Vascular plants soilpH mineral nutrients and microorganisms

Figure 2.3 emphasizes the important influence of soil pH; chemical weathering, for example, proceeding very much faster in the most acid soils. Humification (the breakdown of dead plant material to humus) and biotic activity in general increase towards the mid-range of pH and then drop away as soils become alkaline. Soil bacteria and fungi react differently to variation in soil pH, though it strongly influences both. The optimum pH for soil bacteria is slightly on the alkaline side of neutrality (Clark, 1967); though they commonly tolerate a range between pH 4 and 10, some species have a relatively narrow tolerance to soil reaction. Soil fungi are better adapted to acid soils than bacteria, and bacteria do poorly on distinctly acid soils even when the competitive rivalry from fungi is restricted experimentally by the use of fungicides. This intolerance of bacteria for acidic conditions causes the decay of organic matter in acid soils to proceed relatively slowly. Moreover, although anaerobic conditions are the main cause of mineralization by bacteria (the conversion of organic compounds to soluble inorganic chemicals - see Chapter 7), very acidic conditions, with abundant positive hydrogen ions, lead to the accumulation of ammonia as the ammonium ion (NH4+) rather than nitrate nitrogen (NO3~) simply because the ammonium carries a positive charge. Direct and indirect effects of soil pH on the soil fauna also influence decomposition patterns (see Section 7.2).

The influence of soil pH on soil nutrients varies greatly. As can be seen in Fig. 2.3, the availability of iron and manganese diminishes steadily as pH rises, sometimes to the stage where they limit the growth or performance of particular plants. In contrast, the availability of molybdenum is greater in alkaline soils as is that of most other nutrients until very alkaline conditions are met. Larcher (1975) found the protoplasm of most vascular plant roots to be severely damaged outside the range pH 3-9. Within this range, various plant species show a wide range of tolerance of soil pH, and the resulting changes in soil chemistry, in the same way that they do to drought or waterlogging.

Sir Arthur Tansley observed in the early 1900s that some plants were restricted to calcium-rich soils (calcicoles - calcium loving) while others were not (calcifuges - calcium fleeing); see Lee (1999) for a detailed review of the subject. The best examples are perhaps seen in grassland species but the principle applies equally to forests on acid and alkaline soils. The causes are very complex and have been traced most significantly to the effects of pH, primarily through the agencies of iron and aluminium rather than calcium itself. On acidic soils both aluminium and iron are highly soluble and potentially toxic even at low concentrations but calcifuge plants have effective chelating mechanisms (which combine a metal ion with substances in the root) that prevent harmful levels of absorption. Iron cannot be completely locked up since it is an important nutrient and this inevitably lets some aluminium in despite its lack of a role within the plant (it is interesting to note the paradox that aluminium is the commonest metal ion in soils and the one metal not used by plants). However, on alkaline, calcium-rich soils, similar mechanisms lock up what little iron is available and calcifuges look pale and sickly (chlorotic) due to iron deficiency. Conversely, calcicoles lack such efficient chelating mechanisms and so can scavenge the small amounts of iron available on alkaline, calcium-rich soils; but on acidic soils have no defence against toxic amounts of aluminium. Other factors may complicate the issue in some plants such as tolerance of boron poisoning, which develops in markedly alkaline soils, and excessive amounts of calcium uptake on alkaline soils. Acid secretions by roots into the rhizosphere (Section 5.5.1) may also play a vital role; calcicoles produce an abundance which, although readily broken down by microbes, may mobilize iron, phosphorus (P) and cations and so improve uptake; conversely, calcifuges which produce few acid exudates can be deficient in P on alkaline soils (Zohlen and Tyler, 2004).

The majority of vascular plants are amphi-tolerant (amphi - 'both sides'), competing successfully with others in the mid-range of soil pH. They do best, however, when they are growing at the pH that suits their physiology, so slight changes in pH can result in quite different plant communities or even a monoculture field layer. Species unable to compete at mid-range pH are in nature often confined to habitats where competition is less intense. Though creeping soft-grass Holcus mollis has a physiological optimum around pH 6, in central European woods it is frequently crowded out by other species (Ellenberg, 1988). In English oakwoods H. mollis is unable to compete effectively with wavy hair-grass Deschampsia flexuosa below pH 4.0, so it is restricted to a rather narrow soil pH band by competition from other species.

Wavy hair grass is described as acidophilic-basitolerant (acid loving and tolerant of alkaline soils), and outcompetes the acidotolerant creeping soft-grass. On soils which are not excessively acidic, wavy hair grass also outcompetes acidophilic heather Calluna vulgaris whose roots are restricted to acidic soil horizons. Colt's-foot Tussilago farfara, a ruderal (plant of disturbed areas) sometimes found along woodland rides, is in contrast basophilic acido-tolerant. Interestingly, bearberry Arctostaphylos uva-ursi competes best on acid and basic soils, being uncommon on those of intermediate pH.

Fungal associations with tree roots are almost universal in long-established woodlands, in which mycorrhizas are of great importance (see Section 5.4.1). Fungi of ectotrophic mycorrhizas form a compact sheath of hyphae over the roots, which are stimulated to form the numerous stubby branches commonly seen in beech, oak, eucalypts and pine. Mineral nutrients (notably N and P) and water absorbed from the soil by the fungi are passed to the trees, from which the fungi receive simple sugars. Tree growth is often limited by the level of available phosphate; association with mycorrhizal fungi greatly improves the situation. In difficult situations, such as the establishment and maintenance of agro-forestry in semi-arid regions, inoculation of young trees with mycorrhizal fungi can be a very wise investment. The height of seedlings in their first year from new forest nurseries in New Zealand was always very variable until the soil had become inoculated with suitable mycorrhizal fungi.

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