Microbial Indices of Soil Quality

Microbial Indices of Soil Quality

Summary

A study was initiated to obtain experimental data that would help us better understand the biological aspects of soil quality. The specific objectives of the study are to:
1) measure aerobic and anaerobic microbiological processes in soils obtained from differently managed farms representing major geographical regions of Illinois;

2) using data collected through the Illinois Soil Quality Initiative (ISQI), examine relationships among soil biological, chemical and physical features; and

3) based on the success of biological measures in assessing status of soils under varied farming systems, and the predictive capabilities of chemical and physical measures, develop one or more simple field tests for microbial indices of soil quality.

The status of our nation’s soil is of interest not only to producers, but the general public as well. Many different measurements have been proposed to inventory soil quality. However, the biological component of soil is complex and dynamic, and thus is very difficult to characterize. One of the most perplexing problems in measuring biological properties of soils is that relatively small changes in moisture may cause big shifts in the activities of microorganisms, and the optimum moisture content is unique for each soil. It is therefore difficult to assess what moisture content to use to evaluate soils from different sites.

We chose to examine three microbiological processes under flooded soil conditions. Once a soil is flooded, moisture is no longer a limiting factor. However, excess water generally results in depletion of soil oxygen, thus producing anaerobic conditions. Some soil organisms are capable of functioning under anaerobic conditions, and since soils in Illinois are often periodically flooded, and may be temporarily anaerobic from time to time, information on anaerobic processes in Illinois soils is relevant. The processes under study include degradation of the pesticide trifluralin and microbial reduction of two important microbial alternatives to oxygen, nitrate and iron. Soils were obtained from farms across the state of Illinois. The data gathered thus far strongly implicate a connection between all three of these microbial processes. The results of these studies are being compared with data collected from a large on-farm soil quality project to determine whether these anaerobic measures of soil biological quality are related to chemical, physical and other biological measures taken on the same farms.

North Central Region SARE 1997 Annual Report.