Planting Guidelines for Corn, Soybean, and Sorghum

Planting too early and in poor soil conditions can have a negative impact on yield potential. Planting should be done according to soil temperature and soil moisture as opposed to the calendar date.

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Soil Temperature

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Crops should be planted when soil temperatures are optimal and within the target dates for the region. Corn, soybean, and sorghum seeds will germinate and emerge slowly and unevenly when soil temperatures are less than optimal. Under cool soil conditions, seeds may remain dormant for several weeks, leaving them vulnerable to diseases, insects, and animal predators.

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  • For corn planting, soil temperatures should remain at or above 50 °F.
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  • For soybean planting, soil temperatures should remain at or above 60 °F.
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  • For sorghum planting, soil temperatures should remain at or above 65 °F.
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Chilling Injury. Early planted corn 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 and/or experience large swings (25 to 30 ºF) in daily soil temperatures. Chilling injury can result in failed germination and/or hindered growth of the radical or coleoptile if it occurs within 24 to 36 hours after planting, and corkscrewed mesocotyls and/or leafing out below the soil surface if it occurs during the emergence process.

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Soil Moisture

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Soil should be moist but not saturated for planting. Planting into soils that are too wet has several downsides:

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  • Prolonged exposure to soggy, wet soils deprives the seed of oxygen needed for germination and plant growth and can increase the incidence of disease.
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  • Disk openers slicing through wet soil can create sidewall compaction in the seed zone, which can restrict root growth.
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Seedbed Preparation

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Tillage Precautions. Tilling fields when soil is too wet can cause soil compaction, which can result in decreased seedling germination and restricted root growth and nutrient uptake. Consider staying out of the field if soil sticks to the tires or if equipment is leaving tracks deeper than 1 inch.

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Starter Fertilizers. Starter fertilizer can help meet the early fertility demands of the seedling for nitrogen (N), phosphorus (P), and potassium (K) until the root system develops. Early plantings, fields under reduced tillage, and fields with heavy residue may exhibit a greater response to starter fertilizer.

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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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  • Generally, a starter fertilizer should contain a high phosphate (P2O5) ratio (1-2-1, 1-3-1, etc.) and the P should be highly water soluble.
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  • Starter fertilizer with a 1:3 (N:P) ratio is widely used in P deficient soils.
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  • Nitrogen-only fertilizers may give corn and sorghum plants an early growth response in soils where N is low at the time of planting but P and K levels are adequate. Soybeans may not respond to N in starter fertilizers.
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Figure 1

Figure 1. The 2-by-2 banded placement of starter fertilizer.

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Rates and Placement. Starter fertilizer placement is critical because of the salt effect on the seed and germination. Injury from N and K is possible when seed is in direct or very close proximity to these elements. Burn from starter fertilizers can be anticipated when soils become dry after planting.

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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). For corn, starter fertilizer applied 2 inches horizontally and 2 inches deeper than the seed (2x2) is commonly recommended (Figure 1). The rate of starter fertilizer applied should be reduced if placed closer than 2 inches from the seed.

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Table 1

As a general rule, do not use a starter fertilizer with a salt index greater than 20.0 in-furrow.

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Always be aware of the fertilizer’s specific salt index (Table 1) and adjust the rate accordingly with placement spacing from the seed. The salt index is a measure of the salt concentration that the fertilizer will induce in the soil water solution. As a 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. The salt index is used to compare fertilizer materials, but cannot determine the amount of fertilizer that will cause injury.

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Planting Depth

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Soil moisture factors into planting depth.

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  • Corn seeds should be planted around 1.5 to 2 inches deep when soil moisture is adequate. Planting shallower than 1.5 inches can result in rootless corn or root lodging and can also increase the risk of injury from some soil-applied herbicides. If soil is dry, corn can be planted deeper into moisture, at around 3 inches.
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  • Soybean seeds should be planted at 1 to 1.5 inches deep and no deeper than 2.5 inches. Planting too deep can create added stress on the seedling and can inhibit emergence in stressful situations such as soil crusting and compaction.
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  • Grain sorghum planting depths are similar to corn and vary by moisture and soil type. Depths range from 1 inch in heavy soils to 2 inches in sandy soils.