Considerations When Planting Corn Early

Early planting can increase corn yield potential if conditions remain favorable for germination and plant development. Planting into suboptimal soil conditions can lead to soil compaction, increased disease and insect pressure, and risk of chilling injury. Starter fertilizer may help increase early growth and promote crop uniformity.

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Soil Temperature. Corn requires a minimum soil temperature of 50 °F and moisture for germination to occur. Soil temperatures below optimal will cause seeds to germinate and emerge slowly and unevenly. Seeds that remain dormant for long periods become more vulnerable to diseases, insects, and animal predators.

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Figure 1

Figure 1. Corn seedling chilling injury.

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Chilling Injury. Seeds and seedlings can be at risk for chilling injury if they are exposed to soil temperatures under 50 ºF for an extended period of time during the initial 24 to 36 hours after beginning the germination process. Also, large variations (25 to 30 ºF) in daily soil temperatures can contribute to increased injury incidence. Chilling injury can result in swollen kernels that fail to germinate, or hindered growth of the radical or coleoptile, corkscrewed mesocotyls, and/or leafing out below the soil surface if it occurs during the emergence process (Figure 1).2

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Figure 2

Figure 2. Restricted root growth from sidewall compaction at planting (left); Starter fertilizer injury on seed (center) and radical (right).

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Seedbed Preparation. A proper seedbed is critical for establishing seed-to- soil contact and enabling good root growth. Compaction can occur in wet soils when heavy equipment passes through the field. In addition, double-disk openers on the planter slicing through wet soils may result in sidewall smearing. Compaction may contribute to decreased seedling germination, and restricted root growth and nutrient uptake (Figure 2). To test if a field is fit to plant, take soil from the top 3 to 4 inches; soil should break apart when pressed between fingers rather than forming a ribbon or ball.3

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Starter Fertilizers. Starter fertilizer may increase early plant growth and uniformity, especially for reduced tillage systems and fields with heavy residue. However, the increased early growth may not translate to increased yield potential.1,4 Starter fertilizer may have more impact on yield potential on soils with low phosphorus (P), sandy soils, or some high pH soils.4

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  • When deciding on the right starter fertilizer formulation to use, it is important to refer to soil fertility test results and to choose a starter fertilizer formulation that best matches the soil test recommendations.
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  • Nitrogen (N)-only fertilizers may give corn plants an early growth response in soils where N is low at the time of planting but P and K levels are adequate.
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  • Starter fertilizer with a 1:3 (N:P) ratio is widely used in P deficient soils.
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Starter Fertilizer Rates and Placement. Starter fertilizer​ placement is critical due to the effect of salt on the seed and germination. Injury from N and potassium (K) is possible when the seed is in direct or in very close proximity to these elements (Figure 2). Potential injury from starter fertilizers is higher when soils become dry after planting.

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In-furrow applications, also referred to as "pop-up", requires care to prevent germination injury. Generally, no more than 10 lb/acre of N plus K2O should be applied in-furrow with seeds.1,5 Also, ammonium thiosulfate should not be used in-furrow.

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Seedling injury is less likely when soil moisture is adequate or when starter fertilizer is applied at a safe distance from the seed (a minimum of 1 inch). Starter fertilizer applied 2 inches horizontally and 2 inches deeper than the seed (2x2) is commonly recommended. The rate of starter fertilizer applied should be reduced if placed closer than 2 inches from the seed.

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The fertilizer’s specific salt index should be considered and the application rate may need to be adjusted based on the placement spacing from the seed. The salt index is a measurement of the salt concentration that the fertilizer will induce in the soil water solution. As fertilizer dissolves in the soil, it increases the soil water solution’s salt concentration; the higher the salt concentration, the harder it is for plants to extract the water they need for growth.​