N (0–24”)	– [8 × average ppm NO₃-N in the soil]	Credit for residual nitrate-N already present in the root zone.
3. Subtract the Organic Matter (OM) Credit (0–8”)	– [0.14 × EY (bu/A) × % OM]	Accounts for N mineralized from soil organic matter.
4. Subtract Other N Credits	– other N credits (lb/A)	Includes credits from legumes, manure, or irrigation water.
Final N Fertilizer Rate (lb/A):		= Total N Need – All N Credits

For example, if your expected grain yield was 225 bushels per acre with the top 2 feet of soil containing an average 5 ppm NO₃-N, 1.0 percent organic matter in the tillage layer, a previous alfalfa field with 30% stand (30 pounds N/A credit), and 24 acre inches of irrigation water containing 5 ppm NO₃-N to be applied during the growing season, the suggested N rate is: 

The University of Nebraska includes an adjustment for corn and N fertilizer prices.  They offer a helpful online tool to determine rates based on their algorithm here: https://agritools.unl.edu/tools/nitrogen. If interested, see their factsheet at https://extensionpubs.unl.edu/publication/ec117/2023/pdf/view/ec117-2023.pdf. 

Calculation Step	Formula	Result (lb N/A)
Total N Needed	35 + [1.2 × 225]	305
Residual Soil Credit	– 8 × 5 ppm NO₃-N	–40
Organic Matter Credit	– 0.14 × 225 × 1.0% SOM	–31
Legume Credit	–30	–30
Irrigation Water Credit	– 5 ppm NO₃-N × 0.223 lb × 24 acre-inches	–27
Calculated N Rate		177 lb N/A

Table 3 suggests N rates for irrigated corn at an expected yield of 225 bushels per acre. Fertilizer N rates decrease with increasing levels of NO₃-N in the top 2 feet of soil and increasing soil organic matter content. Suggested N rates in this table do not account for other N credits. Subtract N credits for manure, legumes, and irrigation water from the N rates in Table 3 to determine the N rate for the field. Rates are rounded to the nearest 5 pounds of N/A. For more precise rates, calculate the N rate for your field by using the algorithm above, using the appropriate expected yield. 

Table 3: Suggested nitrogen rates (lb/A) for irrigated grain corn, assuming 225 bu/A, as related to NO₃-N in the soil and soil organic matter content. 

*Average weighted concentration (ppm) in the tillage and subsoil layers to a depth of 2 feet.
Note: Credits for N from manure, irrigation water, or previous legumes should be subtracted from the above N rates.

ppm NO₃-N in Soil*	Soil Organic Matter (%)
	0 – 1.0	1.1 – 2.0	> 2.0
0 – 6	250	220	185
7 – 12	200	165	135
13 – 18	150	120	85
19 – 24	100	70	40
> 24	80	50	20

Timing of N Applications

The corn N rate algorithm presented here has been validated through many field demonstrations and is generally considered a best practice throughout Colorado to determine the quantity of N to apply to a given corn crop. However, the algorithm does not provide an idea of when to apply N fertilizer, but much research has been done on this, which has resulted in Best Management Practices that should be utilized when possible. 

N uptake in corn significantly increases starting at the six-leaf (V6) growth stage and tapers off near the blister (R2) growth stage. By the time the crop reaches R2 growth stage, approximately 150 lb N/A will have been taken up by the crop (from all N sources, not just fertilizer). Ideally, N would be steadily added during these growth stages to match the uptake without overapplying. However, this is not often possible due to limitations in the methods of N application. Therefore, the most efficient approach is by applying N just prior to the rapid growth period 30 to 40 days after planting at the V6 growth stage. Additionally, apply any remaining N fertilizer before tasseling to maximize N use efficiency.  

Fall application of N is not recommended for most soils. Some N may be band-applied in combination with starter fertilizers, but the rate should be less than 20 pounds of N per acre. Use of planter attachments with the standard 2-inch by 2-inch placement (2 inches below and beside the seed row) is preferred for starter fertilizers. Use caution with popup placement (directly with the seed) of fertilizers, including those with K and S, because seedling emergence may be decreased in dry soil, especially at rates supplying more than 10 lb N/A. All sources of N fertilizers are equally effective per unit of N if properly applied. Base your choice of N fertilizer on availability, equipment available, and cost per unit of N. 

Corn roots grow quickly into the soil between the rows. Sidedress N fertilizers early in the growing season to avoid root pruning. Apply N fertilizer during early cultivation (i.e., at V6 growth stage). 

Applying N with Irrigation Water

Application of N fertilizers with irrigation water (i.e., “Fertigation”) is a convenient method and allows split applications to improve N use efficiency. Use in-season soil or plant analysis to determine the nutrient status of the growing crop. If the N status of the crop is low or growing conditions appear to be above average, apply additional N with the next irrigation. 

Nitrogen fertilizers may be applied through sprinkler irrigation systems. Equip all closed-irrigation systems with backflow prevention valves if N fertilizers or other agrichemicals are applied through the system. Urea-ammonium nitrate (UAN) solution is the most efficient N fertilizer to apply through sprinkler systems. Anhydrous ammonia is not recommended for application in sprinkler systems because of N losses as ammonia and problems due to formation of solids in the water. 

Apply N fertilizers in furrow irrigation systems only in fields where a tailwater recovery and reuse system is in place. For high-efficiency surge-flow irrigation systems, addition of the N fertilizer during the next to last cutback cycle improves the uniformity of application. Bubbling anhydrous ammonia into head ditches may result in N losses to the air as ammonia. 

Foliar spray applications of N are not practical since only relatively small amounts of N can be absorbed through the leaves. Also, substantial leaf burn may result if the N concentration in the foliar spray is too high or if sprays are applied during hot, dry weather. 

Modern remote sensing technologies have made it possible to determine corn N status during the growing season more readily. These sensors include unmanned aerial vehicles (UAV; e.g., drones), satellites, and on-ground cameras that use different wavelengths of light to detect developing N deficiency before it becomes visible to the human eye. Such technologies should be considered when available, as they have been shown to significantly improve nitrogen use efficiency when used to inform in-season N fertilization. 

Phosphorus Suggestions

Crop responses to applied P are most likely on soils with low or medium levels of extractable P. Suggested P fertilizer rates (Table 5) are determined from an algorithm related to the soil test extraction used (Mehlich-3 or NaHCO₃) and the method of fertilizer application. The algorithm for determining the suggested P rate for banded fertilizer applications based on each soil test method is: 

P rate (banded, lb P₂O₅/A) = 48 – 1.6x (Mehlich-3-P) 

P rate (banded, lb P₂O₅/A) = 48 – 2.5x (NaHCO₃-P) 

where x = ppm available P in soil. 

The main soil tests for extractable P in Colorado soils are the Mehlich-3 and sodium bicarbonate (NaHCO₃) tests (also known as Olsen P). Values for both tests are given in Table 5. When using the above algorithms to calculate the suggested P rate, a negative P rate means the probability of response is lower at higher soil test levels and application of fertilizer P is not suggested. 

Placement of P fertilizers in the root zone is important because P is not mobile in the soil. Incorporate broadcast applications of P fertilizers into the soil prior to planting. Band application at planting (starter fertilizer) is the most efficient placement method for P, and suggested rates for band application are about half those for broadcast application. Subsurface placement of P is especially important for reduced tillage cropping systems. Use caution with popup fertilizer placement (directly with the seed) because seedling emergence may be decreased in dry soil, especially at rates supplying more than 10 pounds of N per acre. Monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (DAP, 18-46-0), and ammonium polyphosphate (10-34-0) are equally effective per unit of P if properly applied. Base your choice of fertilizer on availability, equipment available and cost per unit of P. 

Soils that have had manure applications will require less P fertilizer because much of the P in animal manure builds up in soils over time. Do not apply manure to high-P soils because of lower probability of crop response to P and to avoid potential surface water contamination with P due to runoff and soil erosion. 

Table 5: Suggested phosphorus rates for band and broadcast applications to irrigated corn.

For more precise rates, use the algorithm in the text relating to the soil test method.

ppm P in Soil	Relative Level	Fertilizer Rate (lb P₂O₅/A)
		Banded	Broadcast
0 – 10 (Mehlich-3) 0 – 6 (NaHCO₃)	Low	40	80
11 – 22 (Mehlich-3) 7 – 14 (NaHCO₃)	Medium	20	40
23 – 35 (Mehlich-3) 15 – 22 (NaHCO₃)	High	0	0
> 35 (Mehlich-3) > 22 (NaHCO₃)	Very High	0	0

Potassium Suggestions

Most Colorado soils are relatively high in extractable K, and few crop responses to K fertilizers have been reported. Suggested K rates related to soil test values (NH₄OAc extractant) are given in Table 6. Low levels of extractable K can cause lodging of corn, but this problem more often is caused by stalk rot than by shortages of extractable K in the soil. Potassium removal from soil is much greater with production of corn silage than grain, but soil minerals generally will release K to replace that which was removed by crops. Use soil tests to monitor extractable K levels in fields mainly cropped for corn silage. Plant tissue testing and symptom identification may also help confirm a K deficiency, as they are rare in Colorado. The main K fertilizer used in Colorado is KCl (potash), and broadcast application incorporated into the soil prior to planting is the usual method. Most K fertilizers can also be broadcast without incorporation in no-tillage systems. Pre-plant application is recommended. 

Table 6: Suggested potassium rates for irrigated corn. 

ppm K in Soil (NH₄OAc)	Relative Level	Fertilizer Rate (lb K₂O/A)
0 – 60	Low	60
61 – 120	Medium	30
> 120	High	0

Sulfur Suggestions

Most Colorado soils contain adequate levels of available sulfur (S). Gypsiferous soils contain adequate S. However, some sandy soils with low organic matter may require S applications (Table 7). Ammonium Sulfate (AMS; 21-0-0-24S) is often used to meet both N and S needs in systems where both are needed and is immediately available to the plant because it releases sulfate (SO₄) as opposed to S. Elemental S (e.g., 0-0-0-90S) is not water soluble nor immediately available to plants. First, it must be oxidized to the plant-available sulfate form by soil microorganisms, and therefore it is recommended to apply it in the Fall to allow time for this to occur.  

Irrigation water from most surface waters and some wells often contains appreciable SO₄-S, so irrigated soils usually are adequately supplied with S. However, some deep well waters are low in S, so water samples should be analyzed for SO₄-S if soils are low in organic matter and S deficiency is suspected. Be sure to consider the SO₄-S content of your irrigation water when deciding whether to apply S fertilizer (Table 7). 

Table 7: Suggested sulfur fertilizer rates for irrigated corn grown on sandy soils only. 

¹ Use Ca(H₂PO₄)₂ extract for SO₄-S analysis.
² Applied in a band next to row but not with seed.

Soil Test (ppm SO₄-S)1	Amount S to Apply (lb/A)
	Soil Organic Matter <1% (Broadcast)	Soil Organic Matter <1% (Banded)2	Soil Organic Matter >1% (Banded)2
Irrigation water with <6 ppm SO₄-S
<6	20	10	5
6 – 8	10	5	0
>8	0	0	0
Irrigation water with >6 ppm SO₄-S
<6	10	5	0
6 – 8	10	5	0
>8	0	0	0

Zinc Suggestions

Zinc (Zn) availability decreases with increasing soil pH, and most Zn deficiencies are reported on soils with pH levels higher than 7.0. Zinc deficiencies of corn have been widely reported in eastern Colorado soils. They are commonly found on soils leveled for irrigation where subsoil is exposed, or on soils with high levels of free lime. Incorporation of manure or treated sewage sludge (biosolids) in these exposed subsoils may correct Zn deficiencies, as well as improve soil structure and health. 

Table 8 shows suggested Zn rates for banded and broadcast applications based on soil test values. Applications are based on the use of zinc sulfate (ZnSO₄), and soil test extractable Zn values are assumed to be determined by DTPA. Apply effective Zn chelates at about one-third of the rate of Zn as ZnSO₄. Band application is more effective than broadcast application; thus, suggested rates are considerably lower for band application. Several Zn sources (both solid and liquid) are sold, and their relative effectiveness and cost per unit of Zn vary considerably. 

Table 8: Suggested zinc rates for band and broadcast applications to irrigated corn. 

*Rates are based on zinc sulfate applications.

ppm Zn in Soil (DTPA)	Relative Level	Fertilizer Rate (lb Zn/A)*
		Banded	Broadcast
0.1 – 0.9	Low	2	10
1.0 – 1.5	Marginal	1	5
> 1.5	Adequate	0	0

Zinc deficiencies also may be corrected by foliar sprays of a 0.5 percent ZnSO₄ solution applied at a rate of about 20 to 30 gallons per acre, but several applications may be necessary. However, it is difficult to prepare this solution in the field so Zn-EDTA or other soluble Zn sources can be used. A surfactant (wetting agent) increases plant absorption of the applied Zn. 

Zinc fertilizers applied to the soil have measurable residual effects, and repeated annual applications will result in a buildup of extractable Zn in the soil. Because of these residual effects, annual soil tests are suggested to assess extractable Zn levels in soil. As soil test Zn increases to higher levels in soil, decrease Zn rates according to soil test results. 

Iron Suggestions

Availability of iron (Fe) decreases with increasing soil pH, but most soils are adequately supplied with available Fe for corn production. Iron deficiencies are most likely to occur on highly calcareous soils (pH higher than 7.8) or on soils leveled for irrigation where the subsoil has been exposed. Visual symptoms of Fe chlorosis are white or yellow striping of younger leaves. Select corn hybrids that have tolerance to chlorosis as this may be adequate for overcoming Fe problems. 

If chlorosis persists, Fe fertilizers may need to be applied. Research by University of Nebraska scientists shows the most effective treatment for correcting high pH chlorosis in corn requires an at-planting, seed-row application of 50 to 100 pounds of ferrous sulfate heptahydrate (FeSO₄.7H₂O) per acre. This treatment requires dry fertilizer application equipment on the planter. Foliar spray applications of a 1 percent FeSO₄ solution at 20 to 30 gallons per acre are not always completely effective in correcting chlorosis, and several applications may be necessary. FeSO₄ solutions are difficult to prepare in the field and other Fe sources may be used. In addition, soil application of manure, compost or treated sewage biosolids can often help to correct Fe deficiencies of crops. Sewage biosolids also may contain some heavy metals; heavy metal loading limits for soil are controlled by Colorado Department of Public Health and Environment regulations. 

Other Nutrients

There have been no confirmed deficiencies of boron (B), copper (Cu), manganese (Mn), or molybdenum (Mo) in corn in Colorado. However, it is important to regularly test soil and plant tissue samples to confirm this. The greatest source of micronutrients in soil is from the decomposition of organic matter.  As a result, soils with low organic matter and low clay content are more likely to be deficient in these micronutrients. Additionally, the use of manure as a fertilizer, or the integration of livestock into the agricultural system, further reduces this risk, as manure typically contains many of these micronutrients. Soils with a pH greater than 7.5 may see a decrease in available B, Cu, and Mn.