Thinking About Adding Nitrogen

Published on 25 Sep 2017, 08:00 AM • by: Dan Davidson, Illinois Soybean Association • 410 Views

African soybean producer wants to know if he should be applying nitrogen to his soybeans.

ILSoyAdvisor, ISA’s soybean management website, has global reach. We get emails from around the globe asking basic questions about soybean production and management practices. Our answers often have to be pretty basic as well, with some background information included. It is always fun to receive and answer these emails and help these growers learn how to produce better beans.

Recently, I received an email from a gentleman in Zimbabwe.

His email included a great question to consider:

I am planning [for] soybean farming this coming rain season in Zimbabwe on leased land and want to maximise returns through high yields. I am reading through your [ILSoyAdvisor] articles, which I find highly educative and informative—more particularly this one. There is a portion where you say, “Adding nitrogen destroys the nodulation process, but that one would need to fertilize with phosphorous and potassium.” Does it mean that no nitrogen fertilizer should be applied at all?”

My response was as follows:

Soybeans can form nodules that house bacteria that capture ammonia from the air and give it to the plant directly. This is called nitrogen fixation.

Soybeans require a lot of nitrogen, more than almost any other crop. Using the English system of measurement, we know that soybeans need 4.5 lbs. of nitrogen per bushel of yield, so a 60-bushel-per-acre soybean crop requires 270 lbs. In metrics, a 60-bushel soybean crop is equal to 4000 kg/ha and 270 lbs. per acre nitrogen is equal to 300 kg/ha.

Soybeans are estimated to produce only about half of their total nitrogen needs, with the rest coming from the soil. So, if soybeans need 300 kg/ha, the plant’s nitrogen fixation mechanism will only supply about half. The other half will come from nitrate reserves already in the soil and from mineralization from organic matter.

At low yield levels, say around 2,000 to 2,500 kg/ha (30 bushels), nitrogen fixation and soil reserves are sufficient. But at high yield levels of 5,000 kg/ha (70 bushels or more) fixation and soil reserves may not be sufficient and supplemental fertilizer nitrogen or manures/compost may need to be added.

Nitrogen can be applied in small amounts preplant to stimulate the plant until nitrogen fixation commences 4 to 6 weeks later. But a soybean’s real nitrogen demand begins during pod fill, after the R3 reproductive stage, so applying nitrogen after pod set begins is a good time.

It has long been thought that nitrate in the soil will suppress nodulation on soybean plants. However, recent data out of universities is beginning to show that this isn’t true—that plants will continue to nodulate and fix nitrogen even with high levels of nitrate in the soil.

My advice—if your yield potential in only in the 2,000 to 3,000 kg/ha range, do not add nitrogen fertilizer. However, make sure that your seed is treated with a rhizobia inoculant—the bacteria that drive nitrogen fixation.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.

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