Nitrogen is not just something we need to add to our soils, it is essential to survival.
'Life as we know it would not exist if it were not for nitrogen,' according to Dr. Jim Friedericks, AgSource Laboratories' Outreach and Education Advisor.
Nitrogen is essential to plant growth specifically because it is a major factor of chlorophyll and needed for photosynthesis. It is also a chief component of amino acids, the building blocks of protein, and it is necessary for nucleic acids such as DNA which allows plants to reproduce.
Plants absorb nitrogen from the soil as either nitrate or ammonium. Nitrogen can be added to the soil in these or similar forms as fertilizer (including ammonia, nitrate or urea), or it can be added as more complex organic forms from manure or crop residues. The organic forms will be changed to ammonium and then to nitrate through biological conversions.
When evaluating nitrogen availability, emphasis has been on measuring the nitrate in soil because that is the final product of most of these conversions. Once N is in the nitrate form in the soil it is easily taken up by plants but it is also subject to leaching and other losses. Ammonia (NH3) added to the soil as a gas is quickly converted to the ammonium ion (NH4+) when it contacts water.
Because of this, spring nitrate testing is important for several reasons, including:
· Verifying that the current N fertilization program will supply adequate nitrogen to this year's crop.
· Measuring fields for nitrate carryover after a drought, or monitoring fields after a wet spring.
· Checking fields that have a history of manure applications.
· Evaluating fields that have had previous legume crops in the rotation for nitrogen availability.
However, remember that both nitrate and ammonium are applied as fertilizer and are available for plant uptake. Nitrogen that becomes available from soil organic matter, crop residue and manure ultimately converts to both ammonium and nitrate as well.
Adding a test for ammonium will give you a more complete look at the nitrogen available for your growing plants, said Friedericks. Because this is a biological process, early in the season more ammonium is present because the nitrification conversion is slower in cooler soil. But other factors that delay the conversion to nitrate are low pH and water logged soil.
'Accounting for the conversion of ammonium to nitrate provides a much better picture of what's available for crop uptake,' said Fredericks. 'Nitrogen levels in your fields might not be as low as you think.'
Add an ammonium test to the nitrate test to gain: A more complete measure of the nitrogen available to a crop; an evaluation of the conversion process from ammonium, or urea, to nitrate; high rates of conversion in warm soil and pH above 7, means less ammonium in reserve; low rates of conversion in cool, wet soil and acidic pH, means ammonium remains longer and potential savings on applied nitrogen and reduced nutrient loss to leaching.
Don't forget ammonium is not mobile in the soil but nitrate leaches downward.
'A second, deeper sample may help you track down the nitrate,' said Friedericks.