Agriculture

Seed-Placed Nitrogen Fertilizer: A Second Opinion Please 

Prepared by John Heard, MAFRD Crop Nutrition Specialist (FPE)
 
By this time of year many growers have calibrated their seeders to ensure proper delivery flow and rates of seed and fertilizer. It should not be too late to reset the safe rates of seed-placed nitrogen fertilizer. Rate guidelines are not carved in stone but are a moving target, largely based on the moisture of the seedbed.  As seedbeds and soils dry out, the same rate of nitrogen fertilizer can cause more stand thinning. 
 
When I am buying something, I like second opinions.  Many farmers also seek out second opinions of safe rates of seed-placed nitrogen.  When opinions differ, they want to know why.  Generally Manitoba’s safe rate of seed-placed urea guideline1 (Table 1) is more conservative (ie. less) than guidelines from Alberta2 and Saskatchewan3 agriculture departments.  One of the reasons is due to the generally higher pH of Manitoba’s soils.
 
Table 1.  Rates of urea fertilizer (lb N/ac) safely applied with cereal and canola seed if seedbed soil moisture is good to excellent.
 
Soil Texture
1 in. spread
2 in. spread
3 in. spread
 
(disc or knife)
(spoon or hoe)
(sweep)
 
 
Row spacing
 
 
6"      9"      12"
6"      9"      12"
6"      9"      12"
 
 
SBU*
 
 
17%    11%  8%  
33%    22%   17%
50%   33% 25%
CEREAL SEED
 
 
 
Light (sandy loam)
  10      0        0
  20     15       10
  30     20      15
Medium (loam to clay loam)
  20     15       10
  30     25       20
  40     30       25
Heavy (clay to heavy clay)
  25     20       10
  40     30       25
  50     40       30
CANOLA SEED
 
 
 
Light (sandy loam)
   0       0        0
  10      0         0
  20     10        0
Medium (loam to clay loam)
   0       0        0
  20     10        0
  30     20       10
Heavy (clay to heavy clay)
  10      0        0
  30     20       10
  40     30       20
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
(From page 9, Manitoba Agriculture Soil Fertility Guide1). 
 
When urea, CO(NH2)2, hydrolyses in the soil it converts to ammonia (NH3) and ammonium, (NH4+). The ammonium ion (NH4+) has a positive charge and so tends to be held on clay and organic matter particles and is less toxic to the seed.  This is one reason why safe rates are greater for clay than sand soils. The free ammonia form (NH3) tends to cause seed toxicity and reduces stand emergence. Under high pH, more of the N is present in the free ammonia form.  So similar rates of seed-placed urea will tend to be more toxic on high pH soils.
 
Any rate of seed placed N will reduce stands and it is often a case of balancing stand loss with flexibility of nitrogen application.  Often stand losses of 15% can be accommodated without affecting final crop yield.  Consequences of stand loss are:
  • Reduced yield in severe cases.
  • Delayed maturity. Cereals may compensate for fewer plants by producing more tillers, which may mature at slower rates.
  • Reduced weed competitiveness.  Herbicide use may need to be intensified.

Another place producers and agronomists can get a second opinion on safe fertilizer rate guidelines is from a spreadsheet calculator developed by South Dakota State University (SDSU) soil specialists4.  It is Excel operated and available at Fertilizer Seed Decision Aid (EXCEL).

This spreadsheet uses data from a number of research sources. It lets the grower or adviser set the level of stand thinning they are willing to accept.  In Table 2, I have used the SDSU calculator to assess the stand risk from our rather conservative Manitoba guidelines in Table 1.  I have used the same examples of furrow opening, row spacing, and the light (coarse) and heavy (fine-medium) soil textures and a moist soil condition.
 
Table 2.  Estimated stand thinning (% stand reduction) from seed-placed urea using Manitoba safe rate guidelines from Table 1.
 
Soil Texture
1 in. spread
2 in. spread
3 in. spread
 
(disc or knife)
(spoon or hoe)
(sweep)
 
 
Row spacing
 
 
6"      9"      12"
6"      9"      12"
6"      9"      12"
 
 
SBU*
 
 
17%    11%  8%  
33%    22%   17%
50%   33% 25%
 
Estimated stand reduction %
CEREAL SEED
 
 
 
Light (sandy loam)
  10      -        -
  9        11       10
  9       9       9
Heavy (clay to heavy clay)
  16     18       13
  12     14       16
  10     12       12
CANOLA SEED
 
 
 
Light (sandy loam)
   -       -       -
  15     -         -
  30     15        -
Heavy (clay to heavy clay)
  20      -        -
  30     30       20
  26     29       25
 
The safe rate of N guidelines for cereals produce an estimated 15% or less stand thinning, so are probably quite appropriate for use. I am surprised that the SDSU calculator projects such high stand thinning to canola when using Manitoba guidelines. This should be of particular concern to canola growers who are shaving seeding rates.
 
Options to improve seed-placed safety in these instances are to:
  • use Agrotain to delay urea conversion to ammonia. NOTE: Agrotain is no longer recommended by the manufacturer, KOCH Agronomic Services, for this seed safety use.
  • replace a portion or all of the urea as ESN polymer coated urea

Of course there are numerous other placement and timing options for nitrogen that do not compromise crop stand and these could all be considered.

This SDSU calculator can assess improvements in safety with Agrotain (NBPT) use.  It also estimates stand injury as seedbeds become borderline moist or dry.  This may become an important consideration if seeding progresses and moisture is not received.  Indeed our Manitoba guidelines indicate that where seedbed moisture is low or weather is hot or windy, those rates in Table 1 should be reduced by 50%. In some cases there is enough nitrogen in our phosphorus based fertilizers to jeopardize seed safety of sensitive crops.  This is covered in a previous article: Safe Rates of Seed-Placed Phosphorus for Manitoba - Narrow Row and Row Crops
 
Stand establishment is critical for Manitoba crops.  If seed-placed nitrogen rates are likely to compromise stands severely, use other options for applying that nitrogen.

References

1Manitoba Agriculture, Food and Rural Development.  2007. Soil Fertility Guide.
 
2Alberta Agriculture and Rural Development.2002. Fertilizer Application and Placement. http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex621
 
3Saskatchewan Ministry of Agriculture. 2009.  Guidelines for safe rates of fertilizer placed with the seed.
 
4Gelderman, R. 2007.  Fertilizer placement with seed: A decision guide. Proc. of North-Central Soil Fertility Conf.  Vol. 23. Pp. 46-54. http://extension.agron.iastate.edu/nce/ncepdfs/2007/nc2007-p46-gelderman.pdf