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Comparison of Soil Phosphorus Extractants as Predictors of Mycorrhizal Dependency¹

ABSTRACT

To determine which of several soil P extraction techniques best predicted the response of citrus (Poncirus trifoliate X Citrus sinensis) seedlings to mycorrhizal fungi (Glomus fasciculatus), 25 nursery and field citrus soils from southern and central California were collected and analyzed for available P by five methods. Mycorrhizal dependency (MD), defined as the dry weight (mycorrhizal plants)/dry weight (nonmycorrhizal plants) x 100, was significantly correlated with the reciprocal of extractable soil P regardless of the extraction method. Those methods whose results depend most on soil solution P concentrations gave best results. Saturation extract P(P_SE, R² = 0.67***), anion exchange resin P(P_AER, R² = 0.57***), and 1:10 soil to water extract P(P_1:10, R² = 0.51***) were all acceptable for predicting mycorrhizal response. Bicarbonate-extractable P (P_B1C, R² = 0.32**) and ammonium fluoride P (P_AF, R² = 0.19*) were less acceptable. Using regression models, based upon the reciprocal of the extractable-P value, a MD > 200% was predicted where P_SE < 0.65 mg L^–1, P_AER < 62 mg kg^–1 soil, P_1:10 < 0.66 mg L^–1, P_BIC < 34 mg kg^–1 soil, and P_AF < 54 mg kg^–1 soil. Where analytical capabilities are adequate, P_SE is the preferred method for predicting response, while P_AER would suffice where higher analate P concentrations are required. Alternatively, inclusion of both P_SE and P_AER gave an excellent correlation with response (R² = 0.82***). Inclusion of DTPA-extractable Mn significantly improved regression equations in all cases except that of P_SE, where DTPA-extractable Zn significantly improved the relationship.

¹ Contribution from the Dep. of Soil & Environmental Sciences and Dep. of Plant Pathology, Univ. of California, Riverside, CA 92521.

² Formerly Research Assistant (now Extension Specialist, Univ. of Idaho), Assistant Professor of Soil Science, Associate Professor of Plant Pathology, and Staff Research Associate, respectively.

Received for publication December 6, 1982. Accepted for publication May 16, 1983.