Organic Versus Conventional Weed Control

A study in the journal Weed Science investigates weed persistence in organic versus conventional farming systems.

by Dani Yokhna
Courtesy Weed Science Society of America
Scientists studied the viability of lamb’s-quarters and smooth pigweed (pictured above with redroot pigweed) in both organic and conventional farming systems.

Weeds are hard to kill—they seem to come back no matter what steps people take to eradicate them. One reason is because of the persistence of weed seeds in the soil. Organic and conventional farming systems both have methods for battling weed seeds, and the authors of a study in the April-June 2011 issue of the journal Weed Science conducted tests to compare the results.

The scientists’ research determined weed-seed viability under both organic and conventional farming systems over a two-year period at agricultural research locations in Maryland and Pennsylvania. To compare the systems, researchers buried two types of weed seeds—smooth pigweed and common lamb’s-quarters—in mesh bags. Weed-seed viability was determined by retrieving seeds every six months over the two-year period.

Depth of seeds in the soil, environmental conditions and soil management are among the factors that affect seed persistence. Under conventional soil management, tillage is an important practice that manipulates the depth of seeds and environmental conditions that can influence weed-seed persistence. Organic soils have enhanced biological activity, with more carbon, moisture and microbial activity that could lead to greater seed decomposition.

The organic soils in the study were higher in total soil-microbial biomass than the soils of the conventional-farming tests. This was measured by phospholipid fatty acid content. But the results of the tests did not lead researchers to conclude that this microbial biomass has a dominating role in seed mortality.

Pigweed seeds showed shorter life spans under the organic farming system in two of four experiments, whereas lamb’s-quarters seeds had shorter life spans in just one of the four experiments. The seeds in the conventional farming system had shorter life spans in two of four experiments.

These results leave an ambiguous answer to the question of which farming system can better eliminate seeds deep in the soil to control weeds from their source.

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