Soil Factors and Vegetation

After reading this article you will learn about the effects of soil factors on vegetation.

Within vegetation much depends on the morphological characters and spatial distribution of shoot parts. A rosette plant is deemed to have fewer chances to reach the required level of illumination than tall species or plants with more vertically directed leaves.

When leaves reach a more exposed position, the energy supply improves but at the same time there is a greater need for root activities. In fact, the seedlings are now in the same situation as those growing from the very beginning on exposed areas.

Freely growing plants and plants partially exposed in vegetation are both affected by physical soil conditions, so it is not necessary to distinguish between the growth response in early and more mature phases of vegetative development: in free-growing plants the growth distribution follows a quite regular pattern from seedling emergence until flowering.

Complications may occur if, in the course of the plant’s development, the nature of the processes that limit whole-plant growth changes as a result of interactions between individual plants (interspecific as well as intraspecific competition).

Hence, the effects of physical soil factors on the vegetative development attention should be paid to:

(a) Differences in response between freely growing plants and plants in dense vegetation; and

(b) Differences in response due to interactions with other factors in the environment.

During germination the optimum soil temperature is quite high, the rate of germination being determined mainly by the biochemical reactions by which the reserves in the seeds are converted into structural tissues in the seedlings. Once the shoots have emerged, there is a distinct drop in optimum temperature. In this phase the absorption processes determine the growth rate.

At a later stage, when the plants have developed a closed-crop surface, the rate of dry-matter production is the same over a large part of the temperature range. Only at very low temperatures at which the leaves are partly wilted does root temperature cause a measurable reduction in dry-matter production. Root-zone temperatures primarily affect root growth and development.

At very low root temperatures (5 and 10°C) there is hardly any extension growth, because leaf water potentials are very low. Despite the markedly reduced increase in leaf area, the photosynthetic activity is much less affected. This leads to an accumulation in the plants of reserves that remain partially available for growth resumption as soon as conditions improve.

The relative insensitivity of the photosynthetic process explains why the effects of adverse soil conditions are less harmful once a closed-crop surface has been attained. All light is then captured and diverted into dry matter.

The degree of adversity of a given soil factor depends largely on the density of the vegetation. In dense vegetation, light is the limiting factor and thus diminishes the importance of soil factors. In open vegetation soil factors are more important.

This might partly explain why annuals or biennials may occur in relatively open vegetation whereas dense vegetations are almost completely composed of perennials with different growth forms.

In rosette plants and in grasses in the vegetative stage these growing points are located in or near the soil surface, and are therefore influenced by the soil temperature. As a result, the growth of such plants is more profoundly affected by soil temperature (Fig. 23.4), since not only root growth root activity but also shoot development, e.g. leaf appearance are directly controlled by this factor.

The temperature at a height of 0.5 cm above the soil surface determined the growth of perennial ryegrass in the field. In addition to leaf-area development, soil factors-including soil temperature-may affect leaf orientation.

This further complicates a quantitative evaluation of the significance of soil temperature when plants are grown in competition. The degree of adversity is also dependent on other external conditions to which the plants are exposed (interaction).

The interaction between light level and the osmotic concentration as well as the temperature of the root medium are shown in Fig. 23.5. The response to a reduced water supply induced by either an enhanced osmotic concentration or by low root temperatures is of minor importance at low light intensities.

The effects of a lower water potential, which persists under such conditions of low light supply, are completely obscured. At increasing light intensities the growth rate of the controls increased correspondingly, and the effect of the adverse factor became manifest at decreasing levels.

It is interesting to note that in the range of maximally tolerable adversity a shift also occurred. Plants which had survived the stress treatments at low light intensities died when treated in the same way at high light intensities. Under low root temperatures this mortality was due to the low water potential which developed as a consequence of the high irradiance.

At high NaCl concentrations the death of the plants might have been caused by the toxic effects of ion accumulation in the tissue. An analysis of the components responsible for these growth responses showed that both morphological (or phenotypical) adaptations (leaf area ratio) and physiological processes (net assimilation rate) are involved (Fig. 23.6).

Plants growth in well-aerated soils (well supplied with oxygen, no accumulation of CO₂) with plants grown in soils with reduced gas exchange invariably shows reduced root growth. Quantitatively, considerable differences between species occur (Fig. 23.7). The penetration of roots into anaerobic layers depends on a number of internal and external factors.

A high level of metabolism, high temperatures, and high organic nitrogen content all reduce maximum penetration considerably, e.g. with waterlogged sandy soils, maize roots penetrated to a depth of 75 cm below the ground -water table at 15°C and low nitrogen nutrition. The depth of penetration was reduced to 20-25 cm at 25°C and a higher nitrogen nutrition level.

In their response to anaerobiosis, maize plants occupy an intermediate position between plant species whose roots hardly penetrate the anaerobic regions in the soil (Phaseolus vulgaris) and others whose roots are much less affected than maize roots (Oryza sativa, phragmitesaustralis).

With respect to the physiological aspects of this adaptation, various plant properties are involved because anaerobiosis is generally accompanied by changes in other soil properties, viz., low partial pressures of oxygen, accumulation of carbon dioxide, a lowered oxidation-reduction (redox) potential, and the accumulation of reduced forms of carbon, nitrogen, sulphur, iron, and manganese, some of which may reach toxic levels.

The degree to which tolerance is achieved on the basis of the following plant properties:

(a) The ability to exclude or tolerate soil-borne toxins,

(b) The development of air-space tissue,

(c) The ability to metabolize anaerobically and tolerate an accumulation of anaerobic metabolites and

(d) The ability to respond successfully to periodic inundation.

Roots of intact plants can oxidize reduced substances in the root medium associated this ability to oxidize the rhizosphere with the ability to penetrate reduced paddy soils. The oxygen diffuses from the atmosphere, via intercellular spaces in shoots and roots, to the root surface.

The limited size of the normally present intercellular cavities, cannot even satisfy the oxygen demands of normal root metabolism, let alone provide sufficient rhizosphere oxygenation to avoid intoxication by reduced soil-borne substances.

Most plants respond to reduced oxygen tension by enhancing the porosity of the cortical tissue: values of about 5 per cent in well-aerated soils can increase to between 15 and 85 per cent, depending on the species (Fig. 23.8) and the degree of anaerobiosis.

The adequacy of the oxygen supply to the active sites determines whether or not normal aerobic respiration can proceed. If it cannot, most plants shift over to anerobic dissimilation processes, and this leads to the accumulation in the plant root of toxic products such as ethanol and lactic acid.

Adapted species have been found to accumulate less toxic substances instead, for instance shikimic acid and malic acid. In wet soils an O₂ gradient will generally be maintained from the soil surface to the lower soil layers. Strongly reduced root growth in deeper soil layers leads to enhanced induction and growth of (adventitious) roots at the stem base, where conditions are less adverse.

It is generally rather difficult to decide which factor limits root development in a particular case. Radial oxygen losses enable some plants at least to improve conditions in the direct vicinity of the roots. Differences between plant species include differences in sensitivity to a number of factors acting at the same time.

Whereas root growth responses of various species to waterlogging tend qualitatively in the same direction although they differ greatly, the concomitant shoot responses show even greater deviations quantitatively.

The effect of a transition from favourable to adverse conditions depends on the nature of adaptive differences. The time required by plant to adapt themselves to the new situation determines their chances of survival. Rapid responses are required to meet a suddenly inadequate water supply or increased transpiration rate, because water in the tissue has to be replaced continuously.

With respect to the mineral supply, much greater variation is acceptable. The same holds for the carbohydrate supply, since a rather efficient feedback between photosynthesis and respiration is thought to exist.

The differences in response determine whether one species will win or hold its ground at one place and others will not. The responses of beans (Phaseolus) and willow cuttings (Salix spec.) to flooding may be mentioned as examples of such differential behaviour.

After a period of growth in well-aerated soil, root elongation in both species is stopped by flooding. Shoot growth, however is affected only in beans, which indicates that root functioning is reduced more in beans than in willows. This is confirmed by the time course of transpiration before and after flooding.

The response depends to a great extent on the situation in the vegetation. The relative importance of a given soil factor for growth is greater for individual in open vegetation than for those in closed vegetation, since in the latter light is more likely to be the ultimate limiting factor.

In such a situation, however, rather small differences in root response may have important consequences for the whole plant, since being a little ahead in the beginning will have considerable advantages in the competition for light as well as in the exploration of soil-bound resources.