My winter wheat has been fertilized with nitrogen, but is so thin it is going to be sprayed out and seeded to soybeans. Will the extra nitrogen harm the soybeans?

Answer provided by John Heard, MAFRD Crop Nutrition Specialist

Our initial experiences of soybeans and high nitrogen soils in Manitoba were negative. High soil N tends to inhibit nodulation while at the same time this nitrogen tends to grow slightly larger vegetative plants. However after flowering that nitrogen is often insufficient to provide full yield and protein potential for the crop. So we adopted a thumb rule that if soils had more than 60-75 lb nitrate-N/acre, producers should consider growing a crop other than soybeans.   The other crops should benefit more from the N than the soybeans, which would still take up the nitrogen but may not express full yield and protein. Nitrogen was also seen to trigger iron deficiency chlorosis in soybeans grown in wetter, high lime soils.

But more recently farmers and agronomists have observed that soybeans may perform well on some high N soils. This may be because soybeans have been grown more often and a native reserve of rhizobium exists in many of these cropped soils. With industry partners (AGVISE Labs and ToneAg Consulting) we made observations at 13 field demonstrations in 2013 where high soil N levels were simulated with N application (Heard et al, 2013). Our observations were:

  • Nitrogen at 50-100 lb N/ac reduced nodulation at all sites, but most severely at the virgin or first time soybean sites. Nodule numbers were still generally sufficient on those fields with a previous history of soybeans (Figure 1).
  • Few sites were harvested for yield, but nitrogen affect was more severe on the first year soybeans (reduced yield or lower protein).

Figure 1: Average rhizobium nodules per root from 13 demonstrations in 2013.


So if one needs to replant soybeans on a field already fertilized with nitrogen consider:

  • Whether well nodulated soybeans have been grown in the past.
  • Treating fields with a history of soybeans as virgin fields by applying full rate of inoculant
  • Even if fields have high N in the spring, soybeans will largely deplete those reserves during the season

One may still wish to avoid planting soybeans if it is a virgin field or if there is risk of iron deficiency chlorosis.

There was no advantage to supplementing properly nodulated soybeans with additional nitrogen at these sites. In US studies, additional nitrogen appears warranted “sometimes” when yields are very high (>65 bu/ac) or when nodulation failures occur due to acid soil, drought or other adverse weather conditions.


Heard, J., J. Lee and R. Tone. 2013. Nitrogen and soybeans: Friends, foes or just wasted fertility? Manitoba Agropnomists Conference 2013.


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Still Lots of N Fertilizer Options when Seeding is Delayed

In a spring when seeding is delayed, field operations should be minimized to permit seeding as soon as possible. Preplant applications of N, in particular may be compromised to advance seeding dates.

There are a wide variety of options for applying nitrogen fertilizer efficiently, which are discussed in the article by John Heard, Crop Nutrition Specialist with MAFRD, available at the following link:


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Banded nitrogen with wide spaced seed rows

Question from a Subscriber of Crop Chatter:  I’m setting up a triple shoot air drill with wide (12.6 inch) spaced seed shanks. Each seed shank has a 3.5 inch paired row seed opener with a granular fertilizer placement tube to put granular fertilizer in the center of each paired row. Additionally, the drill has mid row banding shanks on 25.2″ spacing to apply NH3. I have a 3 compartment air tank.

For cereals, I will place the seed and phosphorus together through the paired row openers, with starter urea placed in the middle of each seed row through the granular fertilizer tubes. For canola, the seed and phosphorus will be placed together with ammonium sulphate (and possibly some urea if a fertilizer blend is used) in the center of each seed row. For soybeans, the seed and granular innoculant will be placed together and phosphorus will be placed in the seed row centers. NH3 will be applied for cereals and canola through the mid row shanks with 1″ openers. The NH3 will therefore be up to about 7″ away from the furthest placed seed.

I’m trying to determine how to best split the total nitrogen rate between urea and NH3. Since NH3 is cheaper and less handling than urea, my preference is to maximize NH3 use. Given that I have heavy clay soil:

1) How long it will take the furthest away plants to be able to access the NH3?
2) How much urea should be applied to keep the crop well suppied with nitrogen until all plants can access the NH3 band?

Answer (provided by John Heard, MAFRI Soil Nutrition Specialist)

Crop roots access nitrogen in mid-row bands in 2 ways:  roots growing  laterally can intercept the band and mid-row placed  ammonia converts to nitrate-N which then migrates as roots consume soil moisture.  Usually mid-row banded nitrogen is available for crop growth by the time it is needed.  Your seed-placed phosphorus fertilizer usually contains nitrogen to keep the seedling nourished.  But we hear on occasion that early season crop yellowing (MN deficiency?) is observed in the situations here – cool, heavy clay soil with wide spaced mid-row bands.  In years with warm, moist or dry soils, root access to bands will likely be quicker than this year.  Fortunately you have an option to place some urea in a band close to, but safely away from the seed.  I should think no more than 10-20% of the nitrogen requirement needs to be supplied there.

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I fertilized my winter wheat last fall. However, I made the decision to terminate the winter wheat field this spring and reseed to canola. What can one expect for fertilizer carry over for this year’s canola?

Answer (provided by John Heard, MAFRI Crop Nutrition Specialist)

The amount of nitrogen remaining for your canola crop  will be affected by  the timing of fall N application, amount of rainfall you have received in May and soil type.  Early fall applied nitrogen (regardless of form) will likely have converted to the nitrate-N form by May of this year.  It is this nitrate-N form that is vulnerable to loss by leaching or denitrification. But fall and overwinter losses will have been negligible since the fall was so dry.  But the heavy rainfall experienced in some parts of the province the past couple weeks may have moved that nitrate-N deeper in the soil profile – especially on sandy soils.  On clay soils that become waterlogged the nitrate-N is lost at a rate of 2-4 lb N/ac/day due to denitrification.

So some of your nitrogen will still be present for canola.  The standard soil nitrate-N test will pick up this form of nitrogen and could be used now.  Or one could seed now but apply a high rate of N in a strip across the field.  If colour differences are apparent by bolting – then make a supplementary N application.   

More details on such a tough decision are covered in the attached article from the July 2011 edition of the Crops e-News.

Do your crops need supplemental fertilizer – July 2011 Edition Crops E-News

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In the United States, farmers receive a nitrogen credit of 20-30 lb N/ac following soybeans. Can we expect the same here in Manitoba?

If a crop like corn is grown after soybeans and compared to corn-after-corn, the nitrogen requirement is less, and is often referred to as a soybean nitrogen credit.  But soybeans do not leave much nitrogen remaining after harvest – either in the residue or in the soil.  It is just that soybeans immobilize or tie up less nitrogen than heavy corn stover. 

With our more diverse crop rotations this benefit after soybeans does not amount to much.  In fact a 50 bu/ac soybean crop provides only a 6 lb N/ac benefit compared to a previous crop of flax.  So we do not suggest much of a nitrogen credit following soybeans.

More details please visit MAFRI’s website at:

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Should I add nitrogen to my corn stalks so they decompose quicker?

Each year we receive this question and the answer is still no.  Grain corn does produce high amounts of crop residue with a high C:N ratio, meaning microbes will immobilize or soak some nitrogen out of the soil during decomposition.  But if the corn has been adequately fertilized with nitrogen for good yield potential, there should remain sufficient nitrogen for orderly decomposition. 

What minimizes decomposition rate is our lack of suitable conditions for microbial activity, specifically heat and, this fall, perhaps moisture. 

 More details please visit MAFRI’s website at:

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Is the soil too dry to apply anhydrous ammonia?

Dry soils are a challenge to in-soil banded nitrogen application, especially anhydrous ammonia.  When anhydrous ammonia is injected into soil the ammonia (NH3) is dissolved in water and reacts to convert to ammonium (NH4+), which is positively charged and held by the cation exchange on the soil particles.

Soil moisture is needed to allow the ammonia to convert and be retained in the soil, however even in dry soils there is usually enough moisture present for this to occur.  The major problem with dry soils is the clods or lumps that form can prevent a good seal, allowing the ammonia to be lost through large voids between clods before dissolution in moisture occurs. Indeed, nitrogen losses on low moisture soils are caused more by poor physical soil structure (soil tilth) than by a lack of moisture to chemically react with ammonia.

Clay soils that are very dry will be cloddy or lumpy and may permit too much gaseous ammonia to escape. The zone of ammonia dissipation from the injection point is larger in dry soil, so although the soil may be difficult to work, deeper injection may actually be required.  Lighter textured soils will have better tilth than dry clay soils and will be more likely to produce a good seal to retain the ammonia.

Slot closure may be better on previously worked  than on uncultivated soils if the soil flows and seals better.  Such is not the case if soils were cloddy.  Some cereal crops were harvested almost 2 months ago and the moisture that has been received may be sufficient to provide good tilth.  Soil moisture and texture varies across the province, as does farm equipment.  The only way to assess your soil conditions is a test run with your applicator. An application pass without N will indicate whether soils are too cloddy and injection slot closure is inadequate. If after making a round with N, you can still smell ammonia from the previous application, make adjustments in depth or closure modifications.  Or wait for rainfall to improve soil structure.

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