- Production -

Manage Fertility

Correct application of nutrients is essential for
healthy, high-yielding corn.

By Pawel Wiatrak and Jim Camberato

EDITOR’S NOTE: Following are highlights from the Soil Sampling Management and Fertility Management section of the South Carolina Corn Production Guide. To view the entire text and additional tables showing recommended rates of nitrogen, potassium and phosphorus for corn grain and corn silage based on yield goals, go to www.clemson.edu/extension/, “rowcrops, “corn” and “guide.” The recommended rates of phosphorus and potassium for dryland and irrigated corn grain are shown here in Tables 1 and 2.

It is very important to perform soil tests and apply lime and fertilizer if needed to maximize yield potential. More information on these topics can be found at www.clemson.edu/agsrvlb.htm on new page 16. Plant nutrient applications of fertilizer and/or lime should be based on soil tests, which help determine soil pH and nutrient levels and the need for lime and/or fertilizer applications.

The following are some considerations regarding the nutrient management practices related to starter fertilizer, nitrogen (N), phosphorus (P) and potassium (K) to help with maintaining high corn yields.

Starter Fertilizers
Research in several states in the Southeast has shown that the placement of some fertilizers near the seed at planting may increase grain yields in years of good rainfall or when corn is grown with irrigation. Usually this fertilizer is placed two inches to the side and two inches below the corn seed. Higher yields will usually be achieved on sandy soils if the starter fertilizer is applied.

Applying phosphorus as a starter may be beneficial if the soil test shows phosphorus to be very low or if corn is planted in a cold soil. Research has shown that applying other nutrients in this manner may help to increase early plant growth, but may not always increase the final grain yield.

Table 1. Fertilizer (P2O5/K2O) recommendations for dryland corn based
on soil test results (reported in K and P) in South Carolina.*
Soil Test Rating Potassium (K20)

Phosphorus
Low
lb/A
Medium
lb/A
High
lb/A
Very High
lb/A
(P2O5)        
Very Low 100-80 100-50 100-0 100-0
Low 80-80 80-50 80-0 80-0
Medium 50-80 50-50 50-0 50-0
High 0-80 0-50 0-0 0-0
Very HIgh 0-80 0-50 0-0 0-0
 
* Source: EC476, Nutrient Management for South Carolina.

 

Table 2. Fertilizer (P2O5/K2O) recommendations for irrigated corn based
on soil test results (reported in K and P) in South Carolina.*
Soil Test Rating Potassium (K20)

Phosphorus
Low
lb/A
Medium
lb/A
High
lb/A
Very High
lb/A
(P2O5)        
Very Low 120-100 120-60 120-30 120-0
Low 100-100 100-60 100-30 100-0
Medium 60-100 60-60 60-30 60-0
High 30-100 30-60 30-30 30-0
Very HIgh 0-100 0-60 0-30 0-0
 
* Source: EC476, Nutrient Management for South Carolina.

 

Nitrogen
Corn plants generally accumulate up to two-thirds of their total nitrogen two weeks before and two weeks after tasseling. Recent research has shown, however, that many stay-green hybrids accumulate even a greater proportion of their total nitrogen during the grain-filling period.

Therefore, nitrogen applications must be properly managed to ensure that adequate nitrogen is available to the plants during this period of development.

For most corn under dryland conditions, a rate from 120 to 150 pounds per acre should be sufficient. For very sandy soils with yield potentials of 80 bushels per acre or less, 100 pounds of nitrogen per acre is adequate. Use from 150 to 180 pounds of nitrogen per acre on river bottoms or on soils with high water and nutrient holding capacities that produce yields of 130 bushels per acre or more.

When corn is irrigated, apply 180 to 225 pounds of nitrogen per acre. Reduce the rate by 20 to 30 pounds per acre when corn follows soybeans in a rotation. When a legume cover crop is grown and turned under before planting corn, reduce the rate of nitrogen applied by 50 to 80 pounds per acre.

When animal manure is applied to the field, adjust inorganic fertilizer nitrogen rates for the nitrogen supplied by the manure. Since animal waste is highly variable in nutrient content (nitrogen, phosphorus and potassium), laboratory analyses are needed to determine its nutritive value. Animal waste analyses for crop land application can be obtained from the Agricultural Service Laboratory of Clemson University. See your county Extension agent for submission forms and sampling instructions.

All sources of nitrogen are equally effective for corn production when applied correctly. To increase the period of availability, growers may consider using slow-release forms of nitrogen or add a nitrification or urease inhibitor to the fertilizer. When nitrogen is applied with irrigation water (fertigation), the additional nitrogen may be applied over two or three applications.

Phosphorus
Phosphorus is relatively immobile and moves very little in the soil profile under most conditions. Most of phosphorus is tightly bound to soil particles and unavailable to plants. Over-fertilization of soils with poultry litter, dairy wastes and other animal manure contributes to build up of very high levels of phosphorus and may result in leaching in sandy soils.

Early in the growing season, deficiency symptoms (purple leaves) may appear during periods of cold weather even when soil phosphorus is adequate because root growth is slowed to a relatively greater extent than vegetative growth. Small ears, undeveloped kernels at the ear tips and twisted or bowed ears indicate that phosphorus deficiency occurred during reproductive development.

The rate of phosphorus fertilizer to apply can only be determined by soil testing. A broadcast application is usually satisfactory, provided the fertilizer is incorporated into the soil. When the soil test for phosphorus is low, or corn is planted in cold soil, initial growth may be improved if part of the phosphorus is banded two inches below and two inches to the side of the seed. This practice may not always result in higher grain yields.

Potassium
Potassium is important to maintain the plant’s salt balance and regulate water and sugar movement within the plant. Deficiency symptoms are usually first detected on older leaves and include the yellowing and dying of plant leaf margins beginning at the tips of lower leaves. Ears will not be filled at the tip, and the stalks will be weak and may possibly lodge. A single application of potassium is sufficient on most soils, but split applications are recommended on very sandy soils.

The rate of potassium fertilizer to apply must be determined by soil testing. Plants will take up more potassium than needed (luxury consumption) if excess potassium is added to the soil. A high rate of potassium application may induce magnesium deficiency on soils with low magnesium levels.

Wiatrak (pwiatra@clemson) is an Extension agronomist at Clemson University. Camberato (jcambera@purdue.edu) is an Extension soil fertility and plant nutrient specialist at Purdue University.


Split Applications Increase N Efficiency

A broadcast application of nitrogen immediately before or after planting may be the most convenient method to apply nitrogen to the soil, but it is the least effective. Nitrate, the most prevalent form of nitrogen in the soil, is a very mobile compound in sandy soils and is subject to leaching during periods of heavy precipitation.

Consequently, nitrogen fertilizer should be applied in a split application to increase the efficiency of nitrogen application. Apply 30 to 40 pounds of nitrogen at planting. The start-up nitrogen application may be combined with a small amount of phosphorus. The early phosphorus application may help to improve its uptake under cool soil temperatures, which generally reduce phosphorus uptake under cool soil conditions.

The best method to apply this nitrogen is to band it two inches to the side and two inches below the seed or on the soil surface, two to four inches to the side of the row. Application too close to the germinating seed or emerging plant may cause severe salt injury. Also, fertilizer salts may cause plants to wilt in low moisture soils by pulling water away from corn roots.

Another method is to apply the nitrogen in a band over the row. If banded over the row, use the lower rate of nitrogen and have the band at least eight inches wide to avoid injury to the seed or seedlings. The remainder of the nitrogen rate for corn should be applied when the plants are 15 to 30 inches high (between six and eight fully expanded leaves on the plants) – just before corn begins rapid growth and development.