Soil solution and other soil analyses as indicators of nutrient supply: a review

This review examines the potential far using soil solution as a tool for ma naging soil fertility. A review of the current use of other types of soil a nalyses indicates that, while their use in some cases is justified, there a re substantial limitations to the development of reliable and widely applic able calibrations. Factors that govern concentrations of nutrients in soil solution and the methods for measuring them are reviewed in relation to the ir use in nutrient management of forest plantations and agricultural crops. Topics include a discussion of (i) nutrient supply and uptake mechanisms; (ii) solution culture studies which define critical concentrations in solut ion; (iii) methods of sampling solution from soils and (iv) estimation of c oncentrations that can be maintained at root surfaces in soil. By inference , nutrient supply would not limit plant growth if concentrations at most ro ot surfaces (e.g, young roots in surface soil) were maintained at or above concentrations needed to maintain high rates of growth in solution culture, i.e. critical concentrations. Several aspects of this method have been val idated for N and P in Eucalyptus nitens plantations. For example, when conc entrations of ammonium (the preferred N source for E. nitens) in the field fell below the critical level of 50 muM, plantations of E. nitens responded to applications of N-fertilizer. This method was also useful for predictin g P deficiency in corn (Zea mays), Eucalyptus globulus and E. nitens grown in soils of widely different P-supply characteristics. The convergence of c oncepts based on the principles of soil nutrient supply and uptake, which l ink soil and solution culture studies, is likely to provide a unifying appr oach for diagnosing nutrient-supply limitations to plant growth and a pract ical tool for nutrient management in forest plantations. (C) 2000 Elsevier Science B.V. All rights reserved.