Results of foliar analyses may differ depending on environmental factors. The date of sampling, the age of the plants, and the type of parent material found below the plantation must be taken into account.

The foliar concentration of N tends to decrease in response to application of biomass ash, as expected from the low N content in the ash and its immobilization in the soil. However, an increase in N concentration has been observed in some peat soils, which can be attributed to a higher mineralization as a consequence of an increase in pH and availability of nutrients (Weber et al. 1985). In a prior study, Solla-Gullon et al. (2006) also detected an increase in foliar N concentration in Pseudotsuga menziesii plantations in Galicia.

In this study, the foliar values of P were prone to increase, as observed by Moilanen et al. (2002), Ludwig et al. (2002), and Solla-GullcSn et al. (2008). This positive response may be the result of the symbiotic association between ectomy — corrhizal fungi that colonize the ash. These fungi increase the solubility of the content of P in the ashes, thereby promoting the uptake of ash by trees (Mahmmood et al. 2003). Use of a diagnostic system based on the N-to-P ratio showed that a balance between N and P was achieved, since it did not exceed the normal ratio ranging from 6 to 16 (Raupach 1967). These values are similar to the average value of 12.0 reported by Solla-Gullon et al. (2008) for Pinus radiata in Galicia.

Foliar K concentration also increased slightly in the WA and WAP plots, confirming the relationship between plant and soil concentration (Zas 2003). Increases in plant K concentration in response to the application of ash have been reported earlier (Moilanen et al. 2002; Solla-Gullon et al. 2008). However, no differences were observed by Hytoonen (2003).

Despite the higher availability of Ca and Mg in the soil, foliar tree analysis did not reveal significant increases in concentration for these elements. This is consis­tent with the findings of some studies in which concentrations did not increase, or increased only slightly (Hytonen 2003; Moilanen et al. 2002; Ludwig et al. 2002). Other studies in which greater amounts of ash were applied revealed increases in the foliar concentration, which lasted for a few years (Solla-GullcSn et al. 2008; Arvidsson and Lundkvist 2002).