FERTILIZATION AND LIMING FOR CORN
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
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
|Corn, sorghum, soybeans, small grain
||100 - 125
||150 - 1753
||175 - 2003
||125 - 150
||175 - 2003,4
||75 - 100
|Grass, grass-legume sod (5 years or more)
||50 - 75
||100 - 1253
||125 - 1503
||75 - 100
||125 - 1503,4
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
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
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).
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).
|Dairy cattle (80% water)
|Hogs (75% water)
|Poultry (55% water)
|Horses (60% water)
|Sheep (65% water)
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.
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
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.
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.
|Soil Test Value
|High (above 60 P, 300 K)
|Medium (60-30 P, 300-190 K)
||0 - 60
||0 - 60
|Low (below 30 P, 190 K)
||60 - 1201
||60 - 120
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
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
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.