When it comes to row crops in the United States, soybeans are a major player. In 2024, farmers harvested these legumes from 86.1 million acres—that’s more than the area of Indiana, Washington, and South Carolina combined. These large acreages mean the crop has a big impact on the economy as well as the environment.

While researchers have looked at how fertilizer applications and tillage practices influence the crop’s impact on the environment, a recent study examined how the planting date can have a large impact on greenhouse gas emissions. The takeaway? Planting just a few weeks earlier can lead to increased yields and a lighter environmental footprint.

The Background

Agriculture makes up about 10% of greenhouse gas emissions in the United States. However, unlike in many industries, the main pollutant isn’t carbon dioxide produced by the burning of fossil fuels. In this world, methane and nitrous oxide are the main gasses to keep an eye on. Since methane emissions are largely linked to animal agriculture, nitrous oxide is the main greenhouse gas in the world of soybeans.

Soil nutrient management, or a lack thereof, accounts for 75% of nitrous oxide emissions in the United States. When you recognize that nitrous oxide’s warming potential over 100 years is close to 300 times larger than that of carbon dioxide, you realize how this lesser-known gas has a big effect on the environment. That begs the question of how when you plant crops can reduce these emissions.

To understand nitrous oxide, we have to understand the nitrogen cycle. When we apply sources of nitrogen to the soil, our goal is that most of it is used by crops. However, when there is too much nitrogen in the soil, soil-dwelling bacteria often convert plant-available nitrate into nitrogen gas, and nitrous oxide is sometimes released as a byproduct. 

Let’s bring this back to soybeans. Corn/soybean rotations are common throughout much of the Midwest. Although nitrogen is rarely applied before soybean plantings, it’s often added before planting corn. Therefore, the first attempts at reducing nitrous oxide emissions involved reducing these fertilizer applications. However, researchers found that reducing the levels below those needed for optimum crop yields didn’t impact nitrous oxide emissions. So, they turned to other tactics.

How to Limit Greenhouse Gas Emissions in Soybeans

Since nitrous oxide is emitted into the air when there is excess nitrogen in the ground, keeping plants growing year-round can limit emissions. Plants take up nitrate (NO₃) and ammonium (NH₄), meaning more plants means less fuel for bacteria to convert into greenhouse gases.

The problem is that a typical corn/soybean rotation leaves the soil barren for nearly half the year. Keeping plants in the ground throughout the late fall, winter, and early spring can limit nitrous oxide emissions.

Planting cover crops like rye or oats into corn in the fall means that plants will keep growing throughout the winter and take up nitrogen. However, since cool temperatures slow bacteria life, not much nitrous oxide is released during the winter. The bacteria begin becoming more active when spring arrives.

That’s where earlier soybean planting dates come into play. Researchers found that planting soybeans earlier in the year reduced nitrous oxide emissions, particularly in the late spring and summer. When cover crops were combined with earlier planting, emissions were reduced 28%.

These earlier planting dates also showed a 16% increase in yields. Part of this may be due to the ability to plant different varieties of soybeans thanks to a longer growing season. These extended-growth soybeans are becoming more widely available, and a larger number of farmers are adopting them.

Details About Earlier Planting Dates

So, how early should you plant your soybeans? Well, it depends.

Unfortunately, research isn’t the only thing driving the ideal planting date. The USDA provides crop insurance that covers farmers in times of loss. Farmers must follow certain guidelines to qualify for coverage, including planting their seeds after the earliest allowable planting date. Therefore, farmers rarely planted soybeans after the dates set by the USDA.

In recent years, the USDA has pushed these dates forward due to the recognition that earlier planting would lead to higher yields. However, they didn’t necessarily account for the environmental benefits. Pushing these planting dates back even earlier in the year would potentially lead to even greater decreases in nitrous oxide emissions.

If your crops are insured, follow the USDA guidelines for the earliest allowable planting date in your area. This may move your planting date back by two or three weeks. While this may sound like a risk and take some getting used to, research has shown this earlier planting date can positively impact both the environment and your wallet.