Is it too early for fall soil sampling?

Currently the traditional and most reliable method of measuring available N for the crop is the nitrate soil test. Fall sampling is most common, and to be effective, it should reflect the amount of N available at planting time. Manitoba recommendations have traditionally been to “delay sampling until soils have cooled to 5oC” so that all the N that will mineralize during the fall will be detected. This has historically been in early to mid October.

But earlier fall sampling may be desirable for number of reasons:

  • sampling is more likely to be done
  • analysis is available for fall fertilizer prescriptions and N application
  • sampling before tillage gives more consistent /reliable sample depths
  • volunteer crop regrowth is less likely to hide available N from test
  • can be used as an audit of the soil’s N supplying ability (after taking into account starting soil N, applied N and N removal) Between 1999 and 2000 we evaluated the effect of early vs later sampling at 8 Manitoba locations on cereal stubble (Table 1). On average soil nitrate levels did not vary, considering that our sampling error within the plots were considered to be 10 lb N/ac. If we had been waiting for average soil temperatures to drop to 5oC, we would have delayed sampling until mid October.
  • Table 1. Mean soil nitrate levels of 8 MB sites.
Sampling date Soil nitrate-N lb/ac in 0-24” Average soil temperature at 4” (1999-2000)
  Mean Carman Brandon
Early Sept 49 16o C
Mid Sept 53 12.5o C 11.5oC
Early Oct 47 7.3o C 8o C
Mid Oct 53 6.6o C 6o C
Early Nov 44 5.3o C 3.7o C
April 51  
May 60  

Soil nitrate levels did start to increase rapidly once spring sampling was delayed into May, due to mineralization in warm soils.

There were 2 instances where fall N levels did change from early fall sampling:

  • 2” of rain on a sandy soil in late October leached some 20 lb N/ac below the 24” sampling depth
  • Aggressive fall tillage (2x) on a high organic matter loam soil, increased soil N by some 25 lb N/ac

So early fall sampling is generally reliable on cereal stubble, but there remain known environmental and management factors that can still influence soil nitrate levels.

But soil sampling early just to exploit a cheap labour force before the school year starts, should not be a deciding factor (Figure 1).

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Figure 1. Hand sampling to 24” may be considered inhumane depending on soil conditions.

Reference:

Heard, J and J. Lee. 2001 The Influence of Sampling Time on Fall Soil Nitrate Levels. Manitoba Agronomist Conf. http://www.umanitoba.ca/faculties/afs/MAC_proceedings/2001/pdf/heard4.pdf

 

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Fall ammonia applications on dry soils

With rapid crop maturity and harvest many farmers will be looking to the next job on their list – fall fertilization. Typically, fall nitrogen is applied by 34-46% of Manitoba farms, and the most popular source is still anhydrous ammonia.

Last year in SW Manitoba, an inadvertent application of ammonia to excessively dry soil caused distress to adjacent landowners, so let’s review a few of the basics regarding N application.

Can soils be too dry for anhydrous ammonia?

Although soil moisture is low, it doesn’t take much moisture for the chemical reaction of ammonia (NH3) with H+ ions from water to convert to ammonium (NH4+).  This positively charged ammonium cation is then held on the exchange complex of clay and organic matter.

But dry soils do affect the physical closure of injection slots and may allow physical escape of ammonia gas. When soils are dry, big clods of soil may form and leave large channels for ammonia to move quickly and escape to the atmosphere. In those situations, ammonia loss can be substantial.

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.

 

What can one do if soils are dry?

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.

Deeper application may help put the ammonia closer to moisture and prevent the dissipation zone from reaching the surface. In the cornbelt where high N rates are applied on 30” spaced shanks, recommendations for dry soils are to place ammonia 6-8” deep, whereas typical ammonia injection depths are 3-4” on the Prairies. Attempts to place ammonia so deep here on clay soils may just produce larger clods.

Modifications to injection knives may offer some help. In-crop ammonia application for corn often uses closing disks or sealing wings (“beaver tails”) on the knives to aid coverage/closure of injection slots.

However, in most cases the farmer is best to wait for rainfall to improve soil tilth.

How do I know losses are unacceptable?

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.

The “white puffs of smoke” are not ammonia gas, but clouds of water vapour. As long as ammonia smells do not persist after application, these white clouds should not be a major concern.

Will fall ammonia banding make my soils drier?

Fall banding can have contrasting effects of soil moisture. Under very dry conditions where snow-cover is limited, the loss of standing stubble through this banding tillage reduces snow trap on the field and may leave the field susceptible to evaporative losses.

However, in areas where snow cover is more reliable, fall banding may provide better moisture than a spring banded application. Spring banding can dry the seedbed, reducing available moisture and seedbed quality.

Additional information on fall N application is posted at:https://www.gov.mb.ca/agriculture/crops/soil-fertility/pubs/fer01s01.pdf

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