It is well known that alfalfa and other legumes in a crop rotation fix nitrogen from the atmosphere into the soil.
What may be less widely understood, however, is the extra boost that more diverse crop rotations offer to long-term yields in the form of increased organic matter.
This phenomenon is described by soil scientists as the “non-nitrogen” benefit, a catch-all phrase that includes increased water retention, better “tilth” or soil structure, and the difficult-to-measure gains from increased diversity in the soil ecology – an improved environment for the myriad bugs, bacteria and fungi that together cycle nutrients in the topsoil.
Besides the fertilizer dealer, nitrogen for crops comes from two sources: the interaction of rhizobium bacteria and legume roots, which together convert the gas that composes 78 per cent of the atmosphere into something that plants can use, and the slow breakdown of organic matter in the soil by other bacteria and fungi.
Studies show that the top six inches of Manitoba soils on average hold about 6,000 pounds of N per acre, according to Martin Entz, a professor in the faculty of agricultural and food sciences at the University of Manitoba.
“That nitrogen, however, is not readily available. It’s in what we call the organic nitrogen pool,” he said, in a presentation at Ag Days 2009.
“Some of it, maybe half a per cent or one per cent, becomes available every year through the biological process of mineralization.”
Depending on weather conditions, some 30 to 60 pounds is freed up by soil bacteria each growing season. This is evident from old studies at Indian Head, Saskatchewan, which found that over 22 years, the organic nitrogen bank in the top six inches fell from 6,936 kg per hectare down to 4,700 kg.
That 30 per cent loss was the result of continuous cropping without putting anything back. In Manitoba, a similar trend has occurred over the past 120 years of agriculture, since the virgin prairie was first put to the plow.
“We grew really nice crops just based on the organic nitrogen, but we depleted it. In the Red River Valley, people grew 25 consecutive wheat crops of 30 to 35 bushels per acre. There was enough nitrogen to support those crops because of this organic pool,” said Entz.
When yields began to fall, farmers adopted summerfallow, which he described as putting “pressure on the sponge” to get two years’ worth of the nutrient.
“It just wrings the nitrogen out a bit faster. Once we used up that nitrogen, we were stuck using the fertilizer nitrogen that we use now, which is made from fossil fuels such as natural gas.”
Currently, three to five per cent of global natural gas production is used to make nitrogen fertilizer, he added, an amount that equals one to two per cent of all fossil fuels produced in the world each year.
Conventional farming uses about 3,000 megajoules of fossil energy per acre, per year. One megajoule is equal to a 35-mm film canister full of gasoline, and nitrogen fertilizer typically accounts for about half the total consumed.
Energy prices are fluctuating wildly, and the long-term trend