ATTRA Question of the Week

M.K
Rhode Island

Answer: I am pleased to provide you with information regarding the effects tillage has on soil tilth. Below is some information taken from the ATTRA publication Sustainable Soil Management.

Tillage can be beneficial or harmful to a biologically active soil, depending on what type of tillage is used and when it is done. Tillage affects both erosion rates and soil organic matter decomposition rates. Tillage can reduce the organic matter level in croplands below 1%, rendering them biologically dead. Clean tillage involving moldboard plowing and disking breaks down soil aggregates and leaves the soil prone to erosion from wind and water. The moldboard plow can bury crop residue and topsoil to a depth of 14 inches. At this depth, the oxygen level in the soil is so low that decomposition cannot proceed adequately. Surface-dwelling decomposer organisms suddenly find themselves suffocated and soon die. Crop residues that were originally on the surface but now have been turned under will putrefy in the oxygen-deprived zone. This rotting activity may give a putrid smell to the soil. Furthermore, the top few inches of the field are now often covered with subsoil having very little organic matter content and, therefore, limited ability to support productive crop growth.

The topsoil is where the biological activity happens—it's where the oxygen is. That's why a fence post rots off at the surface. In terms of organic matter, tillage is similar to opening the air vents on a wood-burning stove; adding organic matter is like adding wood to the stove. Ideally, organic matter decomposition should proceed as an efficient burn of the "wood" to release nutrients and carbohydrates to the soil organisms and create stable humus. Shallow tillage incorporates residue and speeds the decomposition of organic matter by adding oxygen that microbes need to become more active.

In cold climates with a long dormant season, light tillage of a heavy residue may be beneficial; in warmer climates it is hard enough to maintain organic matter levels without any tillage. The passage of heavy equipment increases compaction in the wheel tracks, and some tillage implements themselves compact the soil further, removing oxygen and increasing the chance that deeply buried residues will putrefy. Tillage also reduces the rate of water entry into the soil by removal of ground cover and destruction of aggregates, resulting in compaction and crusting.
Both no-till and reduced-tillage systems provide benefits to the soil. The advantages of a no-till system include superior soil conservation, moisture conservation, reduced water runoff, long-term buildup of organic matter, and increased water infiltration. A soil managed without tillage relies on soil organisms to take over the job of plant residue incorporation formerly done by tillage. On the down side, no-till can foster a reliance on herbicides to control weeds and can lead to soil compaction from the traffic of heavy equipment.

Other conservation tillage systems include ridge tillage, minimum tillage, zone tillage, and reduced tillage, each possessing some of the advantages of both conventional till and no-till. These systems represent intermediate tillage systems, allowing more flexibility than either a no-till or conventional till system might. They are more beneficial to soil organisms than a conventional clean-tillage system of moldboard plowing and disking. The principal benefits of conservation tillage are improved water conservation and the reduction of soil erosion. Additional benefits include reduced fuel consumption, reduced compaction, planting and harvesting flexibility, reduced labor requirements, and improved soil tilth (Sullivan, 2003).

There is quite a bit of research looking at the different types of tillage systems and their associated benefits and disturbances. The Rodale Institute, in Kutztown, PA, has the longest running research project on tillage. Much of their current research focuses on no-till and rolling cover crops. Please visit their website for more information at www.rodaleinstitute.org. In addition, their New Farm website provides information and resources on the different types of tillage.

The University of California, Davis, is another resource that is involved in studying the effects of tilling the soil. In fact, they have worked with the Beltsville Agricultural Research Center, Beltsville, MD, to conduct a long term no-till sustainable research experiment. UC Davis has published information relating tillage and carbon cycling. The following publication and contact may be of use:

CONSERVATION TILLAGE AND CARBON CYCLING:
SOIL AS A SOURCE OR SINK FOR CARBON (PDF/191 KB)
D. C. REICOSKY
Soil Scientist, USDA-Agricultural Research Service, North Central Soil Conservation Research
Laboratory, 803 Iowa Avenue, Morris, MN 56267 USA.
(320) 589-3411 ext. 144; FAX: (320) 589-3787, E-mail: dreicosky@mail.mrsars.usda.gov

There are a few ATTRA publications that provide additional information and resources on tillage:

Conservation Tillage by Preston Sullivan (2003).

Pursuing Conservation Tillage Systems for Organic Crop Production by George Kuepper (2001).

References:

Sullivan, Preston. 2003. Conservation Tillage. ATTRA Publication. Butte, MT: National Center for Appropriate Technology.

Sullivan, Preston. 2004. Sustainable Soil Management. ATTRA Publication. Butte, MT: National Center for Appropriate Technology.