With diesel fuel hovering around $3 a gallon, and nitrogen fertilizer at near record price levels, many farmers have gone to reduced and no-tillage systems for economic reasons. For other growers the move to no-till was more of a career choice that happens to save on fuel and fertilizer.
Nitrogen cycling is critical to any type no-till system, with the key points to get as much N as possible in the plant and leave as little possible to get into streams and waterways. A side advantage is carbon storage in soil organic matter, which reduces total carbon dioxide the air and improves our environment.
Reactive forms of nitrogen are those that are plant available and cause problems with how plants are grown. The atmosphere is 78 percent nitrogen, but it is not in a reactive form that plants can use.
Ammonia is readily absorbed by plants and transforms to nitrates in the soil. During spring planting time, it takes 2-3 weeks for most forms of nitrogen fertilizer to become nitrates. Nitrates are readily available for plants in the soil, and this is the form that causes problems in non-point source pollution of streams. Though it has consistently proven to be a minor player in water pollution, agriculture has done a poor job of conveying this message to the general public.
When farmers go to cover crops as part of a no-till system, there can be problems with immobilization of nitrates. “If we are managing nitrogen in any particular field, we need to think about nitrogen loss pathways,” says Virginia Tech Soil Scientist, Mark Alley. To increase economics to the grower and to improve environmental quality, requires minimization of nitrogen loss from each individual field, the Virginia researcher stresses.
Though each field is different, he explains, there are a number of nitrogen loss pathways that are common, regardless of field conditions. The question is, which is the most significant?
When nitrogen is transformed from a reactive form to a non-reactive form, it doesn't cause environmental problems, but may reduce plant-available nitrogen. Developing a workable nitrogen management program that takes into account the variations among soil types and the interactions between different soils and different plants is critical, Alley explains.
In no-till systems, Alley says, immobilization of nitrogen most commonly comes from a cover crop. That's good, the Virginia scientist says. However, if a farmer wants to add nitrogen to a killed cover crop for the benefit of the crop planted into the cover crop, holding nitrogen in a non-reactive form is bad, he adds.
The key is to know how to manage the movement of nitrogen in the soil. Addressing short-term production concerns can be beneficial to addressing environmental concerns — the two are intricately linked, Alley explains.
Volatilization losses are another pathway for losing nitrogen from a crop. Transformation of ammonium to ammonia causes a loss of nitrogen into the atmosphere and is both an economic loss to the farmer and a greenhouse gas problem that is linked to environmental dangers that create a negative image that farmers don't like and don't need, Alley points out. Surface applied urea fertilizer and manures and biosolids can be both an economic loss, and an air quality problem.
Denitrification, which is transformation of nitrates to nitrous oxide or dinitrous gases in the atmosphere, can be an economic loss, and N20 is another greenhouse gas, Alley notes.
Leaching is the movement of nitrates along and below the root zone of plants, and this is one that really causes farmers problems, according to Alley. “When nitrates move through the soils and get into our water supply, it causes rapid algae growth, and has been long-linked to water pollution,” the Virginia soil scientist says.
“Nitrogen is complex, and the answer for best management for any individual field is probably going to change each year. So, rather than saying we have a method of application, we should say we have a nitrogen management program that addresses questions like is it wet, is it dry, do I have a cover crop and what is going on in this field this year, and how can I best manage it,” Alley stresses.
Organic matter is a critical component of a nitrogen management program. On average organic matter is about 5 percent nitrogen and 60 percent carbon, for an ideal carbon to nitrogen ratio of 12:1.
“If the Federal Government develops nutrient trading programs, this 12:1 ratio and similar numbers are going to become very important to farmers,” Alley notes. For example, he says surface six inches of soil with two percent organic matter has 2 million pounds of soil and 40,000 pounds of organic matter and 2,000 pounds of nitrogen — most of which is not in a usable form for plants, he explains.
Soil in traditional peanut areas of Virginia and North Carolina typically has 0.5 to 1.0 percent organic matter and not much nitrogen. At 0.5 percent organic matter, there is only 22,000 pounds of carbon. “If we do have nutrient trading programs in the future, these soils won't fare too well, unless organic matter is increased,” Alley points out.
Some advantages of long-term, continuous no-till systems, Alley points out, are:
- Increased soil organic matter.
- Reduced sediment loss.
- Increased water infiltration.
- Increased crop production efficiency.
“We are seeing increases in organic matter in the top surface inch of Coastal Plain soils from continuous no-till systems from two percent organic matter to four percent organic matter. In real terms that means increasing organic matter from 6,600 pounds to 13,000 pounds, nitrogen from 333 pounds to 667 pounds and carbon from 3,667 pounds to 7,300 pounds,” Alley explains.
How much these increases are worth to farmers will be determined by the individual farmer and his/her crop production program; and how much value they have in nutrient trading programs will be determined by political policy, the Virginia scientist says.
“On the long-term no-till sites, some continuous up to 14 years, we have seen increases in organic matter in the surface layer of the soil, increases in nitrogen and carbon and decreases in soil bulk densities, which is the pore space in a soil,” he says. Reducing bulk density of soil has a positive impact on root growth and overall vigor of the plant, he adds.
In a drought year, like we have had this year, the difference between four years in no-till and four years in conventional-tillage in one test plot that we looked at in early April was dramatic, Alley says. “There was barely enough moisture for corn to germinate, and to till that soil would require a lot of diesel fuel, you would wear out a lot of chisel points and plow points, and you would lose what little bit of moisture you had. By contrast, in the no-till fields, we could have dropped a corn planter on that field and planted corn,” he explains.
In northern Virginia, the winter of 2005-2006 was one of the driest on record and there is some concern whether growers killed their cover crop on time. The biggest challenge is whether there was enough moisture left to plant corn. This is a good reason for getting cover crops planted early, so they can get growth in the fall in order to capture nitrogen before the winter, he explains, and then having enough cover to be able to kill early in the spring if rainfall is low.
In no-till corn behind wheat, lacing a 2X2 band of starter fertilizer at planting is much more efficient versus broadcast application of nitrogen over cover crop residue. “If you can get 40-50 pounds of N in that starter band, you have a longer window of time to get back to side-dress, you reduce immobilization losses, N is down two inches, and you have no runoff losses. We've seen this program work really well,” Alley stresses.
In corn tissue analysis Alley's research team found the same amount of N from 30 pounds of nitrogen in the band as with 60 pounds broadcast. With nitrogen prices at 35 cents per pound, it is easy to understand why banding starter fertilizer is a good idea, he adds.
“Long-term no till will help us reduce erosion and runoff — we have to do this. It will increase infiltration and improve efficiency of water usage, and more nitrogen is sequestered and reduced leaching in some cases, and in others increased leaching has been recorded — we must understand that process better,” Alley concludes.
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