Biology Today - February 2018

O-horizon

• Surface layer of organic matter, rich in humus.

A-horizon

• Uppermost horizon of soil, called top soil.


• Rich in organic matter. Typically dark colour.


• This horizon is rich in microorganisms and
  has high biological activity.

B-horizon

• Also called subsoil and zone of accumulation.


• May contain soluble minerals, rich in plant
  nutrients.


• Poor in aeration and biological activity.

C-horizon

• Unweathered bedrock.


• Bedrock lies below the soil profile.

O

A

B

C

Horizons

Fig.: Soil profile

• Water vapours : They occur
  in the soil atmosphere and
  make the latter saturated.


• Hygroscopic water : It is
  not available to the plants
  as the water is held very
  firmly by the soil particles.


• Capillary water : It is water
  present inside micropores
  and is available to plant
  roots.


• Combined water : The
  water occurs bound up
  in chemicals and is thus
  unavailable.

Water

held in

soil is

of four

types

Responses to Abiotic Factors

• Living organisms cope with stressful conditions by various methods:

Regulate

• Some organisms are able to maintain a constant body temperature and
  constant osmotic concentration despite changes in the external environment
  are known as regulators. Only birds and mammals and a very few lower
  vertebrates and invertebrates belong to the category of regulators. All perform homeostasis mostly through thermoregulation
  and osmoregulation by physiological adjustments and rarely by behavioural changes.

Conform

• Most of the animals and nearly all plants do not have a mechanism to
  maintain a constant internal body environment. Their body temperature
  changes with the surrounding temperature. These animals and plants
  in which the osmotic concentration and temperature of body changes
  according to ambient conditions of water are called conformers.
  Some species are partial regulators as they have the ability to
  regulate body functions to a limited extent; because beyond that limit
  they become conformers.

Migrate

• The organisms included can migrate temporarily from the unfavourable habitat to more favourable area and return when
  unfavourable period is over. For example, thousands of migratory birds comes in India from Siberia every winter.

Suspend

• Organisms stop their development to survive unfavourable conditions,
  HJ, Bacteria, fungi and lower plants develop thick walled spore
  during unfavourable conditions and germinate in suitable conditions.
  Polar bears undergo hibernation to escape cold, snails undergo
  aestivation and zooplanktons enters diapause stage.

Biotic Factors

• These are the living organisms (plant, animals and micro-organisms)
  who depict interactions not only among themselves but also with the
  physical environment. All kinds of interactions bring about modifications.

Capillary water

Hygroscopic water

Water vapour

Combined water

Gravitational water

Field capacity

Available

water

Unavailable water

Wilting

coefficient

Fig.: Various fractions of soil water