The soils under study are acid, as are most forest soils in the region, mainly due to the instability of the parent rock and to the strong leaching of the base cations. Also, the plant growth itself generates soil acidity. This implies a great obstacle to the growth of forest species since it reduces the availability of essential nutrients such as P (Sanchez-Rodriguez et al. 2002; Merino et al. 2005).

In this study, the application of ash slightly and temporarily increased the pH of the solid soil fraction, resulting in higher availability of elements such as K, Ca, and Mg. The availability was proportional to the quantity of ash applied. The concen­tration of Ca increased greatly after the second application. Thus, a direct relation­ship between the application and the availability of the element was observed.

Nevertheless, the increases were less than those reported for soil treated with fly ash (Ohno and Erich 1990; Saarsalmi et al. 2001; Hytonen 2003; Solla-Gullon et al. 2006).

The behavior of P was different, and a low response to the different treatments was observed. This may be due to the low availability of P in the ash, as well as to the chemical and biological reactions that this element undergoes in the soil. In acid soils, phosphate ions tend to precipitate with Fe and Al, forming insoluble com­pounds and thus reducing the concentrations of H2PO4 and H2PO4 taken up by plants (Garcia-Rodeja and Gil-Sostres 1997). It is also possible that the availability of P decreases owing to microbial immobilization, a process that is accelerated by the application of organic waste to the soil (Salas et al. 2003).