1. Audley DE, Skelly JM, McCormick LH, Jackson WA (1998) Crown condition and nutrient status of red spruce (Picea rubens Sarg.) in West Virginia. Water Air Soil Pollut 102:177–199
2. Ball J, Carle J, Del Lungo A (2005) Contribution of poplars and willows to sustainable forestry and rural development. Unasylva 221(56):3–9
3. Bengoa JL, Rueda J (2001) Variación estacional y espacial de los niveles foliares en parcelas de ensayo de clones de Populus × euramericana y P. × interamericana. Libro de actas del I Simposio del Chopo, Zamora (España), pp 211–219
4. Bennett WF (1996) Nutrient deficiencies and toxicities in crop plants. APS press, The American Phytopathological Society, St Paul
5. Brady NC, Weil RR (1999) The nature and properties of soils, 11th edn. Prentice-Hall, New Jersey