Conservation-tillage alone — without the benefit of a cover crop — doesn't work, at least not in Southeastern soils.
“In the last 10 years, I've changed my mind about conservation-tillage,” says Wayne Reeves, USDA-ARS research agronomist with the National Soil Dynamics Laboratory in Auburn, Ala. “I now know that conservation-tillage alone doesn't work. Erosion rates for no-till actually can be higher on our soils than with conventional-tillage because we have surface soil compaction.”
Soils in the Southeast are under more intensive pressure from climate and degradation due to the loss of organic matter and erosion, says Reeves. Farmers in the Southeast, therefore, must be better at what they do than growers in the Midwest and other regions of the country, he adds.
“If we don't have any residue cover in the fields, it leads to a lot of run-off. So I've changed my attitude about how to approach conservation-tillage. Soil carbon or organic matter and crop residues are the keys to making conservation-tillage work,” notes Reeves.
A lack of tillage isn't what makes a conservation-tillage system successful, he adds. “You hear some people say that after four to six years of no-till, you won't have soil compaction problems, and you'll have ‘ice cream’ for soil, but that's not the case. It's certainly not the case with Coastal Plains soils and their inherent hardpans.”
If you must do tillage, go ahead and do it, advises Reeves. But, he says, it has to be non-inversion-type tillage.
“The main thing to remember is that we must produce residue faster than we decompose it. And, we need to pump carbon into our soils so that organic matter accumulates faster than we're losing it. The only way to do this is to intensify our cropping systems and then use conservation-tillage to prevent that residue from decomposing.”
In Brazil, which has a similar climate to the Southeastern U.S., farmers routinely use cover crops in integrated conservation-tillage systems, says Reeves. Brazilian growers, he says, don't focus only on conservation-tillage.
“If their land is going to be fallow for six weeks, they put in a cover crop. They also use crop rotation because they don't have government price supports. They have low labor costs, but they also have higher transportation costs. Their backs are to the wall, and they do everything possible to reduce their input costs.”
Research in north Alabama's Tennessee Valley has proven the worth of using cover crops in no-till systems, says Reeves. “In a five-year experiment, where we used no-till and planted a rye cover crop each year, we got up to 4-percent organic matter in the top inch of the soil.”
On the heavy clay soils of north Alabama, he adds, Para-Tilling or subsoiling in the fall in the row and planting a rye cover crop works very well. “The tillage drops the organic matter to 3 percent, and the Para-Till increases organic matter to a deeper depth than the strict no-till system due to increased root growth. Root growth deposition of carbon will increase organic matter.”
Whatever occurs in the top inch of the soil is very important, says Reeves, especially in Alabama where water is a major productivity issue in one out of every two years. “The top inch is the most important part of the soil profile because it affects water infiltration.”
A rainfall simulation study, he says, has proven the benefit of cover crops in conserving water. In the study, 2 inches of water were applied on conventional and no-till plots with and without cover crop residue. The Coastal Plains soils in the plots, he adds, were very sandy with low organic matter and a hardpan.
“After applying 2 inches of water, we conserved about 95 percent of the water in no-till plots with residue in place. There was almost no run-off. In the conventional-tillage plot, about 25 percent of the water infiltrated the ground. When we removed the residue from a long-term no-till plot, we still saw 55 to 56 percent infiltration.
“During the cotton growing season, the amount of water from a 2-inch thunderstorm that would go into the soil and be available to your crop in a conventional system would last about three days. With residue in place, it would last about 11 days. When we used a Para-Till, we disrupted the hardpan and increased the infiltration for both the conventional and no-till without residue. Residue has a larger impact on getting that water down into the soil surface than Para-Tilling.”
Moving into a “high-residue” system requires a learning curve, says Reeves. “You can't just jump into it. It's helpful to follow Extension recommendations and check your soil temperature for the planting date rather than following a calendar.”
Long-term weather data from north Alabama indicates that it takes until May 1 to accumulate enough heat units to germinate a cotton plant, he says.
It's also important, he adds, to kill down a cover crop four weeks prior to the planting date. “You need to get a good kill on that cover, and you need to reduce the risk of that cover crop extracting soil water when you have a dry spring. In addition, chemicals in that straw will control weeds but also will control the germination of cotton.”
Another component of a high-residue system is managing the residue, says Reeves. Residue in a no-till system can cause much wetter conditions than in a conventional system, he says, resulting in problems with closing the seed furrow.
“We commonly see more down pressure being placed on the closing wheel, and this can cause problems. This can be avoided by using spoked closing wheels. You can close the seed furrow in a wet situation — on a number of soils — and you can do it with less compaction.”
Hardpans are natural in Coastal Plains soils, says Reeves, but there are ways to handle them in conservation-tillage systems. “I like to see hardpans handled by doing the tillage with a non-inversion method.”
Growers need to keep in mind that cover crops will cost money, he says. Costs per acre could run from $5 to $45 per acre, depending on the type of cover crop used. The cost of planting a cover crop can range up to $12 per acre for custom drilling.
“Sometimes, the cost of nitrogen fertilizer is overlooked in these systems. If you want the kind of residue we're recommending, you'll tie up about 30 to 40 pounds of nitrogen and it'll cost you $6 to $8 per acre in extra fertilizer costs on that cotton.”
Research has shown, says Reeves, that a small grain cover crop will cost about $12 to $15 per acre. “If you're not going to do it to get ample residue, then don't do it at all — you're wasting your money, whether it's $5 or $40 per acre. If you kill it too early, you're increasing your costs and you're not getting any of the benefits. It won't pay off later in the case of drought.”
Researchers, he explains, have been looking at a Brazilian system of high-residue management where a rye cover crop is rolled in one direction and cotton or peanuts are planted parallel to the cover.
“In strip-till cotton, with heavy rye versus no cover crop — using a KMC strip-till rig — our yield differences ranged from 523 to 1,476 pounds of lint per acre. Through the three years of the study, yield differences were 358 to 1,307 pounds of lint per acre. That could be worth about $200 per acre per year, and it cost only about $19 per acre to put in the rye cover crop. And farmers in Brazil are doing this on thousands of acres.”
Conservation-tillage by itself simply can't solve the problem of soil degradation in the Southeast, says Reeves. “We need residue production and conservation of that residue once we produce it.”