“Being the opportunistic SOBs that we are in the industry, we take that free nitrogen, convert it to something, and then charge the guys for it.”
– RAY DOWBENKO
Growing crops to feed the world removes nitrogen from the soil.
To figure out how much needs to be put back in for next year, a farmer can stick a probe in the ground and send a sample off to the lab, or he can think backward to estimate how much the previous crop took out.
For wheat, that amount is roughly 2.5 pounds of the nutrient per bushel; canola, 3.5 lbs.; barley 1.5 lbs.; and oats, 0.75 lbs.
Plants can eat both ammonium and nitrate forms of nitrogen. Farmers feed it to them in the form of animal manure, nitrogen fixed by the natural interaction of rhizobia and legume plant roots, and by hauling it in by the truckload from fertilizer manufacturers.
Up to 75 per cent of crop yields can be traced back to available nitrogen, while less than 20 per cent comes from phosphorus, and just three per cent from micronutrients, sulphur and potassium.
“N2 gas in the atmosphere is somewhere from 72 to 78 per cent of the air we breathe. Being the opportunistic SOBs that we are in the industry, we take that free nitrogen, convert it to something, and then charge the guys for it,” joked Ray Dowbenko, a senior agronomy specialist with Agrium, at a soil fertility workshop here.
Fertilizer plants take N2 gas from the atmosphere, and using natural gas both as a source of hydrogen and heat, combine the two under pressure to make anhydrous ammonia (NH3), the base fertilizer product from which all others are derived.
Adding nitric acid creates ammonium nitrate. The addition of water plus nitric acid makes urea ammonium nitrate (UAN). Further adding carbon dioxide makes urea, and a dash of sulphuric acid makes ammonium sulphate. Adding phosphoric acid makes mono-ammonium phosphate (MAP).
BTUS PER TONNE
“It takes 66 million BTUs (British thermal units) to make one tonne of ammonia, and then you’ve got to add another 33 million BTUs to convert that ammonia into urea,” said Dowbenko. “Sixty to 70 per cent of the cost of a tonne of ammonia is comprised of natural gas costs.”
Although organic matter contains about 98 per cent of the nitrogen in soil, it first has to pass through a mineralization process by which soil microbes break it down into inorganic ammonium and nitrate forms that plants can use, typically at a rate of one to four per cent per year. Every one per cent organic matter represents 1,000 lbs. of potential N per acre.
But the rate of breakdown depends on the ratio of carbon to nitrogen (C:N) and the soil temperature and moisture. The carbon, or biomass, is the food, while the nitrogen portion is the fuel.
If nitrogen isn’t available in sufficient quantities, the process is temporarily stalled as microbes “immobilize” part of it by incorporating it into their bodies. When they die and decompose, they release it in a form useful to plants.
“Show this at a farm meeting and guys will ask, ‘Why do I need any nitrogen?’ But the reality is that our environment doesn’t allow all this to be available, or when the crop needs it.”
Where a C:N ratio of 30:1 favours immobilization, 20:1 favours rapid mineralization. Pea straw is typically under 40:1, while wheat straw is up to 80:1.
A proper balance of nitrogen fertility makes plants healthier, and therefore better able to weather periods of drought by making the stomata – the holes in the leaves – better able to regulate moisture losses through transpiration, which is how plants “breathe.”
One study of water-use efficiency showed that
underfertilized corn yielded 91 bushels to the acre on 18.6 inches of water, compared to a well-fertilized plot that produced 147 bushels on just under 20 inches.
That’s how nitrogen gets used by plants. But how does it get wasted?
One way is through volatilization, which is how part of a farmer’s hard-earned money evaporates into thin air when he broadcasts urea without using a urease inhibitor to top dress winter wheat or forages in warm, damp conditions when there’s not enough rain to wash it into the soil.
“It’s really temperature and moisture that drives all of this,” he said.
Denitrification occurs when the soil is wet. Gasping for oxygen, the soil microbes strip off the oxygen from the nitrate molecule to stay alive. At 5C, the rate could be two to four pounds per acre per day. But if a heavy rain comes later in the summer, when the soil is warmer, the rate could easily triple, he said.
The third, leaching, happens when the soluble nitrate form gets washed downward, away from plant roots. A heavy rain on sandy soil can move the nutrients out of reach and slow crop development until the roots can catch up with it.
Claims that foliar nitrogen sprays are seven times more efficient than ground application don’t hold up to scrutiny, he said.
But tests at the University of Manitoba found spraying in a greenhouse setting saw only 23 per cent of the nitrogen absorbed through the leaves, with the rest dripping off into the soil.
“The plant leaf architecture isn’t designed to capture anything other than sunlight energy,” Dowbenko said, adding that while micronutrients can be absorbed that way, foliar sprays are best used only as a temporary boost or “rescue” application of 10-15 lbs. per acre. [email protected]