The risk and success of such operations depend on soil characteristics, weather conditions and machinery operations.

Opportunities

Usually growers are advised to delay application as long as possible in the fall so soils are cool, in order to prevent or delay conversion of ammonia-form N (NH3) to nitrate (NO3-), which is vulnerable to loss by leaching or denitrification.

On poorly drained soils or highly leachable soils, growers may be advised to completely avoid fall application. But, leaching and denitrification losses occur only under wet soil conditions. The very dry soil profiles this fall mean soils will behave as well-drained sites, reducing the risk of losses from fall nitrogen applications.

With the reduced risk of “wet weather losses,” growers may reconsider making applications on poorly drained soils, and may make “earlier-than-normal” applications on well-drained soils.

This is illustrated in Figure 1, where early-fall applications of banded N on well-drained soils have full yield potential compared to spring applications. (Having N in place in the fall can reduce seedbed disruption and moisture loss from N applications next spring and may also speed up spring seeding.)

Soils also appear to be cooling off, with many already dropping below 10 C in the evenings. The Manitoba Agriculture weather network of 35 stations measures soil temperature at a two-inch depth under sod (posted at http://tgs.gov.mb.ca/climate/SoilMoisture.aspx).

This data may provide guidance in when to start applications. Applications are at low risk if they commence at 7 C to 10 C while soils are cooling. At 5 C there is very slow conversion of ammonia-N to nitrate.

Remember — all fertilizer operations in Manitoba are to be completed by November 10 according to the Water Protection Act.

Challenges

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 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 (Figure 1). 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.

However, deep tillage of dry, clay soils may simply produce larger clods. 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 two 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.

Figure 1.

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MAFRI CROP NUTRITION SPECIALIST

The past few wet years have played havoc with the traditional practices of fall nitrogen (N) fertilization. In many cases the N losses have been high, resulting in low yields and/or protein, and in extreme cases some fertilized fields went unseeded.

So in recent weeks I have had calls from growers either switching to spring, one-pass seeding and fertilizing or wanting some assurance that their fall fertilization program was in order. So a refresher on the practice is warranted.

The major premise of efficient fall fertilization is to keep N overwintering in the ammonium form. This NH4+ form is a cation and is held by the cation exchange surfaces on negatively charged clay and organic matter particles. No amount of rain will dislodge and leach N when it remains in this form. Likewise denitrification is a bacterial conversion of nitrate (NO3-) nitrogen to a gaseous form that is lost. But again the N must be converted to the ammonium form before this is a concern.

Several practices may help keep fall-applied N in the ammonium form:

A slow-release coating on urea will delay urea from escaping into the soil, let alone converting to nitrate (like ESN).

A nitrification inhibitor, like DCD in SuperU, is toxic to the nitrifying bacteria. The initial ammonia shock of a urea or ammonia band temporarily kills bacteria and fungi within the injection zone. The temporary bacterial absence delays the conversion of ammonia to nitrate. The more concentrated the injection zone, the slower the recolonization by bacteria.

Application to cool soils means the bacteria perform slower (see Table 1).

One can check the range of temperatures observed in our soils by accessing the Manitoba weather network data: http://tgs. gov.mb.ca/climate/DisplayImage.

SeveralpracticesmayhelpkeepfallappliedNintheammoniumformCANSTOCK

aspx? StationID=soiltemp. This shows the two-week soil temperature trend for 36 stations across Manitoba. Temperatures are taken at the two-inch depth under sod. Our summer-like conditions have kept temperatures elevated until recently.

Past guidelines suggested farmers should delay applications until soils cool to 5 C. However this ignores the vagrancies of fall weather which may thwart sufficient application before fall freeze-up. Growers need to gauge their starting date by their soil conditions and the number of days required for application. Another consideration for Manitoba farmers is that all fertilizer applications should be completed by Nov. 10 in order to comply with current Water Protection Act regulations. This is the date selected as the start of winter and extends until April 10.

For best performance it is even better that these bands not be disturbed with spring tillage or seeding operations.

But N losses with early application do not always occur. Figure 1 shows that Manitoba research locations that were well drained, upper slopes performed as well or better than spring banded N (sometimes banding into wet, cold soils prior to seeding can muck up the seedbed, reducing stands). However, under wet environments and poorly drained, depressional soil, the N losses and yield penalty with early banding on warm soils are unacceptably high.

Phosphorus

Growers are also asking for pointers on fall phosphorus (P) applications. Some of our high P-removing crops like soybeans and canola cannot safely tolerate their removal amounts when placed with seed. Fall phosphorus application is an option. The availability of this phosphorus is improved if it is dual banded with nitrogen or ammonium sulphate. Spacing of bands should be 12 inches or less so bands are not too far away from future rows. And ideally these bands should remain intact and not disrupted by spring seeding or tillage. Application of some P fertilizer with the seed may still be advantageous if soils are cold in the spring or soils are very low in P.

In summary, there is little new about fall fertilization principles. Band ammonium-based fertilizers to cool soils.

From MAFRI s monthly crop e-news. Growers can subscribe by emailing crops@gov.mb.ca

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TABLE 1.

Nitrification rates of ammonia to nitrate form-N from banded urea (calculated from Tiessen et al, 2003).

Average soil temp. at band depth

1 C

5 C

10 C

15 C

20 C

Days for 50% conversion to nitrate

190

40

20

13

10

Days for 100% conversion to nitrate

380

80

40

25

20

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FIGURE 1.

Effect of date of fall N application on spring wheat grain yields from fall-banded urea relative to spring-banded urea at depressional and upper slope positions at three sites near Winnipeg and one site near Brandon (2001-02).1

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