ISSUED: 2-83
M. S. Smith, K. L. Wells and G. W. Thomas

Nitrogen (N) is the fertilizer element required in largest amounts, and at greatest cost, for corn production. Each bushel of grain harvested will contain almost a pound of N. Properly fertilized silage corn removes slightly over 10 pounds of N for each 1,000 pounds of dry matter. Unfortunately, even more N must be provided by the soil or by fertilizer since only one-third to two-thirds of the N added is recovered in the harvested corn. Most of the unrecovered N is irreversibly lost from the soil.
Nitrogen recovery is not only variable but also unpredictable. The unpredictability is primarily due to the powerful effect of weather on the release of native soil N and on the fate of fertilizer N. This makes it impossible to predict precisely the quantity of N required for maximum yield or maximum economic return. This also makes it impossible to perform a meaningful soil test for N in Kentucky's climate.
The recommended N rates in Table 1 are based on many years of field experiments on many soil types. They are the best available approximations, but they will not apply to all locations in all years. Producers should realize that experience, careful observation and economic considerations can be valuable in fine-tuning these recommendations to their own fields.

Table 1. Recommended Ranges of Nitrogen Application (in lb/A) to Corn.1

Previous Crop
Conventional Tillage


Corn, sorghum, soybeans, small grain 100 - 125 150 - 1753 175 - 2003 125 - 150 175 - 2003,4
Grass, grass-legume 75 - 100
Grass, grass-legume sod (5 years or more) 50 - 75 100 - 1253 125 - 1503 75 - 100 125 - 1503,4
1 Nitrogen rate for irrigated corn: Due to increased risk of available N depletion from crop uptake, leaching and denitrification, apply 25 lb N/A more than the maximum shown in the appropriate recommended range above.
2 Poorly drained soils that have been tilled will require about 25 lb/A less N.
3 N rates can be decreased 25-50 lb/A on poorly or moderately well-drained soils if 2/3 or more of the N is applied 4-6 weeks after planting.
4 Nitrification inhibitors are most likely to be valuable on these soils.

Nitrogen in the Soil
Biological and chemical reactions are continuously changing soil N from one form to another, and each form behaves very differently. The agrinomically important transformations of nitrogen are diagramed in Figure 1.

Organic soil N
Organic soil N is mostly insoluble and not directly available to plants. Virtually all soils contain a large quantity of organic N. A soil that has 3 percent organic matter contains over 3000 pounds of organic N per acre. However, only a small part of this, 1 to 5 percent each season, is broken down to inorganic N forms and so made available to plants. A greater rate of N release can be expected from fresh plant residues and from plowed-down or killed grass and legume sods. For this reason, cropping history is an important consideration when estimating fertilizer requirements.

Inorganic ammonium (NH4+)
Inorganic ammonium (NH4+) is either released by organic matter decomposition or added as fertilizer. Ammonium is a relatively immobile ion, behaving much like K+. Under some conditions, it can be lost from the soil as ammonia (NH3) gas (see "N Sources" below) but it is not susceptible to leaching or denitrification as is nitrate (NO3-). On the other hand, corn takes up NH4+ less readily than NO3-. In most Kentucky soils suitable for corn production, NH4 is rapidly converted to NO3- in a process called nitrification. This reaction is largely completed with in a few days to a month after fertilization.

Nitrate is a highly mobile ion because its solubility in water is essentially unlimited and because it does not interact significantly with the clays or organic matter of most Kentucky top soils (Crider-Pembroke soils are an exception). Since NO3- moves with soil water, it is readily available to plants but also is susceptible to leaching below the root zone. Leaching losses of fertilizer N are most significant on well-drained soils during long-lasting or very intense rainfall. Denitrification (see Figure 1) is a microbiological reaction that can proceed very rapidly when soils become water-saturated. Therefore, it is most important in soils with slow drainage characteristics in the rooting zone.
Some nitrate is lost almost every year in all Kentucky soils but such losses become serious when heavy rains fall within a month after fertilization. Soil drainage is the most reliable predictor of total loss so more N fertilizer is required in poorly drained soils. The tillage system also influences these processes. Denitrification, leaching and immobilization can all be greater in no-till soils, so N rates should generally be increased slightly when using no-till (Table 1).

Fertilization Practices

With only a few exceptions, plants respond equally to all inorganic forms of fertilizer N. Ammonium forms are generally equivalent to NO3- forms because they are converted to NO3- so rapidly. Urea (NH2-CO-NH2) can be an important exception. Urea is rapidly converted to NH4+ in the soil which makes the soil near the granule more alkaline. This, in turn, favors the formation of NH3 gas from NH4. As a result, a large fraction of the N sometimes can be volatilized as NH3 if urea is broadcast on a moist, warm soil. Losses can be minimized by avoiding these conditions or by incorporating the urea. In some locations, urea is available at a lower price than alternative N forms and so it may be more economical to use, despite the greater risk of N loss. Urea should not be considered a slow release fertilizer. True slow release N sources currently are not economically feasible for corn production.
Organic sources of N should not be ignored when estimating N fertilizer needs. Sods, legume cover crops and animal manure (Table 2) can replace a large portion or, in some cases, almost all of the inorganic N application.

Table 2. Fertilizer value of some animal manures. For each ton/A of material applied, reduce fertilizer rates by the amount shown (in lb/A).
Source N P2O5 K2O
Dairy cattle (80% water) 11  12 
Hogs (75% water) 10 
Poultry (55% water) 31  18 
Horses (60% water) 14  14 
Sheep (65% water) 28  10  24 

Nitrogen applied as a liquid, either solution or suspension, is not necessarily more effective than solid N. The choice between liquids and solids should be based on cost, handling and convenience factors.

Probably the most practical and effective method of increasing N recovery by corn is to delay or split the N application. This practice works because young corn plants (up to 4 to 6 weeks) require very little N and most can be supplied by the soil. Also, soils are wettest and so most prone to N losses early in the season. Delayed applications are most beneficial where denitrification and leaching losses are greatest, particularly on poorly or moderately well-drained soils. As a general guideline for these soils, if two-thirds or more of the N is applied 4 to 6 weeks after planting, the total N application can be reduced by 25 to 50 pounds per acre. Fall application of N for corn is never recommended in Kentucky.

Placement has no consistent, significant effect on the efficiency of N fertilizers containing NH4 and NO3- salts. Researchers have sometimes observed benefits of banding or sub-surface application under certain conditions, but the evidence is not yet complete enough to justify the machinery alterations and extra time which will usually be required. Incorporating urea immediately after application will reduce the risk of volatilization.

Nitrification Inhibitors
These chemicals act specifically to inhibit the conversion of NH4 to NO3- and therefore indirectly reduce the chance of early season leaching and denitrification. These chemicals are beneficial only when such losses are potentially significant. In Kentucky, plants have responded consistently to inhibitors only on less than well-drained no-till soils. If a nitrification inhibitor is used on these soils, about 25 pounds less N can be applied.

Phosphorus and Potassium
Both phosphorus (P) and potassium (K) are required in large quantities for good corn growth and yields. A good yielding crop will take up 30 to 40 pounds of phosphate (P2O5) and 100 to 150 pounds of potash (K2O) per acre. Of this total uptake, about three-fourths of the phosphate and about a third of the potash is in the grain, the remainder being in leaves, stalk, roots, husks and cob. So for a grain production system where all crop residues are left on the field, 20 to 30 pounds P2O5 and 35 to 40 pounds K2O are removed from the soil each year. In silage production, all P2O5 and K2O taken up by the plant, except that in the roots and stubble, is removed from the soil.
It is particularly important that adequate P2O5 and K2O be available for plant uptake during the first half the season. By the time kernels start filling rapidly (70 to 75 days after seedling emergence and 10 to 15 days after silking), the plant will have taken up about 70 percent of its P2O5 requirements and nearly 90 percent of its K2O requirements.

Availability from Soil
Both P and K are considered immobile elements since they react with the soil in ways that minimize their movement with soil water. This is particularly true for P since it forms compounds with soil calcium, iron, aluminum, manganese and zinc which are less soluble than the P compounds in the fertilizer. If soil pH is in the range of 6.0 to 6.5, much of the fertilizer P will react to form monocalcium or dicalcium phosphates which are more soluble than the iron, aluminum and manganese phosphates that form at lower pH levels. Therefore, increased P availability is one benefit of good liming practices. Potassium, like NH4, is retained on clays and organic matter by cation exchange. Except very sandy soils, soil cation exchange capacity is great enough to hold an adequate reservoir of readily available K+. For these reasons, leaching of P and K from Kentucky soils is of little importance. In comparison, loss of P and K by erosion of topsoil is of much greater concern.
Corn grown on fields being rotated from a sod may respond less to P fertilization than expected based on soil analysis. This is because P will be released as organic residues from the sod decompose.

The amount of P and K fertilizer required for good corn growth is directly related to the amount of plant-available P and K already in the soil. Using a reliable soil testing lab which employs field-tested procedures is the best way to determine soil content of plant-available P and K. Recommendations made by the University of Kentucky, based on the Mehlich III extraction, are shown in Table 3. The annual amount of P and K taken up by the plant from fertilizer is not likely to exceed 15 to 20 percent of the P or 25 to 40 percent of the K applied.

Table 3. Recommended rates of phosphate and potash application (in lb/A) for corn, as related to soil test value.
Soil Test Value P2O5 K2O
High (above 60 P, 300 K) 0 0
Medium (60-30 P, 300-190 K) 0 - 60 0 - 60
Low (below 30 P, 190 K) 60 - 1201 60 - 120
1 Where soil test values indicate extremely low levels of P in the soil (less than 5) and where fertilizer P must be broadcast and disked into the soil, up to 200 lb of P2O5/A may be beneficial.

Commercial fertilizers are the most widely used source of P and K for corn production. Sources of P most commonly used are triple superphosphate (0-46-0), diammonium phosphate (18-46-0), monoammonium phosphate (11-48-0), and a wide array of other ammoniated phosphates, both liquid and dry. Most commonly used sources of fertilizer P are considered equally effective for agronomic purposes when used at recommended rates and properly applied. Solid and fluid forms of P also are considered equally effective.
Almost all K fertilizer used for corn is muriate of potash (0-0-60). Other available sources are sulfate of potash (0-0-50) and sulfate of potash magnesia (0-0-18, 11 S, 18 Mg). All are considered equally effective.
Organic sources of P and K such as animal manures and sewage sludge may also be used for supplying P and K. Table 2 shows approximate nutrient values of some animal manures. The nutrient content of sewage sludge varies, so analysis is necessary to determine how valuable it will be as a soil amendment. Also, it is important to know the content of heavy metals (nickel, cadmium and chromium) to prevent toxic buildup.

Broadcasting P and K is the most convenient method of application, although at low to very low soil test levels, large amounts are required. Banded applications (2 inches to the side and 2 inches below the seed) can increase agronomic efficiency of P and K, making it possible to decrease the usual rate by one-third to one-half. A "starter" effect, or improved initial growth, is likely to result from a banded placement. This may appear very significant during the early growing season, but in Kentucky it rarely increases yields, provided that P and K are used at recommended rates.

Other Nutrients
Zinc deficiencies in corn are common on Kentucky limestone soils, particularly when soil pH is above 6.5. Soil tests for zinc help to predict where the problem can occur, but not with absolute reliability. Zinc deficiencies can be corrected in two ways: broadcasting 20 to 30 pounds per acre or banding 6 pounds per acre of actual zinc. The former treatment should last at least 4 to 6 years. Banding needs to be repeated annually for 6 to 8 years and perhaps occasionally after that.
Boron deficiencies have been documented in Kentucky but they are probably not common. Plant tissue analysis (see Extension publication AGR-92 for details and procedures) is the best way to test for this deficiency. If the ear leaf sample contains less than 5 parts per million (ppm) and if the soil test value is less than one ppm, applying 2 pounds of boron per acre might be beneficial.
Deficiencies of other nutrients are extremely unlikely for corn in Kentucky. If a problem is suspected, tissue analysis is recommended.

Corn is somewhat less sensitive to acid soils than the crops with which it is usually rotated-wheat and soybeans. Nevertheless, at very low pH, corn suffers from both manganese and aluminum toxicity. Manganese toxicity causes striped leaves and stunted growth whereas aluminum toxicity results in poor root growth with consequent drought injury. Both symptoms are common in soils with pH values of 4 to 5. Above pH 5, no symptoms are likely although improvement in growth may continue to pH 5.5. Above pH 5.5, no yield response of any kind is to be expected. Furthermore, at pH values of 6.5 or higher, corn is very susceptive to zinc deficiency. Therefore, a pH near 6.0 is safest.
Corn can tolerate moderately acid soils, but growers need to keep two points in mind. First, adding N fertilizer to corn greatly accelerates the acidification of soils. Second, no-tillage further adds to this problem. In no-till corn fields, the soil surface can become very acid (below pH 5) within three or four years. Once this happens, corn is susceptible to aluminum and manganese toxicity. In addition, the triazine herbicides (atrazine and simazine) are deactivated by acid soil so that weed control is more of a problem.
The best liming program for corn involves a soil test every two years and lime applications according to soil test recommendations. On the average, one can expect to add the equivalent of about one-half ton of lime per year, but usually added at the rate of 2 to 3 tons every 3 to 6 years.

Quality of Lime
Over the past few years, we have noticed a distinct increase in the amount of coarsely-ground lime that is sold as agricultural limestone in Kentucky. We suggest that farmers check the quality of lime at quarries to find their best source. Lime larger than 10 mesh (1/10 inch in diameter) is useless for agricultural purposes. If a handful of lime is half rock chips of this size, it will be essentially half as effective. It is worthwhile to take the time to find good quality lime with relatively few large particles. The expense will be no greater and the lime will be much more effective.