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Fertility Programs

An effective fertility program is key to achieving yield goals.

By David Wright

Liming increases bacterial activity that breaks down soil organic matter to make soil nitrogen and other nutrients more available to the crop.

Agood fertility program should be based on the soil fertility level as determined by soil tests and yield goal. Fertilization programs not based on soil tests may result in excessive and/or sub-optimum rates of nutrients being applied. Soil samples should be taken each fall to monitor the fertility level.

Coastal Plain soils are naturally acid and infertile. Therefore, substantial quantities of lime and fertilizer are required for optimum yields. Corn cut for silage requires more nutrients than corn grown for grain because cutting silage removes all of the nutrients from the field in the above-ground plant parts. The removal of potassium is especially large in comparison to grain harvest.

Lime

Many fields do not have to be limed for corn if grown in rotation with peanuts, which are normally limed. Fields that have continuous corn or are rotated with other grass crops may become acid due to (1) use of high amounts of nitrogen that is acid forming, (2) leaching of calcium and magnesium, and (3) nutrient removal by high-yielding crops, especially silage. Corn grows well in soil with a pH of 5.6 to 6.2. Soil with a pH below 5.2 can fix plant nutrients, especially phosphorus, in forms unavailable to plants.

Since most bacteria cannot live under very acid conditions, liming increases bacterial activity that breaks down soil organic matter to make soil nitrogen and other nutrients more available to the crop. Likewise, herbicide activity of triazine herbicides is most effective when the soil pH is between 5.8 and 6.5. Magnesium is seldom a limiting nutrient in corn production if dolomitic limestone is the lime source. Corn often shows magnesium deficiency even when soil levels are adequate during the peak period of nitrogen uptake (40 to 70 days after planting), but symptoms usually disappear after 10 to 14 days.

Nitrogen

Nitrogen is very mobile in sandy Coastal Plain soils and can be lost if excessive rainfall occurs. To increase the efficiency of nitrogen recovery during the season, split applications of nitrogen are recommended. Nitrogen is typically the most limiting nutrient for high yields. A rough rule of thumb is that the crop needs 1.2 to 1.3 pounds of actual nitrogen for each bushel of corn produced. About 20 to 25 percent of the nitrogen needs of the crop can be applied at planting as a starter fertilizer near the row. The remaining nitrogen can be applied sidedress and/or injected through the centerpivot systems (fertigation). If all the nitrogen is applied with ground equipment, apply 35 to 45 lbs/A at planting and the remainder when corn is 12 to 15 inches tall. In addition, overhead irrigation may enable N to be applied later in the season through the irrigation water. No yield increase has been found from nitrogen applied after the silk-and-tassel period. If nitrogen is to be injected through the irrigation system, apply 30 to 40 pounds at planting and make a sidedress application of 30 to 50 pounds of nitrogen per acre when the corn is 12 to 15 inches tall. The remaining applications may be made through the irrigation system on a bi-weekly basis until the total required nitrogen is applied in three to five applications and should be completed by tassel emergence.

A typical nitrogen uptake curve shows that corn takes up about 15 lbs/acre of nitrogen by the time corn is about 15 inches tall. It starts a rapid uptake period at this time and will grow about 3 feet the next two weeks with good moisture and take up about 80 lbs/acre of nitrogen during those two weeks, followed by another 50 lbs/acre of uptake in the next two weeks prior to tassel emergence. Therefore, at least 130 lbs/acre of nitrogen will need to be available in the four weeks after corn reaches the 15-inch height range.

Over the next 6 weeks of ear formation, corn will take up another 100 to 150 lbs/acre of nitrogen. However, if nitrogen is adequate until tassel emergence, no yield increase would be expected from additional nitrogen application after tassel emergence. Only grain N content is increased with N applied after tassel emergence.

Phosphorus And Potassium

Phosphorus and potassium should be applied according to a soil test. A 200-bushel crop can take up to 45 pounds of phosphorus and 250 to 300 pounds of potassium. Corn may exhibit phosphorus deficiency symptoms (stunted plants and purpling of leaves) on cool, wet soils that have high levels of soil test P.

Generally, all the phosphate and, on most soils, all of the potash are applied at or before planting. Some or all of the phosphorus requirements may be obtained through the use of starter fertilizer. On deep sands, apply potash in split applications, one-third at planting and the remainder when corn is about 15 inches tall.

Secondary And Micronutrients

Corn requires about 20 to 30 pounds of sulfur per acre. On deep sands, apply sulfur in split applications with N. All sulfur should be applied in the sulfate form. Nitrogen sources with sulfur are usually sufficient to meet plant requirements.

Zinc and manganese deficiency can be prevented by using 2 to 3 lbs/A of the element if called for by soil tests. Do not use zinc unless soil test levels are low since peanuts (if used in rotation) are very sensitive to high levels and can result in zinc toxicity and split stems. If needed, apply preplant or at planting in starter fertilizer as chelates.

Boron deficiencies can occur on sandy soil low in organic matter. Generally, use one pound per acre of boron applied in split applications. It is best to apply boron with the nitrogen applications.

Fertilizer Placement

The main objective in fertilizer placement is to avoid injury to the young seedling and to get proper placement for most efficient root uptake. Band placement of N near the row at planting and on corn up to about 15 inches tall has been shown to be most efficient since the root system is limited. Broadcasting potassium fertilizer is less expensive for labor and just as efficient as banding on soils with medium fertility.

However, our research has shown that 25 percent less P and K may be used if applied in band versus broadcast applications. Generally, all of the phosphorus is usually applied as a starter and the potassium is broadcast preplant or pre- and post-plant applications.

David Wright is a professor of agronomy at the University of Florida, North Florida Research and Education Center.


Make The Most Of Starter Fertilizer

Small amounts of nitrogen, phosphorus, sulfur and micronutrients are often used as a starter fertilizer. The main advantage of starter fertilizer is better early season growth, earlier dry down, and with many hybrids, higher yield. Corn planted in February, March or early April is exposed to cool soil temperatures, which may reduce phosphate uptake.

Banding a starter fertilizer 2 inches to the side and 2 inches below the seed increases the chances of roots penetrating the fertilizer band and taking up needed nitrogen and phosphorus. Starter fertilizer can also be used in a surface dribble for strip-till planting with the solution applied 2 inches to the side of the seed furrow for each 20 pounds of nitrogen used.

Currently, the most popular starter fertilizer is ammonium polyphosphate (10-34-0), a liquid. Monoammonium and diammonium phosphates are dry sources and equally effective. There is generally no advantage in using a complete fertilizer (NPK) as a starter, since applying nitrogen and phosphorus is the key to early growth. If soil test levels for P and K are high, a starter with 30 to 40 lbs/A of nitrogen and 15 lbs/A of P is adequate for starter application. Normally, 10 to 15 gallons of a starter fertilizer containing one-third to one-half 10-34-0 and the remainder as 28-0-0-5 has been effective for early corn growth. Corn will take up around 15 to 20 lbs/A of N and 5 lbs/A of P by the time the corn is 15 inches tall. Therefore, high rates of starter P are not necessary unless it is used to supply all of the P for the corn crop in a low soil test field.