The Case Of The Disappearing N

Certain aspects of achieving crop yield goals are like building a foundation on a new house. Soil testing and soil test nitrogen (N) levels are two major yield goal foundation components. Without this information, it is very difficult to consistently hit desired yield goals.

This analogy reminds of my first paid job in ag (off the farm) delivering anhydrous ammonia to farmers. Many would make their application rate decision when I arrived in the field based on questions like What did my neighbour apply? Unfortunately, less than 10 per cent of western Canadian farmers soil sample, and of those, most sample only one depth (often zero to 12 inches) with a minimal analysis of N, P, K, pH, maybe S and maybe organic matter.

In years like 2011 (and 2010), it s especially important to soil sample every year to at least two depths zero to six and six to 12 inches. Having the data to back up recommendations gives farmers a whole lot more confidence in the ever increasing money laid out for fertilizer. Simply put, soil tests enable us all to sleep better at night, knowing that fertilizer purchases were based on good, quality information founded in actual soil tests. This provides us with an important clue when trying to determine just where all the N goes each year.


Let s dig a bit deeper by examining some real examples, because it s not just soil tests that provide the clues but also a good understanding of the nitrogen cycle and what specific aspects of the N cycle that can cause N to disappear.

Before we get started the main mechanisms of N loss are leaching, denitrification, immobilization and volatilization. The trick is to know which processes are in play in any given circumstance.

The first example comes from Glennis Sigurdson, agri-coach and owner of Soil Solutions at Rolling Hills, Alta. He says: I have sampled a number of chem fallow fields and they are coming back with virtually no N at any depth, and even stranger was one field with N in only the zero to six-inch depth and nothing below. This year had almost perfect growing conditions for the cropped land, very good moisture in the early part, soil got somewhat dry during mid-summer but received several timely rains. So with this type of year, why would there be so little apparent mineralization on chem fallow?

The second example comes from Matt Gosling, agri-coach and owner of Premium Ag at Strathmore, Alta. He says, I just finished crunching out some crop plans and exactly half the tests (13/26) had N readings of one ppm at all three sampling depths. Fields were sampled this spring and field conditions were extremely wet. It s hard to make plans for 40 bu. canola with 100 lb./ac. of actual N needed to be applied on top of all the other balanced pieces in the puzzle.


These are two very different scenarios, one with good moisture conditions and one with very wet conditions, and yet soil tests that make one scratch their heads. What is happening here?

In the first scenario, one would expect mineralization from the organic matter in the top six inces and, because there was no crop, a good amount of N would normally be found throughout the profile but none was found in the soil samples. One potential culprit would be immobilization of mineralized N by heavy crop residues or weed/ volunteer growth. Depending on how the 2010 crop residues were handled and their C: N ratios this, in itself, could account for the disappearance of mineralized N.

This immobilized N becomes a slow release form of N for the next seeded crop. Similarly, any N immobilized by weed or volunteer growth would also contribute slow release nutrients to the subsequent crop. It is always important to make observations of these factors as they can have a marked impact on the soil test interpretation and subsequent fertilizer recommendation.

In the second example, you would expect denitrification and leaching but here we have no N to depth, which could mean no leaching umless the N all moved below the 24-inch depth! This would be an extreme circumstance but is possible.

Saturated soils will slow both mineralization and immobilization processes while optimizing leaching and denitrification. Keep in mind that we can easily lose over 20 pounds of N per day to denitrification. That s the great thing about measuring things and using science instead of recipes sometimes the difference between expected amounts of N and what s actually there can be staggering. N is the most researched nutrient in crop production, yes, but field conditions and actual N rates available can still offer big surprises, despite what we think we know about N.

The results from several fallow fields east of Strathmore (scenario 2 above) suggested rates of upwards of 80 to 100 lb. of actual N to meet a yield goal. Not surprinsingly, the farmer s first reaction was, No way. That was until they saw the soil tests.

Now, do you know how much fertilizer your next year s crop going to need? How do you know? Are you sure?

ElstonSolbergisasenioragri-coachand presidentofAgri-TrendAgrology

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