Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Douglas L. Karlen

Second Advisor

Richard M. Shibles


Evaluation of soil quality requires the development of a minimum dataset that can be used at different levels and scales. Soil organic matter is a major reservoir of plant nutrients, and hence the dynamics associated with it due to management practices are important. Particulate organic matter (POM) has been suggested as one of the parameters that may be sensitive to management practices and hence can be used to monitor the changes in soil organic matter. The objective of the first study was to investigate the temporal and spatial changes in particulate organic matter when a field that had been under the Conservation Reserve Program (CRP) was plowed. The study was carried out at Taylor County site in southwest Iowa, on a field that had been under pasture for 8 years. Soil samples were collected weekly from mid-June until late October. This study demonstrated that cultivation caused a significant reduction in mineral fraction carbon in just three days. Although the mineral carbon level increased slightly thereafter, it remained below that observed in the uncultivated field. Cultivation increased particulate organic matter at the 15 to 30 cm depth, presumably because of the residue incorporation. A synchronous fluctuation between total organic matter in the whole soil and that found in the mineral fraction showed that the mineral fraction did not control the total amount of carbon in the soil. Particulate organic matter contributed to a higher percentage of the total whole soil organic carbon at the beginning of the season than at the end. It is concluded that particulate organic matter is a sensitive indicator for assessing soil quality effects of post-CRP management practices;Nitrogen fertilizer management has received much attention in the last decade because of its perceived environmental on underground water quality. It has been argued that procedures that would improve its management would lead to improved water quality and be economically feasible because of the savings that farmers would have by applying less fertilizer. Recent studies indicate that the chlorophyll meter can be used as an alternative tool for predicting of nitrogen requirement for corn. A study was initiated at two sites to investigate how various soil management practices affected plant nitrogen concentration, and if the chlorophyll meter could be used to improve nitrogen management. At the Larson farm, five fertilizer treatments were used (0, 67, 134, 201 and 280 kg/N ha) for corn grown using conventional tillage. Chlorophyll meter measurements were made at three growth stages. Similar measurements were made at the Nashua site, but at six growth stages. At the Nashua site, treatments included two tillage and seven management practices. Results from Larson Farm showed a good correlation between fertilizer treatments and chlorophyll meter readings at all growth stages. Grain yield was also highly correlated with chlorophyll meter readings. At the Nashua site, there was good correlation between chlorophyll meter readings at V12 stage and yield. Plant nitrogen content at six growth stages were poorly correlated with yield, probably because of drought stress. There was a good correlation between plant total nitrogen content and chlorophyll meter readings at V6. We conclude that the chlorophyll meter can be used as a tool to improve nitrogen management.



Digital Repository @ Iowa State University,

Copyright Owner

Moses Mang'eni Siambi



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133 pages