Whether you should fertilize your soybeans depends significantly on your specific soil conditions and environment.
Soybeans are legumes, and they have a remarkable ability to fix their own nitrogen from the atmosphere. This process occurs through a symbiotic relationship with Rhizobia bacteria that live in nodules on the soybean roots. Typically, this natural nitrogen fixation provides the plant with sufficient nitrogen for healthy growth and optimal yield, meaning supplemental nitrogen fertilizer is often not necessary.
When Nitrogen Fertilization Might Be Necessary
However, the supply of nitrogen from soil reserves and root nodules may not be adequate under certain challenging soil conditions. According to research, under some soil conditions (low pH, low organic matter, low residual nitrogen, large amounts of crop residue), the supply of nitrogen from soil and nodules may not be adequate.
In these specific scenarios, soybean yield may be increased by applying nitrogen fertilizer. These conditions can hinder the natural nitrogen supply because:
- Low pH: Acidic soil conditions can negatively impact the survival and effectiveness of the Rhizobia bacteria responsible for nitrogen fixation.
- Low Organic Matter: Soil organic matter is a natural source of nitrogen as it decomposes. Low levels mean less nitrogen is released into the soil over time.
- Low Residual Nitrogen: If previous crops have depleted soil nitrogen levels, or if there hasn't been recent manure application, the soil may simply lack a baseline nitrogen supply for early plant growth before nodule fixation fully kicks in.
- Large Amounts of Crop Residue: Decomposing residue can temporarily tie up soil nitrogen (immobilization) as microbes break it down, making it less available to the young soybean plants.
Factors to Consider
To determine if your soybeans might benefit from nitrogen fertilization, consider the following:
- Soil Testing: A comprehensive soil test can reveal your soil's pH, organic matter levels, and residual nitrogen. This is a crucial step in understanding your soil's nitrogen-supplying capacity.
- Field History: Fields with a history of low yields despite good management, or those with challenging soil types prone to low pH or low organic matter, might be candidates.
- Visual Assessment: While not a substitute for testing, stunted growth or pale green (chlorotic) leaves, especially early in the season, could indicate nitrogen deficiency, although these symptoms can have other causes.
Conditions and Potential Nitrogen Need
Below is a simplified look at how certain conditions might influence the need for nitrogen fertilizer:
| Soil Condition | Potential Need for N Fertilizer |
|---|---|
| Healthy, Balanced | Unlikely (Soybeans fix N) |
| Low pH | Possible (Affects nodulation) |
| Low Organic Matter | Possible (Less natural N) |
| Low Residual Nitrogen | Possible (Early season N gap) |
| High Crop Residue | Possible (Immobilization) |
In summary: While soybeans typically don't require nitrogen fertilizer due to their ability to fix atmospheric nitrogen, applying nitrogen may be beneficial and increase yield specifically under conditions of low soil pH, low organic matter, low residual nitrogen, or high crop residue. For most conventional soybean production in fertile, well-managed soils, supplemental nitrogen fertilizer is not cost-effective or necessary.
