Earlier studies revealed that Pinus radiata shows a good response to fertilization (Sanchez-Rodnguez et al. 2002; Omil et al. 2005; Solla-Gullon et al. 2008). However, this response varies depending on the age of the trees, on the density, and on the nutritional needs (Zas 2003). The demand for elements such as N and P increases during the first years of life, a maximum of 6-8 years (Turner and Lamber 1986). Ca and Mg, which do not suffer retranslocation before needle fall (Ericsson 1994), may cause problems in forest crops since the demand for these elements by higher plants tends to increase with age (Turner and Lamber 1986).

Analysis of covariance revealed that improvement in soil nutritional status leads to higher tree growth from the second year of treatment onward, confirmingthe findings of other studies (Bonneau 1995; Vesterinen 2003). This is due to the site quality (ecological conditions, soil physical properties, weather, and water avail­ability of the trees): under adverse conditions fertilization does not improve soil nutritional status. Thus, the response was more significant in plots on lutites (of lower site quality) than in plots on migmatites. This confirms the results reported by other authors, mainly for rich soils (Silfverberg and Huikari 1985; Ferm et al. 1992; Emilsson 2006). Production may be improved by application of a higher dose of ash, although there is a risk of leaching contamination and increased heavy metal concentrations.

The application of other types of waste such as slag and dairy sludge was found to increase tree height growth (Virgel-Mentxaka 2002; Omil et al. 2005). The combination of ashes with certain organic wastes has considerably increased the growth of agricultural crops (see Chap. 4, Nieminen 2011). However, the amount of N applied must be taken into account. Prior studies revealed that the tree structure of this species may be affected by N and P stresses (Will 1985). Although height growth is not usually altered, radial growth in both stem and branches was most sensitive to addition of these elements. These changes have important impli­cations such as the loss of apical dominancy in fertilized trees and possible stem distortions, resulting in a decrease in the economic value (Will 1985; Hopmans and Chappell 1994).

3.4 Conclusions

Fertilization with bark ash improved the nutritional status of Pinus radiata planta­tions, and increased the contents of the main macronutrients in the needles. How­ever, some limiting nutrients such as P did not exceed critically low levels, so there is some room for improvement in the fertilization treatments. The effects on the needles were also inconsistent, and delay and the intensity of the effects depended on the time of application. The third application of ash significantly improved the diameter and height (and so the volume) growth in one of the plots (lutites) relative to the control treatment. It may be concluded that only after careful site evaluation ash application is indicated, otherwise the costs may exceed the benefit.