Safe Liquid Fertilizer Rates for Corn

Much of Manitoba’s corn receives liquid phosphorus as a starter in the seedrow.  But we cannot apply sufficient amounts with the seed to meet full crop removal (about 44 lb P2O5/ac for a 100 bu/ac crop).  The safe amount of seed placed fertilizer depends upon soil type, moisture, row spacing and seed furrow opening.  Ontario guidelines base the safe rates of fertilizer on N and K content of the starter fertilizer.  For 30 inch rows no more than 10 lb N/ac should be seedplaced – enough to provide 34 lb P2O5/ac of 10-34-0 liquid fertilizer or 8.5 US gal per acre.  But based on South Dakota  studies such rates could cause stand thinning of 4-9% depending on soil moisture and texture. Most farmers will not be pushing starter rates this high as they should have the bulk of their P needs met through a safer placement strategy.

More on these safe rates of fertilizer is posted on Manitoba Agriculture’s website at:,safe-rates-of-seed-placed-phosphorus-for-manitoba–narrow-row-and-row-crops.html

Submitted by: John Heard, Crop Nutrition Specialist, Manitoba Agriculture


Have a follow-up question?

Volatilization of Surface Applied Urea/UAN

Surface broadcasting of N fertilizers has become popular to increase operational efficiency – increasing the speed of seeding and reducing risk of seed injury. Growers must consider the risks of volatilization loss and take precautions when risk is high.

Volatilization of ammonia from urea or the urea portion of UAN (28-0-0) is affected by several factors:

Volatilization Risk

Often we experience such conditions in Manitoba. Research has shown losses to be:

Table 2. Loss of applied urea in 7 days as influenced by temperature and N source (Grant et al, AAFC, Brandon)

Table 2 Loss of applied N

Ways to minimize losses:

  • Treat urea with a urease inhibitor NBPT like Agrotain Ultra. Several Agrotain formulations are available and some other NBPT products are now available. NDSU staff indicates to expect similar performance as long as the active ingredient NBPT rate per tonne urea is the same.
  • Wait for rain.
  • Banded UAN is less subject to volatilization than broadcast sprayed.
  • The addition of other fertilizers has been inconsistent in their impact on volatilization (ie Ammonium thiosulphate).

Submitted by:  John Heard, Crop Nutrition Specialist, MAFRD

Have a follow-up question?

Dry Seedbeds & Seedling Toxicity

The most critical nutrient for early season growth is phosphorus and for quick accessibility it is best placed with or close to the seed. In some crops, seeding configurations and soil conditions, all the phosphorus a crop needs may be seed-placed. In other cases unacceptable seedling burn and germination damage may occur.

Seed-placed fertilizer rates are restricted by a number of factors:

  • Fertilizer type – and salt index and ammonia toxicity
  • Crop sensitivity
  • Fertilizer –seed placement
  • Soil conditions – texture (affecting cation exchange capacity or CEC) and moisture

The salt properties of fertilizers can draw moisture out of germinating seeds. Crops vary in their tolerance to fertilizer salts with cereals being most tolerant, followed by pulses and lastly by oilseeds. The other hazard is ammonia (NH3) toxicity. High seedling zone concentrations of ammonia are toxic to seedling roots, impairing water and nutrient uptake. The portion of free ammonia in the soil is increased with high soil pH, high levels of free lime or carbonates, low CEC and dry conditions.

Many Manitoba soils started off dry this spring. Growers should assess seedbed moisture conditions and be prepared to make changes – especially if winds continue to dry seedbeds.  Following is a list of guidelines for seed placed phosphorus from a variety of sources:

Table 1. Maximum safe rates of actual seed-placed phosphate (P2O5) fertilizer as mono ammonium phosphate (MAP 11-52-0 or 12-51-0) for Manitoba and Saskatchewan. guidelines.

Table 1 Maximum rates

*Rates are based on disk or knife openers with a 1 in. spread, 6 to 7 in. row spacing and good to excellent soil moisture.
**Rates are based on knife openers with a one-inch spread, 9-inch row spacing and good to excellent soil moisture.

Table 2. Safe limits of seed-placed phosphate fertilizer with 30” row corn based on Ontario guidelines (Ontario Ministry of Agriculture and Food, OMAF3) .

Table 2 Safe Limits of seed placed phosphate

South Dakota State University developed a calculator that takes into account the amount of stand thinning that may occur with different crops, fertilizers, sols, moisture and seeding equipment. It is available online at:

As confirmation, it is important to plug a fertilizer run for 50 feet of row. Mark that row and follow-up with a stand count of the seedlings and root inspection of any missing or damaged plants.

 Submitted by: John Heard, Crop Nutrition Specialist, MAFRD
1 Manitoba Soil Fertility Guide. 2007. Manitoba Agriculture, Food and Rural Development. P. 17 or
2 Guidelines for safe rates of fertilizer placed with the seed. 2009. Saskatchewan Ministry of Agriculture.
3 Agronomy Guide for Field Crops. Publication 811.Ontario Ministry of Agriculture, Food and Rural Affairs. Table 9-21
Have a follow-up question?

My corn is quickly approaching knee-high in height and with the wet conditions it’s been a struggle to get on the field. What are some N application options?

Nitrogen application is critical to corn as it nears the knee high stage.  Up to this point, the plant has consumed  little N but will be taking up 2.5 lb/ac/day as it approaches tasselling.

Broadcasting granular fertilizer such as urea can cause leaf scorching from those granules that end up in the whorl.  Likewise, dribble UAN application can cause substantial leaf burn since droplets either run either off the leaf or into the whorl. And corn may be too large or soil too wet to allow side-dress injection of  ammonia or UAN into the ground.

A safe option is to dribble UAN (28-0-0) using drop pipes so it contacts the ground and avoids the leaves.  This simply means plumbing the high clearance sprayer with nozzles every 30” (rather than the current 20” for pesticides) and attaching flexible hoses to direct liquid to the soil.  In the USA, one brand of  the off-the-shelf units called Y-Drop Applicators have been so popular they are sold out.  If soils are moist and temperatures hot, you should consider adding Agrotain to minimize volatilization loss from the applied fertilizer.

Submitted by:  John Heard, Crop Nutrition Specialist, MAFRD

Have a follow-up question?

GOT YELLOW CORN? Using Tissue & Soil Analysis to Confirm Nutrient Deficiency

Not only are soybeans turning yellow in Manitoba, but corn is as well.   So how do you confirm a nutrient deficiency is causing the yellowing?

John Lee of AGVISE Laboratories provided MAFRD staff last year (2013) with the below information on how to properly determine if nutrient deficiencies are playing a role in the ‘yellowing’ corn seen across Manitoba.   It is a great article so thank you to John Lee!

Using Tissue and Soil Analysis to Confirm Nutrient Deficiency in Corn

A cool, cloudy spring with excessive moisture has affected many areas.  One effect of these conditions has been yellow corn from Manitoba to South Dakota.  Using plant tissue analysis along with soil analysis can help determine if the yellow corn is a result of sulfur or nitrogen deficiency.

AGVISE staff recently worked with a local grower to help him determine why one of his corn fields was yellow while the adjacent neighbor’s field was dark green.  Tissue and soil samples were collected from the yellow corn field and also from the adjacent corn field planted the same day.  The concentration of sulfur in the plant tissue from the yellow corn field was well below the sufficiency range established for sulfur at this stage of growth.  The nitrogen concentration in the yellow corn was good and in the middle of the sufficiency range for this stage of growth.  A tissue sample from the adjacent corn field with dark green color was also tested and found to have sulfur and nitrogen levels well within the sufficiency ranges (see tissue and soil test results and pictures of corn at  There were also a large difference in the soil nitrate and sulfate sulfur levels in the soil samples from the yellow field and the adjacent green field.

The soil type in these fields is a sandy loam which is subject to leaching of sulfur and nitrogen with excessive rainfall like this spring.  The yellow field did not have sulfur fertilizer applied this spring while the adjacent dark green corn did have sulfur fertilizer broadcast and tilled in before planting.  The grower was planning on sidedressing the yellow corn with nitrogen fertilizer, but but now with the additional information from the tissue and soil tests, he is going to include some sulfur fertilizer in the sidedress application as well on this sandy loam soil.

This is just one example of how using tissue analysis along with soil analysis in season can help figure out if symptoms are being caused by a nutrient deficiency and which nutrient is the main cause of the symptoms.

Note:  If agronomists/producers send suspected nutrient deficiencies to MAFRD’s Crop Diagnostic Lab, please send samples for tissue and soil analysis simultaneously to an appropriate laboratory.  Then follow up with Crop Diagnostic Lab on the results from the tissue and soil analysis.

Have a follow-up question?

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.


Have a follow-up question?

Should you plant corn after canola?

Planting corn after canola can result in corn being impacted by phosphorus deficiency, or what is commonly called “corn after canola syndrome”.  Phosphorus enters the corn plant through root hairs, root tips, and the outermost layers of root cells.  Beneficial fungi, called mycorrhizae, enhance P uptake in corn early in the season because the mycorrhizae strands increase the effective rooting volume of plants.  This is extremely important for uptake of immobile nutrients, such as P.

In corn, up to 80% of early season P uptake is by mycorrhizae since the strand network may extend 8-12 inches from root.  Crucifer plants, such as canola, sugarbeets, and mustard, are not hosts for mycorrhizae so the mycorrhizae must regrow from spores.  This is why early in the growing season you might see corn suffering from P deficiency.

Although the mycorrhizae eventually regrow and colonize the roots, the damage done by lack of P early in the growing season may have already occurred.  Yield may be impacted by the early season P deficiency, maturity of the crop may be delayed and grain moisture content at harvest may be higher (leading to higher drying costs).

How Common is Grain Corn Planted After Canola in Manitoba?

In Yield Manitoba 2014, Anastasia Kubinec (Provincial Oilseed Crops Specialist) updated her crop rotation tables based on information MASC contract holders provide to MASC.  These tables provide trends that can be used to help with crop rotation choices.  If we dig into the data from the 2008 to 2012 time period, 22% of the grain corn acres were planted into canola stubble.  In fact, it was the most popular choice for producers, followed by planting corn into soybeans (16%) and into spring wheat (10%).  So the data illustrates there are other factors producers look at when planning their grain corn crop rotations, and not necessarily the influence of beneficial fungi.

What is the potential impact to yield?

The same data source provides details on the yield response of those rotations (see Table 1 below).  Grain corn  yields are lower following canola than soybeans or spring wheat.

Table 1: Relative Yield Response (per cent of 2008-2012 average) of Manitoba crops sown on previous crops (stubble >120 acres)

Previous   Crop  Crop   Planted – Grain Corn 
Hard Red Spring Wheat 100
Winter Wheat 87
Barley 99
Oat 93
Canola 95
Soybean 103
Sunflower 99
Grain Corn 87
Yield (bu/ac)  95

The initial phosphorus uptake can be an issue but is this practice manageable?

To avoid ‘corn after canola syndrome’, producers should grow a crop less dependent on mycorrhizae for P uptake after canola (corn & flax are two of the most dependent crops).

If rotation requires corn after canola, a “Plan B” is to supply high starter P levels to try and overcome any P deficiency problems that might occur.

Variable results are seen with in-crop treatments to remedy phosphorous deficiency.   If products are tried, it is recommended to leave a check strip to determine effectiveness on crop recovery and yield.

For more information, please see the complete article by Anastasia in Yield Manitoba 2014 at:

Submitted by:  Pam de Rocquigny, Provincial Cereal Crops Specialist

Have a follow-up question?

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

Have a follow-up question?

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:

Have a follow-up question?