Effect of Physical Properties of Soil on Plant Growth

After reading this article you will learn about the effect of physical properties of soil on plant growth.

The environment is a complex of so many factors, all interacting with each other, that it is impossible to isolate any one factor that does not influence another. For the study of environmental effects, however, this complex is usually sub-divided into clearly defined units.

On of these units is the soil, which is vitally important for plant growth and development. Soil in itself represents a complicated physical, chemical, and biological system by which the plant is supplied with the water, nutrients, and oxygen it requires for its development.

Although over the centuries plants have adapted themselves to various kinds of soil, the adaptation capacity of certain species is limited. This can be clearly seen when soil properties alter. The nature of the soil determines whether a species will thrive and influences its natural distribution. Within small areas, slight local variations in the soil may be sufficient to affect a plant’s chances of survival.

The physical properties of soil are known to be of fundamental importance for plant growth, but much of the literature on the subject is qualitative or vague.

This is not surprising in view of the difficulty one encounters in attempting to divide the edaphic factors unambiguously into physical, chemical, and biological classes. Most physical phenomena have important effects on the chemical and biological soil properties and processes, and these in turn influence plant growth.

Soil is a physical system and can be described in terms of grain size, apparent density, porosity, moisture content, temperature, and friability. Plant growth is affected by the amount of moisture and air in the soil and by the temperature of the soil.

The composition of the soil can impede or foster root development and shoot emergence. It should be mentioned, too, that the physical features of the soil have certain indirect effects on other edaphic factors such as nutrient supply and pH.

Unlike mobile organisms, terrestrial plants are bound to the soil where the seed has fallen. Plants generally have to cope with a hostile environment and may not survive, but in course of time every type of soil, however hostile, becomes covered with vegetation. Since the type of vegetation depends on the prevailing soil conditions, in a sense each particular vegetation is adapted.

In nature, however, plant species are rarely found on the soils whose physical conditions are optimal for their growth and performance.

Comparative experiments with various plants show that the general shape of the curve representing the response to the degree of severity of adverse conditions (such as oxygen deficiency, soil compaction, low soil temperature, high sodium chloride concentrations) is very similar to all plants, whether or not they are adapted.

Minor differences in a single soil factor are sufficient to cause minute variations in the occurrence of plant species in the field. Comparative experiments have also shown not only that plant have a tremendous plasticity that enables them to survive under adverse conditions but also that species develop different strategies in order to survive.

We do not fully understand many plant-soil relationships because we do not have sufficient knowledge about:

(a) The physical conditions of the soil in space and time;

(b) The differences in a plant’s response at various developmental stages;

(c) The plant’s response to changes in the degree of adversity of conditions;

(d) The extent to which a plant’s response is determined by the interaction of other factors;

(e) Methods to assess the effect of minor differences in response over a long period in a plant’s life-cycle.

One of the serious drawbacks of Eco physiological experimentation is that we can hardly discern differences amounting to less than 5-10 per cent but in nature even smaller differences may determine discrimination in the long run, especially in interspecific competition.

It cannot solve the two main problems in plant ecology, namely:

1. Why are certain types of vegetation (species) restricted to a certain habitat, whereas others clearly prefer another set of conditions?

2. How can diversity of species in vegetation be maintained for a relatively long period (measured by human standards) when so many individuals are all dependent on the same resources, i.e. light, water, and minerals?

The second of these problems is the more challenging one, since we know that in contrast to the diversity in nature, competition experiments almost invariably result in survival of only one of the competing species.

The niche concept, a separation of interests in time and (or) space, was introduced to reconcile this discrepancy, but we shall have to learn much more about the ways in which plants behave before these questions can be adequately answered.

If we want to obtain satisfactory answers we must pay close attention to the complete life-cycle of the species in question, since niche differentiation may show up in only of the life stages (germination, seedling establishment, vegetative growth, generative growth, and dissemination or seed longevity).

Sometimes adaptation to a certain habitat can be due to relatively small differences in number of aspects, none of which alone would fully explain the species’ preference for a particular habitat.

Since the Eco physiological approach is based on experience acquired in the field of crop physiology, possibly essential differences in behaviour between natural vegetations and crops must be taken into account.

In both kinds of populations and individual responds to the complex of conditions but agricultural practice has selected for uniformity of response, whereas natural selection has often resulted in the maintenance of a certain degree of diversity and plasticity within a population. Moreover, the external conditions are much less predictable for natural vegetations.