Wednesday, March 11, 2009

Growth Spurts: No-Till Has Its Limits

Recognized globally as an ideal means of conserving soil and water while also storing soil carbon, the agricultural practice known as "no-till" may not be applicable under all environmental conditions.

No-till farming means leaving residue left on the soil surface after harvest in place rather than plowing it under. Compared to plowing, no-till has myriad benefits: less labor, less machinery wear, decreased fossil fuel consumption, reduced soil erosion, improved soil productivity, increased wildlife habitats and a better method of maintaining and conserving soil water.

No-till is considered a successful carbon sequestration practice when carbon input (storage) exceeds carbon output (loss). Carbon input includes crop residues, winter cover crops, complex farming systems, and use of compost and manure. The output includes losses of carbon by decompositon, erosion and leaching.

Ohio State University soil scientists recently measured carbon levels in no-till fields throughout seven states and found that soil texture, moisture, temperature, and terrain parameters affected the amount of carbon stored on the soil surface.

"No-till is not applicable everywhere as a means of practicing carbon sequestration," says Rattan Lal, soil scientist at Ohio State's Ohio Agricultural Research and Development Center. "There are situations where other carbon sequestration methods would be more effective. I'm not saying that no-till is not good. It is a good practice, but it does not work for all soils, for all crops and all conditions. We must not make carbon sequestration synonymous with no-till. The strategy is to develop a system of soil management in which carbon input into the system exceeds the output."

Lal and his colleagues studied no-till fields in Ohio, Michigan, Indiana, Pennsylvania, Kentucky, West Virginia and Maryland and identified situations where the practice was the most effective in storing carbon and where it was not.

"Basically, those soils that are well-drained, are silt/silt-loam in texture, warm quickly and have some sloping characteristics prone to erosion are excellent candidates for no-till. Clay soils or other heavy soils that drain poorly, are prone to compaction and are in areas where the ground stays cooler may not always increase carbon storage through no-till."

In Ohio, for example, the researchers found that no-till would store carbon on about 40 percent of the state's cropland. In actuality, no-till is practiced on 35 percent of Ohio's field crops, said Lal.

"Globally, no-till is practiced on only 6 percent of the total cropland and mostly practiced in the United States, Canada, Brazil, Australia, Argentina and Chile. There's a reason for that -- because it can be worked into practices in which carbon input exceeds carbon output."

In situations where no-till may not be ideal, there are plenty of other carbon sequestration methods available, including mulching, cover crops, complex crop rotations, mixed farming systems, agroforestry, and biochar (a charcoal-like biomass material).

The study also compared carbon levels between no-till and conventional tillage fields and found that, in some cases, carbon storage was greater in conventional tillage fields. But the key is soil depth.

"If you compare carbon storage between no-till and plowed fields with the plow depth, or the first 8 inches of the soil, carbon storage is generally much greater in no-till fields than in plowed fields. But if you go deeper, say 12 inches and deeper, one may find more carbon stored in plowed fields than in no-till."

Farmers should not measure soil carbon based just on surface depth. Lal recommends going to as much as 1 meter (3.25 feet) below the soil surface.

Lal said that the study is not a criticism of no-till and its benefits, but simply a way of determining where the practice best fits and where other carbon sequestration methods may work better.

"In situations where no-till is ideal, it's a sustainable soil management practice that simply can't be ignored," said Lal. "It saves time, money and wear on machinery and its profit margin is much higher than plowing."

Source: Ohio State University College of Food, Agricultural, and Environmental Sciences

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