How Long Does the Dicamba Herbicide Stay in Soil

Yangzhou pioneer chemical CO.,LTD

by:Richard Hoyt

Dicamba is the common name for 3,6-dichloro-2-methoxybenzoic acid that the Environmental Protection Agency registered for use as an herbicide in 1967. Dicamba is a naturally occurring plant hormone that kills broadleaf plants by causing abnormal growth rates. Several different companies market herbicides with dicamba as the active ingredient. The length of time it stays in soil largely depends on moisture, temperature and the soil type.

Adsorption

Dicamba is applied on soil both before and after planting. It is widely used in corn crops. Dicamba may linger in soil by a process called adsorption, which is when molecules from one substance adhere to surface of another substance. This is different from absorption, which is one substance incorporates molecules from another. The U.S. Forest Service says that dicamba lingering in the soil may damage plants by adhering to their roots.

Time in Soil

Agronomists report the time a substance lingers in the soil by its half-life, the length of time a decaying substance declines by 50 percent. The reported half-life of dicamba in the soil varies. The USFS reports that dicamba usually remains in the soil between one and six weeks. Caroline J. Cox, writing in the "Journal of Pesticide Reform," cites studies concluding that while dicamba typically takes between one and six weeks to break down, it has lingered as long as 13 months in Nova Scotia agricultural soils and 1 year in Florida oak and pine forests. The Toxicology Extension Network, a cooperative effort of land grant universities, reports that the typical half-life of dicamba in soil is from one to four weeks, but studies report variations from four to 555 days. Dicamba leached from the soil in three to 12 weeks in humid areas.

Soil Type

Dicamba adsorbs to soil particles in highly acidic soils, those with a low soil pH. It degrades more slowly in wetlands and hardwood forests. Dicamba dissipates more rapidly in soils that have high populations of microbes. Robert J. Taylor of Cornell University writes that dicamba does not "appreciably" adsorb to a range of soils from heavy clay to loamy sand, but does adsorb to peat. In a study reported in "Weed Science," J. T. Altom and J. F. Stritzke found that dicamba degrades more quickly in soil beneath grass than soil beneath trees.

Temperature and Moisture

Dicamba degrades more slowly at temperatures below 68 degrees. Cox cites studies reporting that the half-life of dicamba in soil at 40 degrees is more than six times its half-life at 80 degrees. Dicamba is highly soluble in water and leaches quickly as moisture increases, but when the moisture in soil rises above 50 percent, the rate of biodegradation declines.

Intermediate Substances

In the process of breaking down into water, carbon dioxide and other simple substances, dicamba forms intermediate structures. One of these 3,6- dichloroslicylic (3,6-DCSA) acid adsorbs into soil more strongly than does dicamba. However, the USFS reports that little is known about the toxicity of 3,6-DCSA and other intermediates