Graduate Thesis Or Dissertation
 

Relationships among phosphorus soil tests, forms of soil phosphorus, and plant uptake of phosphorus for selected southwestern Oregon soils

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  • A diversity in climate, geologic parent material, and soil characteristics exist in Jackson, Josephine, Coos, and Douglas Counties of southwest Oregon. Previous field trials and soil testing results have revealed uncertainties as to the phosphorus (P) status of the soils in this area. Twenty-four sites from nine agriculturally significant soil series in southwest Oregon were studied. Soils were selected on the basis of the parent material from which they were derived, either granitic, basaltic, or sedimentary. A greenhouse pot study with Yamhill wheat as the indicator plant was used in conjunction with a modified Change and Jackson P fractionation procedure to determine the forms of soil P which were most available to plants. The organic-P content of each soil was also determined. The aluminum P (Al-P) and iron P (Fe-P) fractions were the most highly correlated plant available P on most soils, although the organic P fraction of the Coos County soils appeared to be an important source of plant P. The Bray P₁ dilute acid-fluoride, Olsen 0.5 M NaHCO₃, and Morgan's NaOAc P soil tests were used to estimate plant available P. The Bray P₁ and Olsen soil test values were more highly correlated to P uptake by wheat and yield response to P fertilizer than the NaOAc soil test values. The Bray P₁ test was more sensitive than the Olsen test at relatively low soil test values. Also only the Bray P₁ soil test values were significantly correlated to the organic-P content of the Coos County soils. These results indicate that of the three procedures studied, the Bray P₁ method is probably best suited for the estimation of plant available P in the mineral soils of southwestern Oregon.
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