Shaun Casteel has made believers out of people with his phrase: “Green to the eye by the Fourth of July.” He says by July 4, he should see all green looking across a soybean field.

What will the Purdue University Extension soybean specialist see when he looks over soybean fields on the Fourth of July this year? Will he see nothing but green? Or will he still see brown, bare slivers of soil? Those bare spots represent lost opportunity because solar energy isn’t being captured by plants.  

What Casteel sees may partly depend on when the field was planted. The other determining factor is row width.

According to the Purdue University Corn & Soybean Field Guide, there is both good and bad news when it comes to late soybean plantings and the ability to close the canopy by July 4.

The guide is published by the Purdue Crop Diagnostic Training and Research Center. Corey Gerber is the director.

Good news vs. bad news
The good news, according to the guide, is that soybeans planted later tend to close the canopy at a faster rate than the same soybean variety planted earlier in the season. This holds true at all row widths.

The bad news is that even if canopy occurs faster when soybeans are planted later, soybeans that were planted earlier may still close the canopy at an earlier date on the calendar. Here are three examples that illustrate this point.

Example 1. Planting May 1 vs. May 25 in 15-inch rows

According to information in the guide, if you plant before May 5 in 15-inch rows, it will take 50 days from planting for the crop to close the canopy and be “green to the eye.” If you plant May 25, the crop can close the canopy in 40 days, or 10 days earlier.

However, here’s how that math works on the calendar. If you planted May 1, the canopy should close by around June 20. If you planted May 25, 40 days puts the crop closing canopy right around July 4. The late-planted crop gets to canopy faster, but it’s still later in the season.

Example 2. Planting May 1 vs. May 25 in 20-inch rows

It takes 60 days from May 1 in 20-inch rows, so the crop would canopy around July 1, just before Casteel’s magic date. Planted May 25, it takes 50 days, which projects canopy closure around July 14, or mid-July.

Example 3. Planting May 1 vs. May 25 in 30-inch rows

Research says it takes 75 days to reach canopy if planted in 30-inch rows May 1. That puts expected row closure around July 14 again. If you plant May 25, soybeans can close the rows in 60 days. However, by then, it will be nearly August.

This is why most agronomists recommend narrower rows for late plantings. In 7-inch rows, soybeans planted May 1 would canopy in 35 days. Planted May 25, they close rows in 25 days. In both cases, rows would close well ahead of July 4.

Soybean Watch ’17 is sponsored by Beck’s.