Water is indispensable for human, animals and plant life. It is a part of all organisms, some of which contain more than 90 percent. Water is essential part of protoplasm. It is an important ingredient in photosynthesis. About 400 to 500 liters of water is necessary for production of a one kilogram of plant dry matter. Water is also required for translocation of nutrient and dissipation of heat.
Properties of water:
Water molecule contains two hydrogen ions and one oxygen ions. The space occupied by each water molecule is mainly due to oxygen ions while two hydrogen Ions do not occupy practically any space. The shape of the water molecule is sphere and the position of two hydrogen ions is at the corners of a tetrahedron that exists within a sphere.
The positive valences of hydrogen ions are partially neutralized by negative valency of oxygen ion. Thus, one, end of water molecule has positive charge and another end has negative charge. This makes water molecules a dipole.
Water molecules do not exist in individually. Hydrogen in water serves as connecting link from one molecule to the other and it is known as hydrogen bonding. Water sticks to it self with great energy and this property is called cohesion, where as water attaches itself to surface of many substances and this property is known as adhesion. By adhesion, water is held tightly at the soil water interface and water is retained in the soil by adhesion and cohesion. The water molecules hold other water molecules by cohesion forces. Because of these forces, water fills small pores in the soil and is in fairly thick film in large pores.