Studies Probe Soil Microbes

By Marcia Wood
March 21, 2002

Hard-working soil microorganisms help plants get the nutrients they need to thrive. Agricultural Research Service studies in Weslaco, Texas, are helping to reveal exactly how organic fertilizers and conservation tillage enhance the microbes' activity.

Soil scientists Joe M. Bradford and Larry M. Zibilske of the ARS Integrated Farming and Natural Resources Research Unit lead the experiments. The unit is part of the ARS Kika de la Garza Subtropical Agricultural Research Center.

For their tests, the researchers are working with specialists from three Texas companies that produce organic-based fertilizers: Earthwise Organics, Inc., of Harlingen; Ag Organics, Inc., of Houston; and Gardenville Fertilizer Corp., of San Antonio. All-natural, organic fertilizers are those approved for producing certified organic crops.

The scientists are using the fertilizers to grow strawberries--as a model plant--in greenhouses. So far, microbes have released nutrients from the fertilizers at an impressive rate, according to Zibilske. And, soil microbes seem to unlock the nutrients at the time the developing strawberry plants need them the most. Now the researchers want to find out whether soil microbes in organic fields perform differently than those in conventionally managed soils.

In experiments with cotton and corn, Zibilske and Bradford are using conservation tillage. Unlike traditional tillage, conservation tillage requires leaving crop residue on the field surface after harvest where it slowly decays.

Zibilske and Bradford report that conservation tillage improved the uptake of phosphorus, iron and other nutrients by both cotton and corn plants. The decaying crop residue likely nourished the soil microbes and bolstered their activity.

Allowing residues to decay on top of soil doesn’t hurt crop growth or yield in subtropical environments like southern Texas, according to Bradford, leader of the research unit. It also decreases erosion and reduces evaporation of needed water from the soil surface. In addition, not plowing-under the residues helps farmers save on costs of labor, fuel and equipment such as tractors.

ARS is the U.S. Department of Agriculture's chief scientific research agency.