The change in the Mg/Al and Ca/Al ratio in the soil solution during soil acidification (Fig. 5.1) affects root growth (Rost-Siebert,1985). The primary effect appears to be a substitution of Ca and Mg by Al in the secondary cell wall, which affects its extension growth (Jorns,1988). As a result there is a correlation between the number of root tips and the Ca/Al ratio in chemical parameters (Meyer et al., 1986). In addition to the interaction with Al, root growth is also affected by a changing NH₄ cation ratio (Bertiller,1986). An increase of ammonium in the soil solution of the humus layer has been observed in acidifying soils (Fig. 5.4; Schneider et al.,1989) which is probably a result of the recent increase of deposition of ammonium from the atmosphere as well as from a reduced microbial activity in acid soils.

   The Mg/Al and Mg/NH₄ ratios affect not only root growth, but also the uptake of cations. Ca and Mg are reduced at the presence of Al and NH₄ (Schröder et al.,1988; Jacob, 1955). In addition, nitrate uptake is inhibited in the presence of NH₄ (Roeloffs et al.,1985,1988). The ratios of stable isotopes of ¹⁵N/¹⁴N indicate that, under conditions of low base saturation, conifers use ammonium rather than nitrate from the soil solution (Schulze and Gebauer,1989).

   The results on ion uptake of roots indicate that the leaching of nitrate from soils, which contributes strongly to soil acidification, results in essence from a plant-regulated physiological process, namely the selective uptake of ammonium by tree roots. Soil acidification, that initially started by sulfate leaching, had a feedback on the form of nitrogen used by the plant cover. An increasing NH₄/NO₃ ratio in the soil solution resulted in preferential uptake of ammonium. This again had a positive feedback on the leaching of nitrate and the acidification process (Schulze, 1989).

   Low supply of cations alone cannot account for the observed expression of nutrient deficiency symptoms during forest decline, because if all nutrients were equally deficient for optimal growth, plants would adjust their growth within a certain range to match the supply of nutrients and show no deficiency symptom (Ingestad,1987). In order to understand the interaction between growth and cation nutrition Oren et al. (1988) have applied the concept of Timmer and Armstrong (1987) and demonstrated that newly formed needles display nitrogen deficiency symptoms rather than optimal supply (Fig. 5.5). At the same time, Mg had been incorporated to new needles to such an extent that even luxury levels were reached! This indicates that these trees received N far below their optimum level for growth, even though nitrate was leaching out of the soil profile, again pointing to a selective uptake of ammonium.

   Only 30--50% of the Mg incorporated in new growth originates from concurrent root uptake (Oren et al., 1988). The larger proportion is reallocated from available pools in the existing tree biomass (Fig. 5.6), especially from the 1-2-year-old needles. When new twigs and needles start to grow, re-allocation will be proportional to the supply of N and independent of the initial Mg concentration in the old needles. When initial Mg content of needles is low, mobilization of Mg from old needles, in response to N - dependent growth of new needles, will lead to yellowing. Thus Mg deficiency results from a N/Mg interaction during the period of canopy growth (Oren and Schulze, 1989).

   Stem growth tends to follow needle growth, and occurs at times when the available pool of Mg in the biomass is likely to be almost depleted. Therefore there is a strong relation between stem growth and the Mg/N or Ca/N ratio in old needles (Fig. 5.7).

   After termination of growth, uptake of cations is used for storage, which supports canopy growth in the next season by reallocation. Also, the amount of re-charge will determine whether a tissue becomes deficient in the wet season.