This is the VOA Special English Agriculture Report.

Farmers and gardeners have turned the soil for centuries. But many now believe in no-till or reduced tillage, a method also called conservation tillage. The Conservation Technology Information Center says 40 percent of American cropland used this method at last report.

Tilling can remove weeds and the waste from last season's crops and help break down old plant material into fertilizer. But it can also increase the risk of soil erosion by rain or wind.

Tilling also releases carbon dioxide from the soil into the atmosphere. No-till keeps carbon in the soil and avoids the release of heat-trapping gases from motorized equipment. It also keeps water in the soil and protects helpful organisms like earthworms. And it saves money on labor, machinery and fuel.

A recent study reported on almost 20 years of different tillage methods in the American states of Colorado, Kansas and Nebraska.

The researchers say tillage makes soil less resistant to being broken apart by raindrops than plowed soil. They say no-till stores more soil carbon, which helps the soil particles stick together. They found that the first two and a half centimeters of topsoil are up to seven times stronger against rain than plowed soil.

The study also noted that when tilled soil particles come in contact with air, soil organic matter is lost through the process of oxidation.

The findings appeared in the Soil Science Society of America Journal. Humberto Blanco-Canqui from Kansas State University and Maysoon Mikha from the Agricultural Research Service led the study.

Another recent study found that a single tillage does not harm yield or soil structure in land that is normally not tilled. Charles Wortmann of the University of Nebraska-Lincoln says a one-time tillage may be used to correct a problem like aggressive weeds. His five-year study appeared in the Agronomy Journal.

Fabian Fernandez, a soil and plant nutrition expert at the University of Illinois, says no-till means less intensive labor. But he also says it does not mean an end to preparing the ground for planting.

Jimmy Wagner works at the American Plant garden center in Bethesda, Maryland. He says hand tools like tilling forks, shovels and thatching rakes require more work than a rotary tiller, but they work well and cost less.

If you till, wait until the soil is warm and dry enough. If the soil stays together in a ball, it is too wet to till.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Jim Tedder.

tillage: the preparation of land for growing crops（耕作，耕種）

oxidation: 氧化

thatch: to cover the roof of a building with thatch（用茅草蓋屋頂）

rake: a garden tool with a long handle and a row of metal points at the end, used for gathering fallen leaves and making soil smooth（耙子）

Related stories:

World leaders urged to meet development goals by 2015

Pretty flowers lead two scientists to a discovery

Raising goats for their hair

Imported foods raise obesity, health issues for Pacific islanders

（來源：VOA 編輯：陳丹妮）