Low Input Soybeans? Don’t Expect Much!

As soybean acreage moves north and west, there have been some strange myths that have tagged along. Myths like soybeans don’t need fertilizer, soybean seed doesn’t need to be inoculated with bacteria or soybeans just don’t need much management at all! The facts are, if you follow these myths, you better be ready for the “Low Yield” results. If you want to manage soybeans for high yields, you need to have a good long-term fertility plan in place.

Soil Fertility Management

Nitrogen: Fields that have no history of soybean production and low soil nitrate levels will require additional nitrogen fertilizer for high yields. If the soil nitrate test for a 0-24″ sample is less than 60 lb/a, apply the difference up to 60 lb/a. Example: If the soil nitrate test is 20 lb/a, apply 40 lb/a actual N to bring the total to 60 lb/a. Soybean seed should always be inoculated with rhizobia bacteria specific to soybeans. Make sure to apply and care for the inoculants so the bacteria are kept alive. If the soil nitrate level is higher than 75 lb/a do not apply large amounts of nitrogen fertilizer, as this will reduce the ability of the soybeans to fix their own nitrogen later in the season during pod fill. A soybean crop requires about 200 lb/a of nitrogen during the season to produce 40 bushels of grain. Most of this nitrogen should come from nitrogen fixed from the air by rhizobia bacteria living in the pink nodules on soybean roots.

Phosphorus: Soybeans yield very well on fields with medium to high soil P levels. The root system for soybean seems to respond better to broadcast P application better than band applications. Even though a broadcast application of phosphorus appears to be the best application method for soybeans, many growers in the northern areas apply phosphorus fertilizer in a band near or with the seed. In many situations, this is the only opportunity to apply phosphorus fertilizer to the field that year. With a double disk opener with a 6″ spacing, the maximum amount of P2O5 placed with the seed is about 25 lb/a (about 50 lb/a of 11-52-0) Higher rates can cause delayed emergence and stand reduction.

Potassium: Soybeans remove much more potassium from the soil than crops such as wheat. 40 bu/a soybeans removes 60 lb/a K2O, while 60 bu/a wheat removes only 20 lb/a of K2O. The higher removal of potassium by a soybean crop will cause the soil potassium levels to decrease over time, compared to wheat. Potassium fertilizer should not be placed with the seed. Delayed emergence and reduced stand can occur. All applications of potassium should be broadcast or banded away from the seed.

Sulfur: Sulfur deficiency in soybeans is normally associated with sandy lighter textured soils with organic matter levels less than 2.5%. Yield responses to sulfur usually occur in localized areas within a field.

Zinc: Soybeans are not very sensitive to low soil zinc. Recent research has shown no yield response to zinc application on soil testing less than 0.5 ppm. Applying zinc to soils lower than 1.0 ppm is recommended for zinc sensitive crops such as corn, edible beans and flax. When you are broadcasting P & K for soybean production, this may be an opportunity to broadcast zinc at the same time to increase the zinc level in the soil for the following crops in your rotation.

Soil Salinity and Carbonate Level: Chlorosis is a complex problem in soybean production. Two factors that contribute to an increased probability of chlorosis are high soil salinity and a high carbonate level in the soil. With wet cool soil conditions in June, severe chlorosis is much more likely on soils with high salinity and high carbonates. To reduce the severity of chlorosis, choose fields with low salts and carbonate and plant chlorosis tolerant varieties in your maturity range. Soil pH by itself is not a good indicator of the potential for chlorosis or how well soybeans will yield. Soybeans can yield very well on soils with a pH of 7.5 – 8.2 as long as the salinity and carbonate levels are low and the drainage is good.