Soils and trees

Soils are often given superficial treatment, and yet without them forests would quickly cease to function. As well as physically supporting plants, soils act as refuse collectors, processing organic waste and thereby recycling nutrients, a major influence on the productivity of forests. Without functioning soils, forests would rapidly be choked with dead wood and other material, and the bulk of nutrients needed by plants and animals would be locked up and unavailable.

Moreover, soil is not simply a loose collection of'dirt', it is a complex mix of living and non-living components, consisting of air (soil gases: typically 25% by volume), water (25%), mineral particles (45%) and organic matter (5%); the last can be subdivided by weight into around 10% organisms, 10% roots and 80% humus. As described further in Chapters 1 and 7, various soil animals, such as earthworms and arthropods and the micro-organisms, including fungi and bacteria, decompose dead material to release nutrients and form the left-over, rather inert black humus of the soil.

Soil takes a long time to form, usually thousands of years, and its quality is one of the most important conditions governing the growth of trees, smaller plants and associated organisms in any site. As Fig. 2.1 demonstrates, soils have distinct morphologies each with a characteristic profile (a sequence of horizontal layers or horizons from ground surface to unaltered bedrock or sediment). Soils are the product of the integrated effects of five soil-forming factors: climate, parent rock, vegetation and associated organisms, relief of the land and time.

These set the conditions under which physical, chemical and biological processes operate to produce the distinct layers or horizons found in soil profiles. The first three main groups of the processes involved, weathering, translocation and the organic cycle are constructive and tend to build up the horizons, while the last two - erosion and deposition - tend to blur profile morphologies

Iron pan resulting from illuviation

C, parent material

A, showing eluviation

Iron pan resulting from illuviation

C, parent material

R, bedrock

Figure 2.1 Idealized soil profile showing the main horizons. The O horizon is dominated by humus and plant litter at various degrees of decomposition. The A horizons are characterized by humus mixed with the mineral particles and include zones of eluviation, the movement by water of fine particles (such as clays and humus) and dissolved substances down further into the soil. The A horizons are commonly composed of an upper horizon and a lower, darker horizon showing eluviation (A1). The B horizon is where these particles and possibly the dissolved chemicals accumulate in a process called illuviation. Here iron and aluminium oxides may build up enough to form a water-impermeable hard iron pan. Below this the C horizon is made up of weathered parent material not affected or moved by the pedogenic processes, and the R horizon is composed of the bedrock.

particularly on hills and in valleys. Weathering is the process whereby the mineral components of the soil are broken down either physically into smaller particles or chemically, releasing nutrients or other chemicals into the soil. Translocation is the movement by water of fine particles and dissolved substances down into the soil (eluviation) and their deposition lower down (illuvia-tion). Some of these components, especially in the case of dissolved chemicals (solutes), may be taken away by the flow of groundwater, a process termed leaching. The organic cycle is the decomposition and redistribution of organic matter to form distinctive horizons in different soils. Erosion of material from one place by wind or water will obviously leave behind a truncated profile with one or more horizons missing (particularly where wind is involved) or a profile divided by channels and valleys (as with water erosion). Deposition gives rise to a range of new fairly mixed (azonal) soils, named according to the agent of movement: alluvium (water), loess (wind) and colluvium (gravity).

The weathering of fresh rocks releases reasonable amounts of the nutrients required by plants (see Section 2.2.1) apart from nitrogen. This element is typically needed in greatest quantities, its supply is often the main factor limiting forest growth, and it is usually most lacking in young soils. Despite making up 78% of the atmosphere, gaseous nitrogen cannot be used by plants until it has been 'fixed', either as nitrate (NO3) produced by lightning, or more importantly as ammonia by specialized microorganisms (or by the Haber process for artificial fertilizers). Nitrogen fixation is therefore a crucial process (see Chapter 8). Inorganic nitrate produced by lightning and received in rain will gradually build up in the developing soil, but it is significant that plants which establish first, and so commence the organic cycle on which other organisms depend, often have their own nitrogen-fixing mechanisms (see Chapter 8). Such plants release soil nitrogen for their successors when they drop litter, die and decompose, and the forest then depends primarily on organic nitrogen being recycled. The increasing amounts of ammonia and oxides of nitrogen in the atmosphere released from agriculture and industry have resulted in many forests becoming saturated with nitrogen, rather than limited by it.

Organic materials also give rise to humus, which provides a reserve supply of nitrogen, forms part of a base-exchange complex (see Section 2.2.1), and helps retain soil moisture. The speed at which shed tree leaves are disintegrated by biological and chemical processes varies enormously with climate; in tropical rain forests such as the Congo, 50% of such a cohort may be completely broken down - largely by fungi - within twelve weeks, a process likely to take years in Lapland. The nutrient quality of the litter is also important, especially the C/N ratio: litter rich in N breaks down faster than litter poor in N. Inevitably there are feedback links, between the type of litter, the climate, decomposition and availability of nutrients in the soil; this is discussed further in Chapter 7.

Much of the organic matter contributed to the soil by trees is in the form of shed leaves and dead roots which accumulate at or near the surface. Tree roots transport mineral nutrients from within the soil profile. These are deposited on the soil surface through leaf fall or near the surface in roots and the organic cycle makes them available to forest herbs.

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