Allison Piper

Date of Award

2005

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Plant, Soil, and Environmental Sciences

Advisor

M. Susan Erich

Second Committee Member

Gregory A. Porter

Third Committee Member

Timothy S. Griffin

Abstract

Crop rotations, and the use of organic amendments to supply plant nutrients, offer many potential agronomic and environmental benefits. This thesis examines root length density (RLD) of four rotation crops and quantifies soluble soil carbon (C) and soluble soil phosphorus (P). The main objectives were: i) to accurately measure crop RLD, ii) to discover potential correlations between RLD, soluble C, and soluble P, iii) to determine whether additions of soluble C can influence P solubility at a constant pH and iv) to determine whether a slight soil pH change can influence P solubility. Roots and soil from potato (Solanurn tuberosum), barley (Hordeum vulgare), soybean (Glycine max), and a forage consisting of alfalfa (Medicago sativa) and timothy (Phleum pratense) were sampled from the Aroostook Research Farm in Presque Isle, Maine, during the summers of 2003 and 2004 to determine RLD and soluble C and P levels. Half of the plots had a history of amendment with beef manure and half had not been amended. Forms of P in the soil and in the manure were chemically characterized. Laboratory experiments were conducted to determine the effect of organic acids and pH on P solubility. Root length was found to differ significantly between crops. Barley and forage consistently had higher RLDs than potato or soybean crops. Barley and forage typically had higher levels of soluble soil C, but the differences were significant at only three of the five sampling dates. When correlating RLD to soluble C, the r values were marginal and none of the relationships were significant. Thus, RLD does not appear to be a significant predictor of soluble soil C levels. As with soil C, soluble soil P levels were typically higher in barley and forage crops. Significant differences were detected at four of the five sampling dates. However, the correlations between soluble C and soluble P were only significant at two of the five sampling dates. The significant correlations were found in amended soils; manure increased levels of both C and P and enhanced the relationship between these two variables. In laboratory experiments a slight pH change of approximately 0.5 units resulted in no significant difference in P solubility. However, additions of citric and oxalic acids significantly increased P levels. Thus, it is likely that the organic acids are complexing A1 and inhibiting P sorption. These results indicate that P solubility is more strongly influenced by soluble soil C than by small changes in soil pH. Root length was not found to be a significant predictor for soil C levels. However, it does appear that soluble C can significantly increase P solubility, therefore cropping systems that promote increased soluble soil C levels may enhance P bioavailability.

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