Rasel Parvej, Syam Dodla, and Matthew Foster

Starter fertilizer is also called “pop-up” fertilizer and usually applied in the seed furrow or in 2 by 2 band i.e., 2 inches side and 2 inches below the seed depth. Louisiana corn producers mostly use ammonium polyphosphate such as 10-34-0 or 11-37-0 as starter fertilizer and apply it in the seed furrow with their planter. These starter fertilizers mostly contain small amounts (10 or 11%) of nitrogen (N) and high amounts (34 or 37%) of phosphorus (P2O5). Some starter fertilizers may also contain small amounts of potassium (K2O), sulfur (S), and some micronutrients such as zinc (Zn), manganese (Mn), etc. Starter fertilizer is usually applied at a lower rate (5 gallons per acre) for in-furrow application due to the chance of salt injury from ammonium-nitrogen but can be applied at a higher rate (up to 10 gallons per acre) for 2 by 2 banding. Although adequate soil moisture at planting can reduce the likelihood of salt injury, high rates of nitrogen fertilizer is still not recommended for in-furrow application.

The main purpose of applying starter fertilizer is to help the germinating corn seedling to boost up early-season growth by easily accessible nutrients placed near the seeds, resulting in increased yield potential. However, corn grain yield response to starter fertilizer is very inconsistent with no yield response being common across mid-South and Midwest. Therefore, each year Louisiana producers often ask the same question whether they should apply starter fertilizer at corn planting. The most common extension answer is “it depends”. Mascagni et al. (2007) conducted 15 site-years research trials from 1991 to 2005 on starter fertilizer (10-34-0 or 11-37-0) for corn production in northeast Louisiana and found that starter fertilizer increased corn yield by 8 to 25 bushels per acre in only 5 out of 15 trials (Figure 1). They reported that the positive yield response to starter fertilizer came only from phosphorus but not from nitrogen. This was because 5 gallons of in-furrow starter fertilizer (10-34-0) contains 19.8 pounds phosphorus (P2O5) but only 5.8 pounds nitrogen (N) per acre, which is a very small amount to make any yield difference by nitrogen at planting. Note that corn requires 30 to 45 pounds nitrogen from planting to V6 stage (6 visible collar leaves and plant is about 12-18 inches tall); therefore, corn nitrogen requirement during the early season can only be fulfilled by applying N as broadcast (Urea; 46-0-0) followed by incorporation before planting or as sidedress (UAN; 32-0-0) at planting. The most important point that Mascagni et al. (2007) reported was that starter fertilizer increased corn yield only in coarse-textured soils such as sandy loam soils. They also reported that sandy loam soils were cold-natured soils with low organic matter content and nutrient holding capacity, where phosphorus deficiency symptoms were common during early corn growing season (mid-March to mid-April). However, they mentioned some benefits of starter fertilizer regardless of soil type and yield response such as improved early-season plant growth and 3 to 4 day earlier mid-silk stage along with earlier maturity. Therefore, based on the research conducted in Louisiana and other corn producing states, the following points need to be considered before making decision in using starter fertilizers.

1. Soil-test phosphorus level: If the Mehlich-3 soil-test phosphorus level is low (less than 21 ppm or 42 lb/acre) and producers already applied phosphorus fertilizer either in the Fall or Spring, there may be no benefit of using starter fertilizer. This is also true for high testing phosphorus soils (more than 35 ppm or 70 lb/acre) where additional phosphorus is not needed. For medium testing soils (21 to 35 ppm or 42 to 70 lb/acre), if producers do not apply any phosphorus fertilizer, there may be a benefit of using starter fertilizer especially in coarse-textured soils with early planting.
2. Planting date: When corn is planted earlier than the recommended dates (before Feb. 25 in south and central Louisiana and Mar. 10 in north Louisiana), starter fertilizer may be beneficial but again this depends on soil-test phosphorus level (low to medium), soil types (coarse-textured), and early-season soil temperature (low). Cold soil temperature often causes reduced phosphorus uptake by young corn plants, due to slow root growth, resulting in temporary phosphorus deficiency especially in sandy loam soils, even though soil-test phosphorus levels are adequate.
3. Soil type: Starter fertilizer may be beneficial for corn production in coarse-texture soils with low organic matter and cation exchange capacity (CEC) especially for early planting and medium to high soil-test phosphorus level with no phosphorus fertilization in the Fall or Spring. No yield benefit from starter fertilizer is common for corn production in fine-textured soils such as clayey soils.
4. Soil pH: Since starter fertilizer mostly contains phosphorus, soil pH should also be considered before making decision in using starter. Phosphorus availability is maximum between soil pH 6.0 and 7.5. Fertilizer-phosphorus is fixed to unavailable forms as aluminum phosphate when soil pH falls below 5.5 and as calcium phosphate when soil pH exceeds 7.5. Starter fertilizer may be beneficial for soil pH outside of this range (6.0 to 7.5) to ensure maximum fertilizer-phosphorus availability for early-season plant uptake.
5. Plant residue and/or tillage: High plant residue with or without cover crops or no-tillage often causes cooler and wetter soil conditions compared to tilled soils with less plant residue, resulting in poor early-season growth and phosphorus deficiency. Also, high plant residue sometimes results in early-season nitrogen deficiency due to nitrogen immobilization by soil microbes. Considering all the abovementioned factors, starter fertilizer may offer some benefits in these soil conditions.

Overall, starter fertilizer may only benefit corn yield in a very specific situation. If corn price is low and input cost is high, corn producers may not need to spend their money on using starter fertilizer in most fields. However, with high corn prices this year, starter fertilizer, if not too expensive, can be a cheap insurance for early corn planting in coarse-textured soils against the detrimental cold and wet weather conditions often experienced by Louisiana corn producers in March.