Think like a soybean seed planted in soil

Perhaps it’s been a long time since you’ve been asked to pretend. For just a moment, imagine you’re a soybean seed that’s just been planted in the soil. What will trigger the emergence process?

Key Points

Soybeans typically start germination at soil temperatures around 50 degrees F.

Consider the last week of April through first few weeks of May as planting target.

Soybeans must take in about 50% of their weight in water to germinate.

Why think like a soybean? It might come in handy when making borderline management decisions about when to plant or what seed treatments to use.

Talk temperature

One of the factors that determines emergence is soil temperature, notes Shaun Casteel, Purdue University Extension soybean specialist. “Soybeans typically germinate when the soil temperature is around 50 degrees F,” Casteel says. Some varieties may germinate at somewhat lower temperatures, but 50 degrees is a good rule of thumb.

The soybean life cycle is partially controlled by the photoperiod response. Onset of flowering is typically tied to length of night, Casteel observes.

However, especially in the first third of the growing cycle, temperature plays a role in how fast soybeans develop. The difference between soybeans and corn is that for corn, temperature is the key factor throughout the entire season.

Emergence percentages

Think back to that soybean in the soil. Suppose it was planted April 10 under cool soil temperatures. “You could expect it to take about three weeks to reach 70% to 90% emergence,” Casteel says. “The later you plant, typically the less time it takes.”

If you plant in late April or early May, expect 85% to 90% emergence, he says. In general Casteel believes the last week of April through the first few weeks of May is usually a good planting window.

That soybean planted in cool soil on April 15 may be more prone to disease and insect damage. That’s one reason seed treatments have become more popular. However, high-quality treatments with insecticide are expensive with questionable benefits in Indiana, Casteel says. Weigh the cost vs. planting later, or planting at a higher rate if you believe you must start early due to your volume of acres.

Water component

That soybean won’t do much — even if it’s 70 degrees — without moisture. Water intake also triggers germination.

Typically, a soybean seed imbibes about 50% of its weight in water to get the process started, Casteel notes. Corn, on the other hand, will start germination once it imbibes about 30% of its weight.

The combination of wet soil and cool temperatures sets seed up for problems, especially if it’s not treated for specific seedling diseases and insects.

Test your seed knowledge

Try this brain teaser. Which will germinate faster — smaller soybean seeds or larger seeds?

If you said “smaller,” you’re right.

“Smaller seeds must imbibe less total water to trigger germination,” Casteel says. In a crusting situation, smaller seeds will also be less likely to “break their backs.”

Seeds as large as 1,700 per pound were reported for 2010 planting, after a wet fall. After the dry fall of 2010, seeds as small as 3,400 per pound have been noted. Average number of seeds per pound is typically 2,500 to 2,900.

This article published in the March, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.