Colloids: Colloids are extremely tiny particles which are so small that they can not be seen by optical microscope ‘ and remain suspended indefinitely in water. They have the property of being electrically charged and can attract and hold ions of calcium, magnesium and potassium, known as bases. These bases are required by plants for their growth. – Soil colloids are also useful in holding water in the soil. When present in large quantities, they make the soil sticky and tough so that it is difficult to cultivate.