Doctoral Dissertations

Date of Award

5-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

David L. Coffey

Committee Members

Robert A. McLean, Charles D. Pless, Robert D. Miller

Abstract

Production of vegetables and tobacco (Nicotiana tabacum L.) have traditionally been through monocrop, conventionally tilled systems. With the increasing importance of low input practices and conservation, and the declining number of producers, there is a need to develop reduced tillage systems and sequential cropping. This need prompted an investigation into the effects of reduced tillage and sequential cropping of tobacco, tomato (Lycopersicon esculentum Mill.) and broccoli (Brassica oleracea var. Italica L.) production systems. Three tillage systems (no-till, conventional till with a winter cover, and conventional till with no winter cover) and three cropping sequences (spring broccoli followed by tobacco or tomato; spring broccoli followed by tobacco or tomato, followed by fall broccoli; and tobacco or tomato followed by fall broccoli) were used in 1989 and 1990 at three locations in eastern Tennessee. Tobacco and tomato systems were evaluated separately. Also, since broccoli yields were suppressed by reduced tillage, field, greenhouse, and laboratory investigations were initiated to assess the possible allelopathic influences of wheat cover crops on the growth of tobacco, tomato, and broccoli. Field soil samples were evaluated for presence of five organic acids which are known to be allelochemicals produced from decaying wheat residue. Ferulic, p-hydroxybenzoic, p-coumaric, syringic and vanillic acids were applied to these crops individually at various concentrations. Methods of extraction and determination of concentrations of these acids from soils through high performance liquid chromatography were developed. Field samples were then evaluated for quantities of these acids. Broccoli yield and quality were reduced by minimum tillage but were not affected by cropping sequence. Tobacco was also adversely affected by reduced tillage. Tomato yield, however, was not affected by tillage, but fewer cull fruit occurred in no-till. Earlier planted tomatoes had somewhat higher yields and quality than those in later planted sequences. Allelochemicals had negligible effects on tomato and tobacco growth but retarded broccoli growth and dry weight accumulation. Greater concentrations of ferulic and p-coumaric acids were found in soils from no-till plots than from conventional till plots. Methods of extraction and analysis of organic acids were successfully developed and described in detail.

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