Starter Fertilizers for Corn Production

Why Use a Starter Fertilizer?

Starter Fertilizer Elements

Optimum Nitrogen and Phosphorus Rates in Starter Fertilizers

Starter Fertilizer Placement

Published by
North Carolina Cooperative Extension Service

Corn starter fertilizers have been used successfully to increase early plant growth, nutrient uptake, and yields in research trials and on the farm. They also promote earlier maturity, improve southern corn billbug control, and help suppress weeds through earlier shading. Use of starter fertilizers is increasing in North Carolina and the southeastern United States. This fact sheet presents the principles of successful starter fertilizer use, research results relevant to North Carolina, and management suggestions for corn producers.

Cool air and soil slow corn plant and root growth in the spring. When root growth is restricted, corn plants often turn purple. Early season purpling varies with variety, but most often it is a symptom of an induced phosphorus (P) deficiency. The deficiency is described as induced because it may occur on soils that test high for phosphorus. It occurs because phosphorus moves slowly in the soil. If roots do not grow enough to reach soil phosphorus reserves, the plant begins to starve for that nutrient.

In most cases, phosphorus deficiencies are temporary, and symptoms disappear as soon as soil temperatures rise to a point where root growth is stimulated and the plant can reach more phosphorus. Grain yields are not reduced every year by root stunting, but yield losses can be significant in years when temperatures are low. Additional factors that limit root growth can also induce phosphorus deficiencies. Some common causes include soil compaction, herbicide injury, and insect or nematode damage to the root system.

Starter fertilizers may be used to overcome slow root growth and the potential for reduced nutrient uptake. Starter and "pop-up" fertilizers involve at-planting placement of a small supply of nutrients near the seed (for starter fertilizer) or in the seed furrow (for pop-up types) so seedling roots can rapidly reach the nutrient source. These fertilizers are not intended to supply all nutrients needed by the crop. Their primary purpose is to provide an accessible nutrient source for root and plant growth when adverse conditions occur soon after planting.

A field's yield potential, soil nutrient reserves, and fertilizer management programs will dictate the best combination of nutrients for use in a corn starter fertilizer. It is best to fine tune your starter needs by comparing various nutrient combinations and starter rates on your own farm. The best tools for evaluating starter fertilizer performance are a comprehensive plant tissue analysis program and comparison of final yields.

Most research in the Southeast supports the practice of including nitrogen and phosphorus in corn starter fertilizers. Research in South Carolina compared several elements in corn starter fertilizers applied 2 inches below and 2 inches to the side of the seed position (known as a "2-by-2" placement) on coastal plain soils. The results showed that nitrogen in combination with phosphorus was superior to both broadcast and banded nitrogen alone.

Furthermore, the addition of micronutrients, potassium, or sulfur to the starter did not improve yields over the nitrogen-phosphorus combination. Soil tests at these experimental sites did not call for micronutrient applications. If soil tests, field history, or past tissue analyses predict a micronutrient deficiency, adding a suitable micronutrient source to the starter fertilizer may be warranted. Lower micronutrient rates are needed for banded starters than for broadcast applications. The most common micronutrients needed on corn are manganese and zinc. Recommended starter rates are 0.75 to 1 pound of actual manganese or zinc per acre when soils are deficient. If boron is needed, do not exceed 0.5 pound per acre of boron in a 2-by-2 band.

Support for including both nitrogen and phosphorus in starter fertilizers has come from several southeastern states. Participants in a regional workshop on corn starter fertilizers observed positive yield responses to starter fertilizers in 78 percent of research and on-farm tests.

Conclusions from the workshop stated that, over time, positive responses to starter fertilizers appear to be more consistent for nitrogen alone than for phosphorous alone. However, combining nitrogen with phosphorus in a starter fertilizer is generally the most attractive option.

Materials commonly used in starter fertilizers include diammonium phosphate (DAP 18-46-0), monoammonium phosphate (MAP 11-48-0), and ammonium polyphosphate (APP 10-34-0). All of these materials have higher phosphorus than nitrogen contents. In a preliminary study of starter materials, researchers compared two commercial liquid starter fertilizers with analyses of 10-34-0 (nitrogen, phosphate, and potash) and 3-18-18 to a blended 10-10-0 starter. The commercial starters were applied at a rate of 8 gallons per acre and the experimental blend at 30 gallons per acre. In that one-year study, the blended starter delivered 30 pounds of nitrogen and 30 pounds of phosphate per acre and produced yields 10 bushels per acre higher than the other materials.

Two later studies in South Carolina and Alabama examined nitrogen and phosphorus rates in corn starter fertilizers. Both programs were conducted on Norfolk soils with high to very high phosphorus levels. Results from six tests were similar for the two states. Corn responses to starter fertilizers were more consistent for nitrogen alone than for phosphorus alone, and yields were generally greater at all nitrogen rates with the addition of 10 to 20 pounds of phosphate (P₂O₅) per acre (Figure 1). From these studies, it appears that there is an advantage on high-phosphorus soils to decreasing the amount of phosphorus in the starter fertilizer and increasing the amount of nitrogen. Optimum levels in these studies were achieved with 10 to 20 pounds of P₂O₅ and 30 to 40 pounds of nitrogen per acre. At this increased concentration, the starter nitrogen can eliminate the need for all preplant broadcast applications of nitrogen.

Figure 1. Effect of nitrogen and phosphorus rates in corn starter fertilizers on grain yields. (Note: Data are averages for six tests obtained on sites in South Carolina and Alabama.)

This fertilization management scheme would improve nitrogen use efficiency, reduce potential nitrogen leaching losses, and provide a mechanism for reducing phosphorus application rates on soils that test high in phosphorus. The desired starter fertilizer analysis can be achieved by blending dry granular materials such as DAP with ammonium nitrate or urea. Another option is to use fluid starter fertilizers by blending products such as a 10-34-0 fertilizer with 30 percent urea ammonium nitrate (UAN) solution. In areas where sulfur is needed, ammonium sulfate can be used in dry blends and fluid sources such as S-25 can be substituted for 30 percent urea ammonium nitrate solution. Before large-scale mixing, always test fertilizer materials for compatibility.

On soils testing medium or low in phosphorus, higher phosphate rates may be needed in the starter fertilizer. On medium-phosphorus soils, adding 30 to 40 pounds per acre of phosphate should be adequate. As a rule of thumb, use about two-thirds of the recommended broadcast rate. On low-phosphorus soils, consider a combination of starter and broadcast phosphorus because the majority of the root zone is likely to be deficient in phosphorus.

Starter Fertilizer Placement

The adoption of starter fertilizers on the farm is often hampered by problems that arise when installing application equipment. The following section offers several placement options.

Figure 2. Placement alternatives for corn starter fertilizer.

Two-by-Two Placement. The most common starter placement is 2 inches to the side and 2 inches below the kernel at planting (Figure 2a). This precision placement method, called 2-by-2 placement, requires specialized equipment generally consisting of either reverse knives, double-disk openers, or courters with drop tubing behind them. Each of these equipment packages has advantages depending on the tillage system used (for example, no-till planting or conventional tillage).

All 2-by-2 placement units should be mounted in a manner that will allow them to "float" with the planter. Planter bar and unit spacings often make it difficult to install additional fertilizer attachments; thus, many are placed on forward-mounted tool bars. Forward mounting of starter fertilizer equipment decreases the precision of the placement on rough or rolling ground. These mechanical constraints and the expense of application equipment for placement has prompted researchers and producers to try the alternative placement methods, as described in the following paragraphs.

Below-seed placement (Figure 2b) is suited for in-row subsoiling units. When fluid fertilizer materials are used, a drop nozzle or stainless-steel tube can be attached to the subsoil shank with adjustable locking collars that allow a fixed placement at any depth below the seed.

With liquid application units, care should be taken to prevent the liquid fertilizers from adhering to the subsoiler shank. When this occurs, the liquid starter drips down to the subsoiler shoe and is unavailable for early plant growth. Splatter shields can be welded at specified depths on the subsoiler, or a K-3 or K-5 flood nozzle can be mounted on the end of the drop nozzle with a 45-degree elbow directing the nozzle spray away from the subsoiler shank. Either method will prevent the fluid from dripping down to the subsoiler shoe.

When granular fertilizers are used, materials should be allowed to fall freely into the subsoil track from a height 6 inches above the soil surface. Soil movement prevents granular materials from dropping all the way down to the subsoiler shoe. Below-seed placement is not as precise as a "floating" 2-by-2 placement or any surface placement because of draft bar movement.

In-furrow placement (Figure 2c) of fertilizer with the seed requires simple equipment but may result in fertilizer injury (Table 1). Fertilizers applied with this technique are commonly referred to as "pop-up" fertilizers. Pop-up fertilization is not recommended in North Carolina. However, some growers working with organic soils have used 5 gallons of 10-34-0 fertilizer per acre placed in the furrow, and researchers in Tennessee have found that little injury occurs when small amounts of pop-up fertilizers are used in clay soils.

Plant growth and nutrient uptake can be influenced by placement method (Figures 3, 4, and 5). In general, placing the fertilizer closer to the seed gives the greatest plant response. To prevent fertilizer salt injury, the total amount of nitrogen or nitrogen plus potash (K₂O) should not exceed 80 pounds per acre, especially if the fertilizer is placed within 2 inches of the seed. It is also important to ensure that application equipment is aligned uniformly so that starter fertilizers are placed the same distance from each row. Large plant growth variations between rows may be observed with placements differing by only 1/2 inch.